FEATURE INTERVIEW

This summer 2004 I had the opportunity to have an in depth chat with Russell Joyner, Pratt and Whitney's Discipline Chief for Propulsion Systems Analysis & Integration.

BB: Hello...This is Bruce Behrhorst of nuclearspace.com and I'm speaking with Russell Joyner, Discipline Chief, Propulsion Systems Analysis, Pratt & Whitney. This is Pratt & Whitney's thermal nuclear rocket entry: "TRITON", TRI modal capable Thrust Optimized Nuclear propulsion & power system for advanced space missions.
  Russ...Could you explain to our readers the Tri-modal nuclear rocket system design that has resulted from the collaborative effort between, the NASA Glenn Research Center (S. Borowski), Pratt & Whitney (R.Joyner), University of Florida (S. Anghaie), Aerojet (M. Bulman, T.Neill), RENMAR(S.Bhattacharyya) and SAIC (D. Pelaccio).

How is this particular design so different from the ESCORT (excore scalable cermet orbital repositioning technology) and the GE-710 reactor Nuclear Thermal Rocket (NTR) concept of the ' 60's?

RJ: First off... The idea for "TRITON" comes from Greek mythology and is based on a mythical creature with god-like powers, Triton was supposedly the son of Poseidon and Amphitrite and lived with them in a golden palace in the depths of the sea. He would ride the waves on horses and sea monsters and he carried a twisted conch shell, upon which he would blow either violently or gently, to stir up or calm the waves. Triton is represented as having the body of a man with the tail of a fish, but sometimes also with the forefeet of a horse.¹
  The whole idea of using the mythical creature "Triton" to describe this evolved nuclear thermal engine is because how Triton possessed three strengths and the TRITON engine has three positive attributes in the design. It relies upon a fast spectrum nuclear reactor design approach, which has payoffs in creating a higher power density reactor and with the CERMET (UO2 dispersed in Tungsten) fuel form it provides for less fission particle release. When used in TRITON nuclear thermal rocket, the fast-spectrum CERMET reactor provides thrust in propulsion mode by using Hydrogen which is gasified by the high temperature of the full-power reactor and the gaseous hydrogen is accelerated out through the nozzle giving you Isp values (i.e. thrust divided by mass flow) of greater that 900 seconds for nominal exit temperatures of 2,600-to- 2,700 K. This approach, provides you with the high thrust typical of liquid chemical rockets of today. Basically you can do fast finite burns at high thrust when in use in space within hours to provide the velocities needed for planetary mission. For the TRITON design we size the reactor for the energy to heat enough hydrogen to provide around 15,000-pounds of thrust. This means we can reduce the total system cost by using designs of the components based on today’s RL10 family of in-space chemical engines.

 Part two, is power production, the power production comes about by doing a unique design form to each individual 'fuel element' using special channels or passages manufactured in the 'fuel element' to permit both the flow of the gaseous hydrogen and gaseous helium and xenon at the top of the reactor. During the different operating modes. both can pass through into the reactor and help cool it. Each element is cooled and also provides heat to the working fluid (i.e. the hydrogen or the helium-xenon mixture) as it passes through each of the different elements. Within the baseline reactor for producing 15,000-pounds thrust is about 100 of the fuel elements.

 When in the baseline propulsion mode, the hydrogen gets heated, super heated actually to temperatures of greater than 2500^oK(Kelvin) and then it exits out the bottom or aft end of the reactor to a choked region we call, 'the throat' and then is expanded and accelerated out through the nozzle. That's the thrust part I had referred to earlier.


  The other nature of the 'fuel element' is it has an internal (in the center of the element) 'Energy Transfer Duct' (ETD) or 'Turnaround duct' that permits doing power mode when not thrusting. The reactor is basically idling atless than 1% of the maximum power for the general values we've looked at for given certain Mars missions. You typically only need 25-50kW electric power to run the spacecraft systems and to keep any cryogenic propellant cool for the long 180-220 day journey to Mars and that basically means you 'size' the reactor for propulsion mode since that is where the highest power increment is needed. We are essentially talking about something on the order of anywhere (reactor design) in size 300-to-500MW power thermally to put out heat for the hydrogen for producing 15,000-30,000lbs of thrust. Then the reactor can run at 100-400 kW thermally to produce 25-100 kW electric.

So, what happens is each reactor 'fuel element' goes to idle mode and you're only producing 100kW thermal optimized for 25 kW electric power production. It's essentially turned down...way down, in terms of power the reactor is producing thermally. It then has enough power to heat a ' working fluid' in this case in TRITON as I've said earlier we use a mixture of Helium and Xenon . The mixture is at a molecular weight of 40 that's re-circulated and then goes up to drive a turbine that's connected on a shaft to (compressor) much like current jet engines do today operating with air, but for us it's a helium-xenon gas. Thus turning the turbine using the high temperature (i.e. ~1,100-Kelvin) helium-xenon mass flow provides the working power to drive the compressor and the helium-xenon gas is cooled as it expands out into a space radiator and then back to the compressor.

After it exits the compressor the temperature has gone down considerably and the gas mixture is brought back to the reactor. An alternator is attached to the shaft of the turbo-compressor unit to generate the electrical power for the spacecraft.  Thus we have a closed power production cycle (Closed Brayton Cycle [CBC]) when the reactor is just sitting there idling; the second operating mode of the 'TRITON' engine. The power output can be scaled to a specific mission need using one CBC unit or multiple CBC units can be used to provide redundancy for longer space missions with higher power needs.

The third mode, thus the [name] TRITON. Is when we use thrust augmentation on the engine with Liquid Oxygen (LOX) .The LOX is combusted with gaseous hydrogen that has exited the reactor core supersonically downstream of 'the throat' at optimum injection points at within the regenerative section of the nozzle. This gives us 200% more thrust by the addition of the LOX flow that is combusted downstream in the nozzle. It is similar to the way it's done in the 'afterburner' of military jet engines today. Most military jet engines use 'augmenters' as we call it in the military world . In current military jet engines, 'augmenters' , which apply additional fuel to the hot combustion gases, provide additional combustion in a duct behind the last turbine stage and before the nozzle to give added thrust on take-offs or for other mission requirements. Our Aerojet partner and we took the same methodology and applied it to the nuclear rocket engine. Aerojet has actually tested the LOX-injection for hypersonic engines and in a configuration that is similar to that needed for the TRITON engine.  The reason that's important is we can keep the core small, very small, we feel we don't need a core producing 2000MW. We want it producing 300MW keeping it scaled down in size making it cheaper by reducing the amount of Uranium production that has to be installed, reducing materials cost, manufacturing and test facility cost as well. �It allows the whole rocket engine systems to stay small in terms of what components could be used without having to use very expensive large high mass flow turbopumps like the Space Shuttle's main engines. You can in effect stay small like the upper stage turbopumps on our RL10 engines.

� So, the three operating modes can be thought of as: 1) Pure thrust mode using hydrogen when you want high Isp >900seconds moderate thrust 15,000 to 20,000lbs thrust . 2) Power mode when you're idling for electrical power to the spacecraft for a human mission or for supporting electrical propulsion for a Mars Crew Transport Vehicle ( MCTV) . 3) The third mode, which really benefits you when you're in a deep planetary " gravity well" at low departure orbits and the spacecraft is trying to push out large payloads similar to Mars landers and 6-8 crew habitats for conjunction-class Mars missions. Mars missions at this point have higher payloads that drive you to be to be lower in thrust-to-weight and the vehicle would not perform as well due to higher gravity losses. So, in this case on a particular mission you can be mission optimized by the added feature of LOX--augmentation and get higher thrust off the core engine without having to oversize to non-optimum higher thrust-levels. It also eliminates the tedious, and expensive process of building a new re-designed second core nuclear thermal rocket engine for higher payload fraction missions . Also for the higher payload fraction vehicle configuration you'll still get fast transfer times like with typical high-thrust nuclear thermal rockets. In other words leaving the proximity of the planetary orbit within less than an hour instead of taking weeks or months to spiral out characteristic of electrical propulsion systems. The other nice feature is a common reactor core that allows you the flexibility of three different mission approaches-this is a key feature to keeping mission cost affordable.

 Lastly the mythological nature of TRITON is available to lead human based missions across the cosmos much like Triton the Greek mythological deity did for the Argonauts in the spirit of discovery and exploration.

 How it's different from the ESCORT design?

 ESCORT originally was design based on some United States Air Force flight requirements. They wanted a long life, high-Isp propulsion system that could also develop power while sitting at high orbits (i.e. > 1,000-km). It would move things around quickly back in the days when we thought we had adversaries that could be shooting at our satellites. It also was to develop high power for systems like large aperture radar systems.

 So, this was the nature of the ESCORT system to have thrust and power for military satellites . The original ESCORT design used liquid metal coolant in the power mode and while in propulsion mode would use either ammonia or hydrogen. It was 'sized' for real low thrust, 500 to 1000 pounds for moving satellites for orbital transfer and was actually too small a thrust size for most manned earth escape or interplanetary missions. The fuel form design was slightly different; it still used the fast spectrum reactor design based upon a CERMET fuel. Primarily, the power mode coolant and its integration were different than what we're now using in TRITON. TRITON is specifically designed for human interplanetary missions for exploration where ESCORT was designed as a long-life repositioning system for orbital transfers, station -keeping and to provide low-power levels.

BB: Is there a 'fail safe' operation in the event the reactor core must be shut down exiting a planetary 'gravity well' or on approach to a 'gravity well' ?

RJ: There are several features that we have adapted and evolved into the current 'TRITON' design to handle risk mitigation for the Uranium Dioxide (UO₂) fuel element core in a Nuclear Thermal Rocket (NTR).
  We have approached this by providing an integrated, robust design the uses dual turbopumps (turbopumps provide coolant flow to the reactor in propulsion mode).
  In thrust mode where you have high power operation, is where this concern has been typically addressed.
The safety features that have been taken into account for risk reduction entail constant supply of reactor coolant by using dual turbopumps. This means turbopumps with their moving parts like bearings, shafts, turbines etc. may cavitate and over speed, if for some reason one of the turbopumps showed signs of malfunction or not operating within appropriate parameters, you could effectively shutdown or bypass the offending turbopump and still have coolant flow going to the reactor. This is one of the key features for propulsion mode operation to make sure coolant is available to flush; the reactor if it needs to be shut down when it has gotten to the full thermal power level. In power mode it's [core] sitting at an idling power-level so the amount of time for the reactor to over-heat if starved of coolant (i.e. He/Xe gas) is extremely negligible because you are running the reactor core at nearly half the maximum temperatures the core is design for. So, if in the event of something like let's say, a minor leak in the radiator during power-mode operation, you can do a shut-down of the reactor from a very moderate control state without over-heating the reactor core. Other failure mode mitigation would be to have a segmented radiator design, or have a coolant purge circuit in the design, or actually split the coolant circuit to provide redundancy. We also have several valve arrangements so that in the event of leakage in idle power mode you could shut a section of the radiator down; the temperature of the reactor is so low it would cool down on its own. This works to our favor in the TRITON design because the CERMET core materials have high maximum operating temperatures since it's designed for exit temperatures near 2,700-K in the propulsion mode.
  Another feature is the nature of going to a fast spectrum reactor. It allows issues such as criticality and impact immersion (e.g. wet sand or salt water) to immediately be mitigated because of the reactor neutron flux levels and the use of only a reflector and no moderator to thermalize a bulk of the neutrons. Essentially it helps to 'poison' the internal nature of the reactor so in the worst case event at launch, if the reactor were to end up in sand or saltwater it will keep it from resorting to a super-critical state. If it shuts down after a brief period of operation, like for some reason and I had to shut it down during an early phase of a human Mars mission, the 'burn-up' (fission product build-up) is so low. Even if I run it for only 5 minutes or, 10 minutes I'd have built up only a minuscule amount that could barely be measured with regards to build-up of fission products in the core. So if it did for some reason re-enter the earth's atmosphere, the radiation levels are only slightly higher than typical naturally occurring levels. Now, you would have to methodically go through a full risk analysis, or a whole mission point-to-point to define the 'What if scenarios' along the mission's plan to properly build in aborts for all the most probable failure modes.
For example, one 'What if scenarios' would look at the failure modes for an orbit capture high-thrust burn at a planet Mars or for Lunar transport. In essence, an inventory of reactor core fission product build-up vs. mission time would be needed and you would map those relative to mission abort requirements. The result would be a total fission build up product inventory to know were risks lay thus, and use that to plan for mitigating risks for future human missions with a nuclear thermal rocket system. You would also work to build additional redundancy or robustness in the reactor design architecture.
Another part that adds additional safety aspects is the nature of the fuel form itself, the CERMET fuel form has a lot of thermal shock resistance by going to a Uranium Dioxide -Tungsten matrix and it's much more resistant to any hydrogen gas impinging or embrittlement.

BB: You've specified a Ceramic-Metallic matrix uses a tungsten UO₂ and Gadolinium(Gd302) mixture with cladded refractory high temperature metal alloys such as Tungsten/Rhenium which I realize has an extreme heat spectrum, fast flux tolerance and overall is rugged and can take punishing temperature extremes.

RJ: With the hydrogen gas...that's key and the Tungsten is used as the primary variable of fission product retention. It also protects the UO₂ fuel from the corrosive properties of the superheated hydrogen coolant/propellant.

BB: Is the bottom of the 'TRITON' core next to the nozzle chamber similar to a KIWI-B4 in core architecture?

RJ: It's on a Tungsten metal grid, each of the elements sit and fit in that grid which are encapsulated in a 'can' approach, simply similar to a 'tennis ball' canister.

BB: Are the CERMET Tungsten fuel elements porous in nature to allow for hydrogen gas flow?

RJ: No, it's of solid nature.

BB: So, Hydrogen gas surrounds for example, a single CERMET tungsten fuel element unit.

RJ: Not quite, the hydrogen flows through each of the elements that have several Tungsten-Rhenium coolant tubes or channels and the fuel element is also cladded on the exterior. Each has a Tungsten-Rhenium cladding that wraps each individual fuel element.

BB: A typical question, you're blowing H₂ through these elements at supersonic speeds. How do you deal with vibration, scouring and chattering etc. could an element break off within the core block?

RJ: That's an excellent question. First the gas speeds only get sonic at the throat and the hydrogen gas exits the reactor core at low subsonic conditions. We had a design we looked at in 1992 (i.e. XNR2000 and a bi-modal design in 1998. We called it the "CPPS design" (Common Propulsion and Power System). In that design we noted we would test how to alleviate acoustical feedback that was a result of resonance conditions due to high pressure, high temperature, and high hydrogen flow rates. Several of our designers with the 'TRITON' engine looked at that and advised the best remedy would be to tie it down from the bottom and block it at the top so that you're constantly in either a compression or tension state preventing the element's instability. The nature of an external 'can' or pressure vessel holds it in compression as well. So in past solid -core designs, elements were either tied down from the top or tied down from the bottom. They had a tendency to want to sit there if they were tied down from the bottom, but when each individual fuel element was hit with the right frequency with coolant flow matching the frequency of the fuel element, their alignment resulted in the element wanting to literally 'jump up' out of the reactor core. Vice versa if you tie them down from the top alone they would have a tendency like 'wind chimes' to rattle against each other.
  So, we looked at the "TRITON' in this case, and we're using the legacy Rover/NERVA designs knowledge about how how they did it and what was driving them to their conditions. We employed the matrix advantages of CERMET fuel, since it is so dense by nature of the materials in the matrix. In locking our fuel elements from the top and from the bottom it still allows for thermal expansion and it keeps the fuel elements from wanting to resonate like it did with older designs that just had graphiteor carbide based fuel element blocks. We looked at that and tried to take a design approach on the current 'TRITON' design that allows us to fabricate it in two stages with the fuel element sitting within a grid-type bottom of the 'can' and then the pressure vessel they fit in and gets locked down from the top with another retainer.

BB: So, essentially you have modified the elements to avoid supersonic resonance feedback.

RJ: Yes, but in other words we have adjusted loads-paths to stabilize the fuel elements and designed a more stable pressure-vessel for the fuel elements to fit in.

BB: The CERMET fuel elements I presume can't be oxidized by the gases that it comes in contact with?

RJ: UO₂ is non-reactive and the Tungsten prevents it from being eroded when coming in contact with hot hydrogen or hydrogen based elements. It is the stable nature of UO₂ with hydrogen that provided one of the reasons it was selected for Naval Nuclear Reactors, Hyman Rickover had the Navy designer examine many forms of dispersing Uranium within metals and oxides and the Navy didn't want to use pure Uranium metal because it could readily oxidize very easily, especially in water if you're doing pressurized water reactors with water as a coolant. UO₂ on the other hand is already 'rusted' since it's in an oxide base and so its behavior is going to be predictable. Now the other part of this, is taking the UO₂ and cladding it with Tungsten to makes it highly corrosion resistant especially for high temperatures. This is the advantage of using UO₂. And Gadolinium in the mixture helps us add a touch of 'poison' like I said, toward fission product build up; it also helps alleviate some of the tungsten's reactivity toward hydrogen as well, - it's an all around good mix.

BB: TRITON's exhaust plume essentially would not have UO₂ particulate down to a micron (1µm) size?

RJ: That's right, Tungsten is the primary material for the fission product retention. It works quite well in terms of hot hydrogen. Hydrogen was the biggest problem that they were looking at for graphite even in composite fuel forms. It hasn't been shown that binary or ternary carbides are going to be able to pull together very well in hot hydrogen either yet. They do show high resistance to high temperature-right? But, when you talk about hot hydrogen compatibility that is still an issue. And hydrogen right now is still the best propellant to be considered for propulsion because of its low molecular weight and thus high Isp - that results.

BB: At the AIAA conference I noted the BWXT company representative; the company whose responsibility it would most likely be to manufacture "TRITON engine" fuel elements mentioned CERMET fuel elements would take 7 years to pass full manufacture certification. Is that correct? How long would it take to pass certification?

RJ: I believe his comment was in reference to an FOC (full operational capability) fuel element that they fully tested, qualified and was in full production within that time. They have indicated in prior discussions they could do two demonstrators testing within a year in relevant environment and have a prototype fuel element manufactured and ready for testing within two more years. The manufacturing process also has to be tested as well so this adds a couple more years. So, there are certain approaches to the fuel element design 'life' that you hope to achieve for the components that you're putting in toward the CERMET elements and the total time to get a CERMET element ready for a TRITON engine could most certainly be way less than seven years. The BWXT company with its extensive experience has done a very good job with projects that I've seen. They've actually indicated that there could be a 'full-up' testing to evaluate the fuel form in a nuclear furnace and that maybe in some Department of Energy (DOE) lab. Then the fuel elements are delivered to a reactor pile for what's called a 'bundled test' with the actual TRITON design number of fuel elements.

BB: One risk assessment study, ("An examination of emerging In-space Propulsion Concepts for One-Year Crewed Mars Missions" by Dennis G. Pelaccio, Gerald A. Rauwolf, Gaspare Maggio, Saroj Patel, Kirk Sorensen) of a round trip Mars human mission (beginning year 2018) based on risk criteria: In-space environment crew exposure, propulsion system crew hazards, system degradation, system complexity, ground and spacecraft hazards, in-space assembly complexity and safety, crew transfer, abort option/capabilities, disposal requirements.
Comparing propulsion type options with mission:
| Chemical | BNTR (Bimodal Nuclear Thermal Rocket same as BNTP) | High power-NEP (Nuclear Electric Propulsion) | VASIMR (Variable Specific Impulse Magnetoplasma Rocket) | Momentum Tether | SEP (Solar Electric Propulsion) | SEP Chemical |
They elected BNTR as a most attractive option as a mature technology, modest Initial mass at LEO (low earth orbit) of 685 metric ton with 32 launches required. Thirty-two launches seems a bit much.
How would TRITON improve on those findings?

RJ: They must have been using a Delta II booster for that many launches...Ha, ha, ha? Generally most Mars mission nuclear thermal rocket (NTR) designs have had no power production capability. If you used Graphite composite fuel forms, the most proven of the NERVA legacy solid core NTR designs, the pure NTR would most likely weighed less than a Bimodal propulsion and power system approach. Previous Rocketdyne reports showed such results. The nature of the TRITON engine is that it uses the tactic of an enriched U235 fuel (in the UO₂matirix) and allows the thrust-to-weight to go up relative to the other types of propulsion strategies proposed. So, you have a bit higher thrust-to-weight with the TRITON engine because of its design approach for a pure NTR. This helps to offset the extra mass for having the Closed Brayton Cycle (CBC) units and radiator for producing power. The higher power density of the CERMET fueled fast spectrum reactor core is really the key to getting higher thrust-to-weights and Isp's that exceed a NERVA type design at the same thrust level.  All this helps mitigate the additional weight issue that comes with taking along the power generation system at the system level. In the end it provides for a smaller Earth departure mass or smaller Mars mission spacecraft.

BB: To sum up, your saying, the ceramic-metallic matrix Tungsten UO₂ with cladded refractory metal alloy Tungsten/Rhenium (W-Re) is much better than Graphite, Tricarbide composite or Particle Bedded Reactor (PBR) options?

RJ: Generally, yes. If I could make a lighter Tricarbide composite fuel and keep as non-reactive in the presence of hot hydrogen that may start to compete with the durability and temperature performance of the CERMET. If I could come up with an ideal mix with excellent cladding that worked the same way as our fuel element form 'recipe' using a Tricarbide composite maybe I would incorporate that. But remember there hasn't been any testing with hot hydrogen at the quantities or levels of temperature with the Tricarbide composites like that done with the CERMET (UO₂-W) in the GE-710 program. �Also the PBR progressed far during the SNTP effort in the late 1980's, but still had many issues to resolve regarding the fabrication and low thermal capacity of the reactor core.
The evolution of an advanced fuel form for the TRITON issomething to study, but presently, there has been no testing history at all for the other promising high-temperature fuel forms that warrants moving from CERMET. Only the CERMET fuel form tested back in the GE-710 program (high temperature gas reactor program) in the late '60's is the only one that did hydrogen testing on ceramic-metallic fuel form using UO₂/Tungsten and showed that it was the only fuel form with any lifespan capability in hot hydrogen.
A Bimodal feature in a NTR allows you to reduce some of the major spacecraft power system elements that you would need by reducing the size of the typical photovoltaic or fuel cells that are sized to handle all the power demands of the entire spacecraft. It takes some of that weight off the spacecraft. This is the advantage, because if you put nearly all of your power production at the end of the spacecraft where you are power-rich with an NTR based spacecraft, then you're downsizing the PVA's (Photovoltaic Arrays) required. Then you would only have the PVA's as a backup to give you a few hundred watts or so to execute critical life support mission objectives.

BB: People tend to say,"... A reactor, is a reactor, is a reactor."
There are actually very large differences from the radioactive inventory point of view. A typical nuclear rocket would have a radioactive inventory that was about at 1 part in 106 of what a standard commercial reactor would have at the end of its life.
In fact a typical nuclear rocket mission would only produce a few Curies of Cesium and Strontium, which are the most troublesome radioactive isotopes. Put that in the context of what we now have on Earth, which is something like 10 billion Curies of Cesium in the radioactive spent fuel in storage pools and something like 100 million Curies already in the biosphere. In a typical NTR mission you would produce only 10 Curies of Cesium. It's very small and it's way out in space; besides astronauts would be radiation hardened for voyages requiring NTR.
What in your estimation are plume-expelled by-products of the fission process in TRITON?

RJ: Well...Let me paraphrase your question since I don't have that technical information readily in front of me. Do I think that the level of fission product that would be released in the exhaust of the TRITON rocket would be dangerous to a crew on a full duration Mars mission or similar to a crisis-type RTG type (plutonium) re-entry? I think the risk is a lot lower because of the nature of the fuel element forms we use and the fact that by using the cladding the way we do retain a lot of those fission products. We also retard the fission product build up by going to a fast spectrum reactor. So the level of scale is going to be considerably lower for a Bimodal or Trimodal design in the case of the TRITON engine. The TRITON fast spectrum nuclear thermal reactor produces less than it would as a thermal based nuclear thermal reactor design that does a lot of its fissioning in the low end of the spectrum versus the high end. If I had the opportunity now with a full-up TRITON development program I could give you general test results on TRITON so we had quantitative number for comparison purposes. I really can't give you a scaled-off number from older graphite NERVA type reactor that didn't have any way to retain those fission products as well since it wasn't cladded with Tungsten like the TRITON.

BB: What about fission gas release percentage?

RJ: Fission gas release is a direct function of the atom percent burn up. And for the UO₂ Tungsten cladded CERMET fuel form we are down below 1/10 of a percent of burn up over the whole mission life. That takes you down to where fission gas release percentage is less than a 1% and that's hard to even reliably predict until we do testing, it's considerably low. Personally, I don't want to be spewing out a lot of fission product even if it's out in space even if no one (human) is behind me or nor would I want to be putting it out near another planet. It's not a good thing to design or allow for high fission product in the exhaust. The other point is in terms of testing. I would like to have fission product retention high so I can retain it all. The way I want to test this with a small engine so that my total product production is so low anyway. It'll be easier to retard release by exhausting into a duct and then cleaning the duct or to the point were it has no release. Basically I don't have problems with fission product release or major fuel swelling over time with the TRITON engine operating for hours during high-thrust and years at power mode levels producing less than 500 kW thermally.

BB: Is there any value in designing 'Swirl vanes' placed inside nozzle chamber in an effort to create vortex toward skimmers for the purpose of cleaning radioactive material from plumes?

RJ: Bruce, I really don't have a way to postulate on that. I know how we've used it to enhance combustion. The use of swirl for combustion purposes because of centrifugal force in terms of delineating the density of gas products. As an indirect consideration you would think it's a positive thing to swirl to help you capture any higher molecular weight or higher micron elements toward the outside so you know where they are instead of them being dispersed. It seems like it would be something to investigate beyond a thesis paper since if I do that you have a better potential of knowing where they are. Thus I can more easily trap and contain them. It's something to be studied.

BB: You have expressed in your paper TRITON: Trimodal capable Thrust Optimized Nuclear Propulsion & Power System for Advanced Space Missions (AIAA 2004-3863). "The TRITON uses the same expander thermodynamic cycle for turbine power in the propulsion mode as the Pratt & Whitney RL10 Liquid Hydrogen-Oxygen chemical engine use on today's expendable rockets."
Will the TRITON be built on the philosophy of expendability?

RJ: The mission designers will dictate whether we expend or reuse the TRITON. Pratt & Whitney will then design according to the mission and vehicle architecture requirements.

BB: In a previous interview I was given some insight ideas on the expendability or reusability it says,
"The key thing here that NASA, to my knowledge has never done any mission studies on what an NTR engine with 10 hours of engine life with a capability 60 stops & starts means for future human exploration missions. If they haven't done it for 10 hours, it certainly hasn't been done for 20 hours or 30 hours of operation. That's a very critical point. What that does is forces you into thinking in terms of mission systems. You would think I've got 30 hours of engine life. How are we going to use those 30 hours of engine life for various missions? Now, let's just take it simple. Assume for mission #1 your going to plan an hour of engine operation out to whatever it could be Mars, Moon and then your bring the engine back to Earth orbit. That’s another hour. So if you have 30 hours of engine life, you're getting 15 missions out of it. Now you have to assume also, if you're coming back from Mars or the Moon or whatever you want to avoid the problem that plagues the trucking industry and that's "Deadheading" - coming back without payload."
Wouldn't it be cheaper in money costs to reuse the TRITON engine for several missions?

RJ: Yes, if the reactor fuel form can be proven to have sustainability of 'life'. I'll pick a number like you mentioned 30 hours maybe even longer at high temperatures...Ok, because obviously running at low temperatures where the reactor surface temperatures are running at 1600^oK or lower versus with the propulsion mode running at 2600^oK-to-2700^oK there is significant 'life', we're talking years. In the propulsion mode is where your statement is applicable for describing running at higher exit temperatures for the higher fuel surface temperatures. The CERMET fuel testing showed maybe up to 50 hours of operation at 2850^oK or higher. If I allowed the reactor to run at 2650^oK or 2700^oK, then I may have a reusable fuel form that can last for at least for 30 missions. The liquid rocket engine hardware that we're bringing in from Pratt & Whitney legacy RL10 has been shown to operate for tens of hours and we run them several times before it's used in a mission today. A nuclear thermal rocket will need very long life turbopumps and using proven turbopump designs that can be compatible with an NTR is the most affordable place to start. Essentially, you would have a bimodal or trimodal nuclear thermal rocket with nominally some number like 30 hours of life in it. Like you were saying, it would have multiple missions in it. Now, you are 'deadheading', if you don't perform another mission with it. But, I'm reusing it to come back with the crew on a very fast return that is not over 1 year or 1&1/2 years. We're talking in terms of about less than 9 months and - that's a positive thing. One scenario would maybe be to stop at lunar orbit so you have even better risk mitigation in mission planning with respect to fission build up to prevent inadvertent return if there is a failure from multiple burns to capture within the earth-moon sphere. I'd stop at a lunar orbit ~ 384,400km, away from earth in large elliptical orbit and have a 'space tug', or 'space bus' to bring it down to lower orbits. This system would be the beginnings of a robust interplanetary transport system.

BB: Like some sort of nuclear powered 'space rail' system to Mars and back?

RJ: Yeah...Kind of like a prototype of a eventual viable system to go back and forth to Mars on many missions, then on its last 'burn' in an unmanned automated fashion have it escape the earth-moon cis lunar area and send it into the Sun for safe disposal. This strategy would get into space operations being very cost effective and its tied to the performance of the fuel element and core design. You have to get the lifespan and reliability out of it based on how you envision the way you operate the engine system. Throughout an entire lifespan of the TRITON engine we would be getting data on the condition of our engine system as it switches from power mode to propulsion mode and back again over and over etc. in successive missions.

BB: In power mode could you in effect run 25kW or 50kW electrical propulsion thrusters (Hall, Ion, HiPEP, NEXIS) for such things as attitude adjustments, maintenance of artificial gravity assist spacecraft rotation etc. and provide quality life support for crew in transit?

RJ: Oh...Bruce, you're right on my preferred concept. It's what I call the "Hybrid approach" It is where I 'size' the reactor for the levels needed for propulsion mode and I would run the power mode at much higher output levels approaching 1 megawatt, I'd probably would design it to produce 100-500kW electrical power for let's say, maybe 8 or more large Ion thrusters, or Hall effect thrusters and use them along the trajectory for pushing me faster to the planets, not simply for station-keeping. I'd use it to further reduce trip times en route to ultra long destinations like Jupiter because once I've done my initial 1 hour long escape burn in the typical nuclear thermal propulsion mode at high thrust, I simply go to power mode and pump electrical power to those electrical thrusters and keep accelerating to reduce the heliocentric transit times and reaching the vicinity of my destination and turn around to present myself for orbital insertion to my destination.

   All this is possible because it allows me to use the beautiful combination of a single common reactor permitting both thermal propulsion high speed travel from the 'high thrust' when I need it in dealing with planet 'gravity wells' and then be able to produce electrical propulsion based 'low thrust' to keep accelerating on those long missions while still providing additional power for spacecraft and cryogenic maintenance of O₂ and H₂ propellants. It's in my estimation the most integrated cost effect system. The idea is to keep the reactor small, 'sized' at 300MW thermal. I could have two TRITON engines and then I have redundancy now for even more interesting mission opportunities. In some Mars mission adaptations I have actual employed three TRITON engines, imagine the interplanetary transport that would provide.
This  multi-engine strategy allows me to do two things; mitigate the issue of propulsion aborts so I still have very robust propulsion capability using one of the other engines if one of the 'high thrust' or 'low thrust' systems needs to be shut down.

BB: Why does TRITON use 8 Boron carbide absorber rotating drums when XE -prime in project rover/NERVA demonstrated temperature sensors alone no neutron sensors and the idea that turbopump control from start-up through full power, shutdown, restart, half power and idling could be achieved. No need for control drums which only added weight and moving parts that could malfunction?

RJ: The reason is safety. It allows me to have a robust control system. Legacy stages (e.g. the XE flight stage design) looked at just using the fuel element and the graphite materials and other materials were acting as a heavy moderator as well. They were trying to get away from using dynamic control. From a safety point of view you want to be able to control the reactor to a slow start and definitely the ability to have active controls lets you know you're tailoring start up and shutdown in a safe way instead of relying on the natural reactivity of the materials. You're right; they add weight to the system. They're there to try and mitigate risk and not go for maximum performance. This feature gives you the most optimum performance for the lowest risk approach for the level of mission requirements needed.

BB: What would a TRITON single engine system cost?

RJ: I was hoping you'd forget about asking that question... Ha! I know in workshops we tried to put numbers together based on historical programs that existed and where we thought we could get started. Basically we're starting a lot of it from ground zero. I have isolated most of the high cost items. I'm going to give you round numbers. To get yourself to the first prototype I think any where between $600-to-$700 million dollars. If you have to add any new facilities to the whole prototype development to get the first one to fly including facility costs, rig testing, design activities and fuels testing you're probably anywhere at $800 million to approaching a billion dollars for a single full up prototype system. Ready to go. Well...you know I still think this is affordable. If I think of the amount of money we spend as adults on beer, cigarettes and adult material it's over thirty billion a year up to one hundred billion dollars per year. If I take the number I just gave you with nine zeros as a point to think about divide that by an eight year program that's about $125 million dollars per year average expenditure to try and get to the means of power and propulsion that you can build interplanetary missions around. Even if I doubled that let's say, making it two billion dollars over eight years I'm still under $300 million dollars  annually of program cost to get a fully capable robust flight system flight ready for human Mars missions. Compared to what we are spending today for frivolous entertainment and consumables, it seems a small investment to start moving us off the planet earth and try to give the human race some survival opportunities by exploring and colonizing the rest of the solar system.

BB: Well...NASA spent a total on Project Rover/NERVA of $567.7 million.

RJ: Their total spending alone, If you go and look approached $2 billion dollars!

BB: Yup...you're right.

RJ: That's NASA and the AEC starting at ground zero with like, I believe 21 programmatic test items before they got to their goals. Between NERVA and KIWI programs they spent that alone it those fiscal year dollars. DOE spent $300 million plus on NERVA, NASA spent $350 million that's almost $700 million dollars just on NERVA alone. That's over what...18 years?

BB: Well...from 1955 to 1973.

RJ: Right...that's what I call 'ground zero' effort they started from nothing, no focus they were testing as they went along and adding as they got more and more experience to what it was they were going to focus on. First they had to prove nuclear reactor would work for propulsion, and then they had to figure out the optimum size. If you look at the furnaces they built and the approaches they did we learned from that legacy and the mechanical designs. How they tested it and how you'd want to test it now before you flew it. There are a lot of good things to be learned from the money that was spent for Rover/NERVA.
If you look at true dollar cost versus a robust interplanetary transportation system it would be something I would invest my money into to think that our species could actually voyage to distant points in our solar system and return to earth safely and efficiently is well worth the cost. There are also a lot of technology jobs that would come out of this endeavor too.

BB: Is Pratt & Whitney entertaining any international cooperation with developing the TRITON engine?

RJ: Not directly, the opportunities for TRITON have not presented itself to go this route yet if we could get coordination so that it wouldn't violate ITAR issues and treaties. Do I think it's possible? My answer is, yes. We just have to wait and see...Do you know what ITAR (International Traffic in Arms Regulations) is?

BB: Vaguely, smells too much like excessive bureaucracy an impediment to cooperation with nations that are able in space exploration capabilities such as in space nuclear technology.

BB: A political question, who would better support the development of a nuclear thermal rocket program in a fully funded space nuclear program; Bush or Kerry and who in Washington D.C. show interest in its development Nelson, Brownback, Rohrabacher or McCain?

RJ: Well...I'm still trying to figure that out. In the House and Senate, generally many support this level of technology development and it aligns itself with the most educated and those that are in the scientific avenues of subcommittees. You see them understanding that if you want to have human missions through the solar system. If you are pro-exploration and the expansion of humans within the solar system then you would support this technology because it would reduce mission time, reduces the risk to astronauts and those traveling in the spacecraft so it reduces the time in transit thus you're less exposed to cosmic rays, and lessens the chance of life support system equipment malfunction. So they recognize the positive attributes to a space propulsion and power system that can push a lot of mass and do it in the least amount of time and reduce the risk to the mission. There are members in the subcommittees that have nodded their heads. Now between presidential candidates? It's hard to tell.

BB: Kerry has already made public statements to the effect, as president he will not support and will end the National Nuclear depot at Yucca Mountain, Nevada.

RJ: Your right, he's voted no, he voted towards ending support for the International Space Station many times when those votes came up. I'm leery of his vision for expansion of humans beyond low earth orbit. I believe you have to have a vision. Leaders try and emulate John F. Kennedy. He had a vision and he realized early on that U.S. technological capability was under threat by a then able Soviet Union adversary. He took a personal interest by actually going out and visiting the Jack Ass Flats, Nevada project Rover/NERVA nuclear rocket facilities. JFK definitely had an interest in technology that would benefit the United States and the peaceful use of nuclear power.

BB: Would Pratt & Whitney consider production of mini nuclear rocket engines for reconnaissance satellite station keeping for platforms to monitor illicit fissile material for terrorist attack prevention and safety for U.S. seaports, airports, borders and other sensitive U.S. Territories?

RJ: Quite simply, I don't know how my particular design would fit with that use, but I would be willing to study the design requirements and see how the TRITON engine could scale to meet the mission.

BB: Do you or the company feel you can get qualified trained personnel from young students to work on your projects?

RJ: Absolutely, the work we have been doing here on the TRITON engine and the TRITON design with one of the most visionary liquid propulsion companies manufacturing rocket propulsion for over 40 years. When we brief interns they say, "Gosh, I didn't know people in Pratt & Whitney were working on Nuclear rocket engines how could I get to work on this project?" The visionary approach of trying to do something that extends human kind off this planet as in a Human Diaspora or migration with in our solar system excites many of the educated young students. Even younger students that my wife teaches science to now are excited by the idea of human spaceflight through the solar system. You instruct them on going to Mars, employment opportunities, exploration of the unknown in space and students get excited as they see themselves participating in this activity in either the design of something or being one of the humans to travel across the expanse of space. Many dream of going to new places like settling Mars and exploring Jupiter and the other planets.

BB: So, you're confident that you can draw from a qualified work force right here in the United States?

RJ: Yes, I think we can. We have good training in major schools; University of Florida has an excellent nuclear engineering program and facility. Georgia Technical University excellent in mechanical, engineering and aerospace. Embry-Riddle Aeronautical University has great aerospace engineering, Stanford University, Massachusetts Institute of Technology (MIT), Purdue University, Pennsylvania State University. I could go down the list of excellent technical universities that are delivering quality individuals that would just love for us as a nation to be working to develop propulsion systems that take us to Mars and beyond.

BB: Last question, how did you personally get into this field of endeavor what motivated you to seek a career in nuclear rocket development?

RJ: My forte is space systems building. I have always been involved in doing mission analysis my technical training allowed me to do the mission analysis and realized nuclear thermal rockets and higher energy systems was a way to make changes in space travel technique from fantasy to reality. Traveling in space through space is my focus instead of traveling up to space and returning. I guess it goes back to when I was raised in the ' 60's as a young boy growing up on a farm in North Carolina and Virginia. I use to lay on the roof of my house gazing at stars thinking maybe at some point all this schoolwork I'm doing might put me in a career with space stuff. Since I loved mechanical things and like to know how something works technically, I naturally evolved to wondering what it takes to make these devices that can transport us safely through space.
That's how I got inspired going to school and getting an aeronautical engineering degree, and later getting a masters in space systems to understand better how to integrate spacecraft systems and how to investigate and design how they would work. I had the benefit of having several interns from many Universities each summer that I could work with to help keep me be inspired.
They keep telling me... This is the right thing to do.

References:

1.Greek mythology online


AIAA paper 2004-3863
"Triton", TRI modal capable Thrust Optimized Nuclear Propulsion & Power System for Advanced Space Missions.
C. Russell Joyner, Joseph E. Phillips III, Robert B. Fowler, Dr. Stanley K. Borowski.


A Closed Brayton Power Conversion Unit Concept for Nuclear Electrical Propulsion for Deep Space Missions,
Claude Russell Joyner II, Bruce Fowler, John Matthews, STAIF 2003

Photo credit: Pratt & Whitney Company, United Technologies, West Palm Beach, Florida...