I’m writing this letter in support of NASA’s proposed nuclear fission reactor initiative called Prometheus.

This letter will be divided into two sections:

-Environmental
-Alternative concepts

Environmental:

NASA has wisely chosen the US Navy/Department of Energy’s Naval Reactors group to partner with in this endeavor. Naval Reactors has the experience, facilities and resources to do the design, development, prototyping and production of the required light weight fission reactor required for Prometheus.

Naval Reactors has a proven track record of safety and reliability in both the design of and operation of fission power reactors. It is doubtful that any new facilities (new construction) will be required for the design, development or prototyping stages of Prometheus. Expansion of existing facilities or construction of new facilities may be required for testing and production stages of the new reactor, but said new construction would likely be at the existing Naval Reactors site. Thus little or no additional threat to the environment would be presented by this new undertaking by Naval Reactors.

Transportation of components of the new reactor system will pose no additional environmental threat, than the existing and totally acceptable threats posed by the US Navy and the Department of Energy transporting reactor systems and components around the country at this time.

Launching the Prometheus spacecraft with the new fission reactor system, will pose no additional environmental threat, over and above the threats posed by the launch of any other large spacecraft.

Alternative concepts:

I firmly believe Prometheus as contemplated today is far too modest of a baby step. Although the step is in the right direction, it is not the bold step that is required.
I propose nothing new. I only propose doing what large numbers of people with in NASA and the Space Community have known and have said for the last 30 years. NASA needs to develop and fly a nuclear thermal propulsion / electricity generation system using a bi-modal reactor.

The use of nuclear thermal energy as direct propulsion is not a new idea; in fact the idea is almost 100 years old. The concept goes all the way back to 1907, when the concept of using nuclear energy to propel a spacecraft was first envision by Robert Goddard, the father of US rocketry. The US military started serious study into the application of nuclear energy to space propulsion in 1955. The means to use nuclear energy as a heat source for direct thermal propulsion was developed and actively worked on by NASA/DOD during the 1960’s and early 1970’s. The result of this work was the Nerva-Peewee solid core reactor, nuclear thermal propulsion system. Nerva-Peewee was well along on the developmental path when funding for the program was cut in 1971. In fact, Nerva-Peewee was very close to being flight certified, when the program was canceled in the 1971.

It is not to say that Nerva-Peewee did not have problems. Erosion and cracking of the fuel core elements plagued the Nerva-Peewee reactor during the entire life of the program. The erosion rate was progressively reduced as the reactor design matured, but erosion was never completely overcome. Cracking of the fuel rod elements was partially overcome by advancements in material science, but again was never completely solved.

Never the less, Nerva-Peewee did demonstrate the concept of nuclear thermal propulsion was sound, and more importantly the concept was clearly shown to be very workable. Nerva-Peewee demonstrated the ability of nuclear thermal propulsion to produce in excess of ¼ million pounds of thrust, with a specific impulse (ISP) in the 850 to 875 range, for duration periods of up to 90 minutes. What is really tragic is that this all took place over 30 years ago! NASA could have been flying a nuclear thermal propulsion system for well over 25 years now.

The key factor in the above is the ISP of 850 to 875. The very best that can be achieved using “relatively” safe chemical propellants (hydrogen-oxygen) is an ISP in the 450 to 465 range (Space Shuttle Main Engines have an ISP of around 460). There would be a slight boost in ISP (to about 475-480) by using hydrogen-fluorine as propellants, but no one in their right mind would ever consider doing so, as hydrogen fluoride is extremely toxic.

The benefit of almost doubling your ISP by using nuclear thermal propulsion is, a far greater mass can be accelerated out of Earth orbit by any given vehicle of a given mass. As example: if chemically propelled vehicle “A” has the ability to accelerate a 10 ton payload from low Earth orbit to escape velocity; then nuclear thermal propulsion propelled vehicle “B” (with a equal initial vehicle mass as vehicle “A”) could accelerate a 25 to 30 ton payload from low Earth orbit to escape velocity.

But all of the above is just the beginning of the story. As stated above, Nerva-Peewee did have problems with reactor core erosion and fuel rod cracking.

Enter Project Timberwind, the Fluidized Pebble Bed Reactor. Timberwind was a DOD program that was started in the mid 1980’s, to develop an advanced fission reactor system, suitable for use as a heat source for electrical power generation on spacecraft.

Timberwind was very successful, in that the pebble bed reactor solved the problems that were inherent with the Nerva-Peewee reactor designs; in other words, no erosion or fuel rod cracking. Additionally the Timberwind reactor was found to be very much adaptable for use as the core of a nuclear thermal propulsion system, with an expected ISP in the 1000 range. Unfortunately, Timberwind died of funding cuts in the early 1990’s, before a demonstration prototype system could be completed and tested.

The story does not end here either. As part of the mid 1990’s “Space Exploration Initiative”, serious consideration was once again given to nuclear thermal propulsion. Additional in depth studies into the feasibility of nuclear thermal propulsion along with the electricity producing bi-modal fission reactor, was conducted at the request of both NASA and DOD by:

Idaho National Engineering Laboratory
Grumman Aerospace
Boeing Defense and Space Group
Babcock and Wilcox
NASA’s Marshall Space Center

The above studies clearly showed the potential of nuclear thermal propulsion. The studies concluded that an ISP of around 1100 was obtainable when the Pebble Bed Reactor was running in the propulsion mode, and the Pebble Bed Reactor had the ability to generate significant amounts of electrical energy while running in the idle mode.

Since the mid 1980’s dozens upon dozens of papers have been presented at the annual Joint Propulsion Conference, on the subject of nuclear thermal propulsion and electricity generation using fission reactors on space vehicles.

The potential advantages of nuclear thermal propulsion over the proposed Prometheus Ion propulsion are many. Thermal propulsion offers a high thrust level, which allows a rapid acceleration out of low Earth orbit to a transfer orbit, and a rapid deceleration into orbit around the target body. Chemical propulsion systems also offer the high thrust for rapid acceleration, but also require the use of cryogenic fuels to do so efficiently. Cryogenic fuels have major problems with long term storage upon a spacecraft, so the storage of cryogenic fuels on a long mission is very questionable.

Nuclear thermal propulsion systems do not burn fuel, they use a reaction mass. The reaction mass would most likely be a fluid of some sort, for ease of storage and transport from the storage vessel to the reactor. Although liquid hydrogen (a cryogenic fluid) would offer the highest possible performance, a cryogenic reaction mass is not required, nor is it desirable because of the long term storage problems. Ammonia, methane or even water would be excellent choices for the reaction mass.

Electrical power could be generated from heat produced by the reactor via a Sterling engine, a low pressure turbine, or even by the tried and true and totally fool proof thermopile, that is used in the radioisotope thermal generator (RTG).

With in the last couple years, a new technology has been envisioned, studied and tested. That new technology is Plasma Based Propulsion. NASA’s Marshall Space Center along with the University of Texas (and other organizations) has demonstrated the feasibility of using a nuclear fission reactor to heat hydrogen gas to a plasma state, and then accelerate the plasma by use of a magnetic nozzle. The fission reactor would also produce the energy needed to produce the electricity for the magnetic nozzle and plasma containment system. Plasma Based Propulsion has the potential of developing ISP numbers in excess of 10,000! I’m quite certain ammonia or methane would work equally well and possibly better than hydrogen in a Plasma Propulsion System.

Clearly Prometheus’ current proposed plan to use a fission reactor to just produce the electricity needed to operate a space probe’s electronics, sensors and an ion propulsion system, is not the bold step NASA needs to take at this time.

In closing, I’m very distressed to see the funding cuts already starting for Prometheus.

NASA’s revised spending plan “Updated 2005 Operating Plan” that was presented to Congress May 11th 2005, cuts funding for Prometheus from 431 million to 260 million, a cut of 171 million.

Unless NASA takes the very bold step; I’m afraid this very important part of the future US Space Program, will go where so many other important programs of the past have gone. And that is, they have gone no where.

One of the original Mercury Seven astronauts said it best: “there are no bucks without Buck Rogers”. Nuclear Thermal Propulsion or the newly envisioned Plasma Propulsion could be the “Buck Rogers” NASA needs to get the general public behind the US Space Program once again, and to thus get the bucks out of Congress to make it possible.

It is time for NASA to stop talking and start doing.

Respectfully;

Gregory Sakala