Opening the Next Frontier
by Anthony Tate

Part 6: A brief technical interlude


This should be quick, so hang with me. If you are already comfortable with rocket jargon, feel free to skip ahead to section 7.

Rockets are measured using totally different units and measurements than more familiar machines, like cars. Cars use horsepower and miles per gallon. Rockets use Specific Impulse, DeltaV and Thrust.

Everybody knows what MPG means, but a quick explanation of rocket terms is needed.

First is Thrust. Thrust is how hard a rocket pushes itself along. It is usually measured in pounds force(pounds) or kilograms force or newtons. If one rocket produces a million pounds of thrust, and a second rocket produces three million pounds of thrust, the second one is three times as 'strong' as the first one. So, thrust is sort of like horsepower.

Second is acceleration. Acceleration is measured in meters per second squared, or more crudely in 'gravities.' A gravity is roughly 10 meters per second squared. For simplicity, I will use gravities. If a rocket has exactly as much thrust in pounds as it weighs in pounds, then it accelerates at exactly one gravity, or 'g.' Using the two rockets from above, if the one that makes a million pounds of thrust weighs a million pounds, then it can accelerate at one g. If the second one weighs 2 million pounds and makes three million pounds of thrust, then it accelerates at 1.5 g's. Simple! Now, as the smaller rocket example shows, if you have equal or less thrust than your rocket weighs, you can't get of the ground. Bigger thrust is usually good. Acceleration is sort of like power to weight in cars. If a tiny motorcycle has 100 horsepower, and a big car also has 100 horsepower, obviously the motorcycle will accelerate faster than the car will.**

Third is Specific Impulse. Specific Impulse is often abbreviated as Isp. Isp is a little more complicated, but it is very important. Isp is sort of like the fuel efficiency of a rocket. It is easiest to explain with an example. The two giant rockets we use to launch the Space Shuttle have an Isp of about 250 at takeoff. What this means is that for every pound of fuel they fire out the back in a second, 250 pounds of thrust is generated. Simple! Another way of looking at it is if you have an Isp of 250, you can make one pound of thrust for 250 seconds. High Isp is very important for efficient rockets. Isp is very like fuel economy for a car. If one car has a very old motor that makes 100 horsepower but gets 5 miles per gallon, and a second car has a new motor that makes the same 100 horsepower but gets 50 miles per gallon, which one would you rather have?

Fourth is DeltaV. Very cryptic sounding, isn't it? Measuring distance in space is very different than measuring distance on Earth. Since there is no air or anything else, once you have built up some velocity, you just keep going. So, the only limit on how far you can go (assuming you are patient) is your ability to speed up at the beginning of the trip and then slow down at the end of the trip. This change of velocity is how you measure the ability to get from one place to another in space and is called deltaV. DeltaV is measured in kilometers per second(kps), or meters per second for small amounts. An example is that it takes about 8.5 kps to go from the surface of the Earth to a low Earth orbit. (As our further talks will show, getting that 8.5 kps is pretty tough.) DeltaV is sort of like the fuel tank of a car. If you have a car with a fuel tank that will take you 100 miles, and a second car with a fuel tank that will take you 200 miles, the second car will take you twice as far on one tank of gas. Simple, isn't it!

**correction:
To find the acceleration of the rocket one must divide the Net Force by the Mass of the rocket. Alternatively one can still divide the Thrust by the Weight of the rocket but there will be upward acceleration only if the ratio is greater than 1.
The equation for the latter case will be: T/W = 1 + a/g , where T - Thrust, W - Weight of rocket, a - acceleration of rocket, g - acceleration due to gravity. I assume that both Thrust and Weight are forces.

correction courtesy: Plamen Ivanov, Vassar College

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Contents:

1: The Frontier Spirit

2: What went wrong.

3: Where do we go next?

4: So, why aren't we going?

5: Dealing with the Devil

6: A brief technical interlude

7: So how good is Nuclear, anyway?

8: Heat, temperature, and cooling.

9: But isn't this dangerous?

10: Prometheus would be proud of us.

11: Ok, that all sounds nice, but this is just fantasy, right?

12: But isn't this just too big?

13: But doesn't this thing make nuclear waste?

14: Conclusions