Nuclear Rockets in the real world

[mike wightman]

I've just found this article and it makes for interesting reading. The rest of the site has some good ideas too.

 

[savage]

I read through this and a couple others that have been around recently. Always like pro-nuclear articles.

For interstellar travel the fission engine is a benefit. Landings and takeoffs are another matter.

He appears to be discussing an open design where the the propellant passes through the engine. There are a few problems with that design.
1. It does radiate the launch site and local atmosphere. The environmentalists wouldn't buy it.
2. Nothing like melting the landing site to molten rock for an interesting end to the trip.
3. Difficult to keep the reaction materials in the reactor if part of its open.
4. The environmentalist will have a fit.

Just a few thoughts. The closed designs appear a little more sound. Pass the propellant around the reactor but performance is easily cut in half.

Well I'll have to review the site in more detail later.
Cheers.

Savage

 

[aramis]

THe article in question is NOT pro-open engine, bbut "Lightbulb" engines, which are closed gas fissionables inside a transparent torroid which you pass the reaction mass through the center of (and possibly also around) to get radiative heating of the reaction mass.

Since the strongest radiative heating is in the central "Hole" of the torroid, that will be the highest heating point. Preheating could be andled outside, or reflecctors could be used. For visualization reference, think of a tube the shape of a doughnut. That's the torrid, make it out of silica. Push a silica pipe down the center, through the hole of the doughnut... that's the absorbtion core, where radiation in the UV and IR ranges is absorbed.

So, you have NO significant long-term radiation problem. Also, the thrust systems will NOT slag properly designed and FLOODED landing zones... at least no worse than the shuttle does with it's incredibly high temperature exhausts...

Now, a failure of containment WILL be a minor long term problem.

All in all, it isn't a bad set of ideas. Well thought out, and well spoken, if a bit long winded. (the point isn't made til page nine or so...)

Now, it's not a launch you really want to watch, either... sunburn city.

 

[Anthony]

More like eye damage. Of course, he's being a bit optimistic on the amount of fissionables required, above-ground nuclear tests have been banned for a long time for very good reasons, and there are a bunch of technical problems involved in creating a nuclear lightbulb, particularly one able to get off the ground (most designs expect a thrust to weight ratio of well under 1).

 

[TheDS]

Well, one way or another, it's a pretty good read. Of particular interest to me was page 6, which explains some of the important definitions used in space travel, things that we quite often don't seem to understand in our discussions here, or often misunderstand. I'll post the useful part now:

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!

I can never seem to keep straight some of this stuff, particularly Specific Impulse, and his example is blindingly simple to understand as compared to other explanations I've seen.

 

[aramis]

one of the more interesting things is that NASA has been trying out some HUGELY HIGH Isp low thrust systems... Ion Thrust. A nuclear power plant can power these wonderfully.

As for not bing able to get off the ground with an accelleration less than one G: I've flow a few aircraft with forward accellerations sub-G...provided you have enough rollout you can atain any spped your streamlining allows for; thus, in theory, with a high enough Isp (Like air-reaction mass for a "Lightbulb") and lifting wings, one CAN get up to where a stronger reaction mass is needed than air incedence will provide... supplement with water or some other high expansion fluid... and you can get up to an orbital velocity.

Many forget that If Isp is suufficiently high, 1.01 G's Local will eventually get you to orbit.

Additionally, if you can build up enough speed that your sub-Gl thrust will keep your velocity from dropping below positive before you reach orbit, again, you can get there with High Isp.

Isp can be remembered as Index of Sustainable Propulsion, even tho' it reaally is Specific Impulse

 

[Theophilus]

Two Questions
1. ISP of 250 for the "big giant rockets". I assume this means the SRB's, I thought the three liquid fuel rockets had an ISP of 435? (with 450 being the maz for liquid rockets)

2. Isn't ISP more correctly defined as the efficiency a (set amount of) fuel burns to produce(a set amount of) thrust during a set amount of time?

 

[Straybow]

Yes, 250 is the SRBs and ~450 is the main engines.

Isp is measured in seconds, and technically it is the length of time one pound mass of fuel can generate one pound thrust. But practically it works the other way around (pounds thrust for one second) for reasonably powerful propulsion modes.

There's a fact article in Analog (the Dec 03 issue?) about a new propulsion system using a magnetically "frozen" plasma magnetosail. It uses the sun's magnetic field to provide acceleration.

Initial tests have gone well, and calculations show that Isp of 20,000 is easily within reach. Double that might be possible. Like ion engines this is a continuous low thrust mode, but the thrust is several times larger than ion engines.

A 500 kg probe could make the trip to Saturn in something like 6 months, using only 50 kg of reaction mass (to create and maintain the plasma bubble) and a thermal radioisotope power plant.