Brian (digibri) wrote,
Brian
digibri

Head UP, young man (woman)!

Here's a remarkably well written essay (with good layman's explanations of the science) about what a nuclear fission powered rocket would look like, how it would operate, and why it wouldn't be emitting a lot of radiation into the atmosphere. He covers a lot of bases, the technology was pioneered and tested 40 years ago, engineering advantages, propulsive advantages, and cultural considerations.

Highly recommended!

B.

Note: The article is currently "slashdotted" and so I've included the full article text below the cut.


Opening the Next Frontier


by Anthony Tate





Part 1: The Frontier Spirit

America loves its legends. George Washington in Valley Forge. The Wild West. World War II. The Man on the Moon.

But lately, it seems the legends have stopped.

Sure, we have the Internet to play with now, and computers are changing the world in ways we can scarcely grasp as of yet. The Soviet Union is no more, and despite our current travails with terrorism, a certain comfortable familiarity has us in its grip.

Where is the next legend? Where is the next frontier? Or are we just going to go comfortably off into retirement?

If the 'entertainments' of the kids these days are any indication, no way.

Extreme sports, fun little things like 'base jumping' and other diversions indicate that the next generation of Americans are harkening back to their roots in a big way. America is ready for the next challenge, refreshed, revitalized, and shaking off old fears and inhibitions.

But what could have caused our recent doldrums?

Why have we not gone back to deep space, that logical 'Final Frontier,' for so many years after Apollo? I believe it was a confluence of several factors, most of which have now passed, that caused us to huddle close to the bosom of Mother Earth for these past decades.

Part 2: What went wrong.

To be blunt, it was the 70's.

After the turbulent change of the 60's, the 70's were just a hard time for America. The Cold War dragged on and on, no end in sight. Vietnam was a horrible, bloody mess, deeply misunderstood to this day, and bitterly divisive even in the aftermath. Watergate destroyed the faith of millions in their own government. The Oil Embargo shocked the economy as well, causing the nightmarish condition of 'stagflation.' Cultural upheaval became the norm as gains in civil rights were cemented into place.

With that litany of bad news, there is little wonder that the public lost interest in space. When you are scared for your job, your children, and whether or not your paycheck next year will still cover the rent, idealism and exploration goes out the window.

Also, lets be honest, landing on the Moon in the 1960's was an incredible feat. That entire rocket, the whole plan, was designed, built, and flown using less computing power than you have in your PC. Genius level effort was used to make that program possible, and the chance of disaster was perilously high, even by the comparatively relaxed standards of the day. In other words, Saturn was ahead of its time, by many years.

If it wasn't for the Cold War imperative to beat the Soviets, we'd probably be looking to go to the Moon right about now, all things considered.

Add in the fact that science itself was throwing up massive roadblocks, and there is little surprise to be had from the seeming 'retreat from space.' The rocket fuel used in the Saturn V moon rocket at launch was BETTER than the rocket fuel used to launch the Space Shuttle today. Why is that? Well, it's simple: The chemical fuels used in the Saturn V are among the best fuels that chemistry allows. Science is remarkably inflexible: unlike in the movies we can't just 'whip up' better rocket fuels. Chemistry is pretty stubborn that way.

So, exploring further in space was not important to the country while we had other problems to deal with, and making rockets better than the SaturnV was pretty much impossible.

So, NASA went sideways for a while. The Space Shuttle is a remarkable system, but it is at its core a compromise. So while it is good at many things, it is great at nothing. But nonetheless, the Space Shuttle kept America in space, and slowly we were building momentum to move forward once again away from the Earth.

Then Challenger blew up (and now we've lost Columbia and her crew as well).

Now, to the doughty folks who made Apollo fly, that disaster would have been a learning experience, and development would have continued. To the folks in the 80's it was a stunning, shocking, stomach-churning event. See, the counterculture of the 60's had grown up by the 80's, and was wielding considerable political and social clout. Why spend money on dangerous rockets when that same money could be put to better use performing good deeds?

But as always, American restlessness asserted itself, slowly and surely. The Shuttle flies again. A new Space Station, terribly crippled for now but THERE, flies.

And a new generation, who think slamming themselves into cement looks like fun, are looking around and saying, 'So, like, where do we go next?'

I find this very promising.

Part 3: Where do we go next?

Oh, there are MANY places to go and things to do. Lets take a quick look at a few.

Mars: The obvious place is Mars. NASA is exploring Mars in great detail, and has found literal oceans of water. Water is like treasure undreamed of in space, finding oceans of the stuff so close by is a tremendous draw. Mars has never been touched, who knows what wonders and riches lie on the ground there, waiting to be gathered up. And think of the ROOM! Mars is a small world, but without those huge oceans to cover it, the land area on Mars is as big as all the land on Earth. What a land rush, just waiting to happen!

Luna: Yes, we've already been to the Moon, but there is treasure there, as we have recently realized. Nuclear fusion is the dream of many for clean energy here on Earth, but the best fusion fuel that we can imagine is called Helium 3. Earth doesn't have any Helium 3 to speak of, but there is lots of it on the Moon. Once we get fusion mature enough to burn Helium 3, treasure lies on the Moon. At today's energy prices, Helium 3 is worth billions of dollars per ton. The Moon has lots more than a ton. Oh, and you remember what I said about water? Well, the Moon has water, too. Not a lot, but its there.

Lagrange points: The Earth-Moon system L4 and L5 points are two stable points in space in the same orbit as the Moon, but a sixth of the way before and behind it in its orbit. They have been looked at for decades as great places to build stuff in space, such as huge solar power satellites. The power could be sent to Earth, or the Moon, or used right there. Plus, there are the L1 and L2 points. The Earth-Sun system has a different L2 point which would be perfect for deep space astronomy. As a matter of fact, we are already planning to put the replacement for the Hubble Space telescope there. Imagine how much more reliable that replacement would be if we could actually go out there and do work on it. As it currently stands, if something goes wrong, that new instrument is useless. Imagine if the same fate had befallen the Hubble, as it so nearly did!

Asteroid 1982DB: I bet you've never heard of this one. This little lump of rock and metal has the distinction of being an asteroid that is one of the easiest to get to from Earth. I pick it because we've known about it for 20 years, but it is far from alone, there are a hundred other known asteroids almost as close, or even closer. Even one small asteroid is enormously large. 1982DB has more mass than every car in the United States, and it's already in orbit above our heads, and very close by indeed. Enterprising souls have worked out how to 'nudge' 1982DB and use the Moon to capture it and bring it into an orbit around Earth. Suddenly, those astronauts on the Space Station would have plenty of stuff to work with. The dollar value of such a second moon is almost impossible to overstate. Steel, carbon, oxygen, silica, nickel, rare earths, all in huge abundance, all in space already. Plus, the few looks at asteroids we have gotten so far show us that more precious metals are there in abundance. Gold, platinum, silver, copper, all are in asteroids in huge amounts, close by, waiting for us to go and get it. Plus, mining in orbit means no pollution on Earth! Also, note that not one of the hundred asteroids I mention above is in the Asteroid Belt. We have no need to look that far afield for incredible amounts of stuff.

Jupiter: Talk about your SIGHTSEEING! The moons of Jupiter are like a whole other Solar System. There is so much to do and see at Jupiter, I can't imagine why we aren't trying to get there NOW. Plus, the resources available in the Jovian system of moons hugely exceeds everything I have described so far.

There are many other places to go and things to do, if only we can get there.

Part 4: So, why aren't we going?

The mood of the country has changed, at last. The mass swell of the 60's counterculture is passing through its lifecycle, and slowly clearing the way for new, younger, more aggressive leaders and thinkers. Slowly but steadily America is looking for those new frontiers. We are tired of resting on the laurels won for us by our grandfathers.

But one huge hurdle remains. While America's mood is looking for adventure, and our technology has emphatically progressed to the point that we can tackle hurdles of this size with less than a Herculean effort, science has not altered its rules one little bit.

And that is why the Space Shuttle uses fuels that are not even as good as the ones the moon Rocket used. We are stuck.

Sure, there is research into high energy metastable fuels. Liquid Ozone has been investigated as a possibly better oxidizer, but even with our better technology we can't make it stable enough to use.

Metallic hydrogen also has great potential, but we can't even make it last a few seconds, never mind make rocket fuel out of it. Spin-stabilized triplet helium also has huge potential, but it's even harder to make than metallic hydrogen. Sadly, it looks like chemistry is just not going to be amenable to our desires, at least not anytime soon. By anytime soon, I mean 'this century.' Metastable fuels are HARD.

So, it looks like we are stuck, despite our newly adventurous mood.

Well, not quite, but getting past the stubbornness of chemistry requires that we cheat. Or maybe make a little deal with the Devil.

Part 5: Dealing with the Devil

What is the Devil? Well, it's everybody's favorite villain, nuclear power.

Oooh, scary, isn't it?

Nuclear power has gotten a terribly bad reputation. According to the press you read, it is dangerous, poisonous, pollutes the Earth, kills bunnies, and generally is Bad. Why, oh why, would anyone even consider using such a terrible thing?

Because it is very, very powerful, like any terrible thing should be.

Remember, chemistry is failing us here. Chemical rockets are just about as powerful right now as they are ever going to get. We have two options for going out into the rich bounty of space. We wait a very, very long time for metastable propellants to finally be developed. Or we use our shiny new technology to tame that nuclear Devil and put it to work. Kind of like fire.

A few words about nuclear power and radiation. Despite all the bad press, nuclear power has killed almost no one compared to risks that we take without thought every day.

Yes, Chernobyl was a very bad accident. But bad accidents happen all the time, and are often much, much worse than Chernobyl was.

For example, Bhopal, India, makes Chernobyl pale in comparison, but we don't stop using all chemicals. According to the UN, burning coal kills 2 million people a year in India. For that matter, burning coal in the United States belches thousands of pounds of Uranium into the air you are breathing right now, millions of times more radiation than nuclear power plants do.

As for nuclear power plants: Every nuclear power plant in the United States was designed decades ago. We have put more effort into building better cars than we have into building better nuclear power plants. Compare the safety, comfort, and efficiency of a car today to a car in the 1960's. Air bags? Seat belts? Anti-lock brakes? Traction-control? Engine control computers? Air conditioning? Remember when cars had manual chokes? How about dieseling? Kids today don't even know what 'pre-detonation' means, much less why its bad for a car. In other words, we could do much better with nuclear today if we wanted to.

I think the Cold War scares about nuclear bombs were the worst, though. Don't get me wrong, nuclear bombs are very scary things, but they are not the only use of nuclear power. We have been throwing out the baby with the bathwater for many, many years. It's time to be sensible about nuclear power. Given that the young people in America these days seem much less timid than their 'counterculture' parents were, I think we can finally look at nuclear power with honest eyes, not ones clouded with irrational fears.

Now, as it turns out, the smart and brave folks who built the Saturn V also knew that they were using the best chemical fuels we were likely to see for a very long time, so they experimented with nuclear powered rockets. They made tests of solid cored nuclear rockets using pure hydrogen as fuel. The NERVA and ROVER programs were only two that experimented with this sort of concept, and even using the primitive means available 40 years ago, the rockets they built were twice as efficient as the best chemical fuels we use today.

That sturdy foundation is what will take us to space for real this time.

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!

Part 7: So how good is Nuclear, anyway?

VERY good. Fission and fusion and antimatter have the ability, once we build them, to get us anywhere in the Solar System with ease. Chemicals on the other hand can barely get us into orbit.

For this discussion, I will stick to nuclear fission. Fusion/antimatter is more complicated, and I like simple.

As I mentioned above, way back in the 60's NERVA and ROVER made nuclear powered rockets. These rockets were thoroughly tested and were able to generate as much as 250,000 pounds of thrust, with an Isp of 900 seconds or better. The best chemical fuels in use today are liquid hydrogen and liquid oxygen, the stuff burned by the three Main Engines on the Space Shuttle (SSME's). The SSME's produce a maximum of about 450 Isp.

NERVA did this using technology that still used vacuum tubes. And not because they 'sound better' than transistors.

Now, technically speaking all rockets that use a reactor to heat up a gas to make thrust are called Nuclear Thermal Rockets (NTR's). An NTR like NERVA is what is called a solid core NTR, since the reactor core was a heavy solid mass of ceramics.

The efficiency of any NTR is limited by the difference in temperature between the core and the gas. The bigger the difference, the more efficient the rocket is. I'll repeat that, because it is an important principal: A nuclear rocket is more efficient when the reactor runs hotter.

NERVA was pretty hot, basically running just barely under the temperature that would start the core ceramics melting. The smart guys who came up with this concept way back then were not satisfied with that however. They came up with an even more efficient system, in which the core of the rocket was not a huge solid mass of ceramic, but it was a cloud of Uranium HexaFluoride gas. Since the core started out as a cloud of gas, it couldn't melt! Therefore it could get much hotter than a solid core rocket, and would thus be much more efficient.

This idea was dubbed the Gas Core Nuclear Rocket, or GCNR for short. The way a GCNR kept the gaseous core in a single mass was the height of simplicity:

Imagine a pot of hot water. You stick in a spoon and begin stirring it in a circle, as fast as you can. Soon, a deep funnel shaped hole appears in the center of the water. If you then crack an egg into the pot, it settles quickly into the bottom of the funnel, which is called a vortex, and cooks all the way through without ever touching the pot itself. Now imagine the water is a buffer gas, and the egg is the Uranium Hexafluoride fissile mass. Simple, isn't it.

They built test models of the GCNR many years ago, and discovered a little problem. Since the core was a hot gas, when you pumped the fuel gas through it to get it hot, the radioactive core gas would leak out through the exhaust. This is a real problem. Luckily, they were able to figure out a way to get around this issue. To fully understand this concept will take a little explaining, but bear with me.

Part 8: Heat, temperature, and cooling.

Now, in a Nuclear Thermal Rocket (NTR) we have to get the heat out of the nuclear reactor part of the engine and into the gas we plan to shoot out the back. In a solid core reactor this is done using conduction. IE, you drill hundreds of holes right through your reactor core and pump the gas through them. The gas picks up heat by rubbing along the inside of the reactor, and then blasts out the back. This worked great, but as I said above, you can't run the reactor very hot, since it melts.

With a gas core reactor, you can use a combination of conduction, where the hot reactor gas rubs against the cold fuel gas, and convection, where small amounts of the hot core gas mixes with the cold fuel gas. This is more efficient than conduction alone, with the huge problem that now you are leaking radioactive core gas out of your rocket. This is bad for a lot of reasons.

Luckily for us, there is a third way of moving heat around, and that is radiative. Under most conditions, radiative heating is very small compared to conduction or convection, and can be ignored. However, the inside of a GCNR is not 'most conditions.'

The way this works is simple. If you turn on an electric stove element, and put your hand off to one side of it but not touching, you still feel the heat. That is radiative heat transfer working.

In a GCNR, the core is run SO hot, it lights up like a lightbulb, and then gets much, much, much hotter. The energy being given off goes above red hot, even goes above white hot, until the core is blazing away in the deep ultraviolet. Yes, it gets so hot you can't see it any more.

At those huge temperatures, the normally small radiative heat transfer mechanism grows until it is easily big enough to get the energy from the core into the reaction gas all by itself. You no longer need to mix the two gases together, and you can keep them separate. But how can we do that, if the core is so super hot?

The answer is fused silica.

Silica is very transparent to ultraviolet light. If we treat the core like a real lightbulb and put a dome of fused silica glass around it, the glass lets basically all of the ultraviolet energy shine right through. Even though it seems impossible, the smart fellows back in the 70's actually built test models of this type of system and made it work. Given the technology we have today, we can make fused silica of such perfect transparency that this works great.

A GCNR with one of these bulbs in it is called a nuclear lightbulb. With today's technology we can build these pretty easily.

Part 9: But isn't this dangerous?

Of course it is! Cars are dangerous. Planes are dangerous. Storms are dangerous. Anything powerful is dangerous.

The Space Shuttle is dangerous, for example. The Space Shuttle generates about 100 gigawatts of power when it is launched, or as much as 50 big nuclear power plants. Plus, the exhaust gases left behind by those huge rockets are not very safe to breathe, either. Not to mention the number of workers who have died in accidents while getting it ready to fly.

Power is dangerous. But the way to stop dangerous from becoming deadly is to be aware of the danger, and take steps to avoid that danger. This is also called risk mitigation. Since the Space Shuttle has flown well over a hundred times so far, we have had a LOT of practice in risk mitigation in this kind of a system.

I can already imagine folks reading this saying, 'Yeah, but the Space Shuttle isn't nuclear!' This is true. But nuclear radiation isn't some magical, evil thing. Radiation is part of nature. The Sun is radioactive, but we aren't all dead from it. The Sun is dangerous too, as anybody with a sunburn will tell you, and many people die of sunstroke and sun poisoning every year. Yet nobody advocates living in caves because of the dangerous Sun. In other words, we know how to handle radiation, we just have to be careful.

Let's examine exactly how much risk we have from using a nuclear rocket to get to space.

Suppose we have a nuclear rocket. Suppose it has 10 pounds of radioactive nuclides in it.

'Radioactive nuclides' is not the fissile fuel, it is the waste generated after that fissile fuel has done its thing and the atoms have split. Surprisingly enough, fissile fuel before you use it is not terribly nasty stuff. You wouldn't ask it home for dinner, but it is not 'kill you in your tracks' stuff either. No, the nuclides left after you use it is the mean stuff, which is why we want to discuss it now. Ten pounds of radioactive nuclides doesn't seem like much, does it? You could hold it in one hand, easily.

Well, it is a lot, trust me. To put it into perspective, all of the radioactive nuclides that were released by Chernobyl were also about 10 pounds worth. That's all. Just ten pounds was enough to kill nearly 40 people and generate a terrible panic among hundreds of thousands of others.

Sounds pretty bad, doesn't it?

Well, let's compare our ten pounds of radioactive nuclides to something else, like the Ivy Mike nuclear bomb test which took place on November 1st, 1952. This is a real test, you can go look it up. Now, when Ivy Mike happened, it obviously released radioactive nuclides into the air. How much?

1023 pounds worth, that's how much.

Holy Cow! That old nuclear test back in 1951 was 100 times worse than Chernobyl! There must have been terrible casualties because of it! How did anybody survive such a huge release of radiation? Thousands of people must have died!

Well, no, as a matter of fact, not a single person died, or was even hurt, by that huge release of radiation. Why not?

Because we knew it was going to happen, and we planned for it. In other words, the risk was mitigated.

Now, given the many, many years of experience we have with launching rockets like the Space Shuttle, I can confidently say that we treat rockets much more like a nuclear bomb than we do a quiet power plant. The only reason Chernobyl was a disaster was because it was a surprise. If we had had even a day to get ready, nobody would have been hurt, and nobody would have died.

A nuclear powered rocket, even a HUGE one like I am about to describe in the next section, is not very dangerous on a global scale because we can launch it from a safe place (like the middle of the Pacific Ocean), and we can be prepared for any problems that occur because we know exactly when we are going to launch it. We can tame this Devil.

We have tamed much bigger ones in the past, and the world did not end 50 years ago, did it?

Part 10: Prometheus would be proud of us.

In this section I describe a huge nuclear powered rocket launcher. I will repeat and expand upon many of the points I made above, because I don't want to throw cryptic acronyms around. I want people to understand just how powerful we can make this rocket if we decide to do it.

The effective use of nuclear power in space transportation allows a paradigm shift in our thinking. All boosters which have been built to date have been shackled by the low efficiency of chemical fuels. Using chemicals it is possible to get off earth, but only barely. Every gram of structure must be trimmed, exotic materials and cutting-edge techniques are a necessity, and safety margins must be as slim as we dare if success is to be achieved.

Nuclear power changes all that. Nuclear is VASTLY more energetic than chemical. We no longer must guard every gram of mass. Much more "margin" can be included. Much more safety can be designed into the machine.

Let's examine a large heavy lift booster. There are other kinds of nuclear rockets we could build, but we desperately need a heavy lift booster if we are to excite people, catch their dreams, and actually do big stuff in space.

The most powerful booster America has built to date was the Saturn V. The size and weight of the Saturn V are easily accommodated by existing infrastructure.

Lets use the Saturn V as a "template" for a nuclear powered heavy lift booster. We will make the launcher roughly the same size, weight and power as the Saturn V, and let's see how the performance compares.

The most important difference between our new booster and the Saturn V is in the engines. The Saturn V used five massively powerful F1 engines in the first stage, burning kerosene and liquid oxygen. The mighty F1 produced 1.5 million pounds of thrust. Despite its large size and power, the F1 was a very "relaxed" design. It ran well inside the possible performance envelope. The reason it did so was to increase reliability. This is a sound design principle, so I will apply it to the new launcher wherever possible.

For an engine, I will designate a Gaseous Core Nuclear Reactor design, of the Nuclear Lightbulb subvariant. I like the gas core design for a number of reasons, and the nuclear lightbulb variant for several more.

To recap, the efficiency and power of the thruster is based on the difference in temperature between the fissioning mass and the reaction mass. If you run a solid core NTR much above 3000 C, it melts. This provides a firm "ceiling" on how efficient a solid core reactor can be. A gas core design STARTS melted. In addition, since all of the structure of the fuel mass is dynamic, a gas cored reactor is inherently safer than a solid core device. If a "hot spot" develops in a solid core, disaster ensues. If a hot spot develops in a gas core, the hot spot superheats and "puffs" itself out of existence. A gas core reactor is expected to operate at temperatures of 25,000C. The much higher temperature gradient makes the thruster inherently more efficient.

Second, a solid core reactor has a "fixed" core, since it is solid. A gas core reactor does not, and the radioactive fuel is easily "sucked" out of the core and stored in a highly non-critical state completely out of the engine! The fuel storage system I propose is a mass of thick walled boron-aluminum alloy tubing. As I said above, the fuel proper is uranium hexaflouride gas. UF6 is mean stuff, but we have decades of experience handling it in gaseous diffusion plants, and common aluminum and standard seals are available which resist attack from it. It is stoichiometric, fluorine is low activation, and UF6 changes phase at moderate temperatures, allowing it to be converted from high pressure gas to a solid and back again using nothing fancier than gas cooling and electrical heaters. This naturally makes dealing with the engine easier.

In addition, the design of the gas core allows the addition and removal of fuel "on the fly." The core can also have its density varied by control of the vortex, which directly affects criticality. Both of these elements allow very potent control inputs to be applied to a gas core reactor which are very stable and unaffected by the isotopic condition of the fuel mass.

Also, to repeat, due to the extremely high temperature gradient in the motor, the main cooling of the fissioning mass is not conductive but radiative, a mode which is inherently less susceptible to perturbations. (Having no working fluid for cooling means no material characteristics for the working fluid must be considered.) This radiative cooling mechanism is what allows the "lightbulb" system to work. The silica bulb just has to be transparent enough to let the gigantic power output of the fissioning core flow through, while keeping the radioactive material of the core safely contained inside the thruster. No radioactive materials leak out of the exhaust, it is completely "clean."

Third, a gas cored reactor has several potential "scram" modes, both fast and slow, and the speed of the reaction is easily "throttled" by adding and removing fuel or by manipulating the vortex. A 'scram' is an emergency shutdown, usually done in a very fast way. For example: a gas cored reactor can be fast scrammed by using a pressurized "shotgun" behind a weak window. If the core exceeds the design parameters of the window, which are to be slightly weaker than the silica "lightbulb," then the "shotgun" blasts 150 or so kilos of boron/cadmium pellets into the uranium gas, quenching the reaction immediately. A slightly slower scram which is implemented totally differently is to vary the gas jets in the core to instill a massive disturbance into the fuel vortex. This disturbance would drastically reduce criticality in the fission gas. A third scram mode, slightly slower still, is to implement a high-speed vacuum removal of the fuel mass into the storage system. Having three separate scram modes, one of which is passively triggered, should instill plenty of safety margin in the nuclear core of each thruster.
Extensive work was done on gas core reactors, and 25 years ago several experimental designs were built and run successfully. There were technical challenges, but nothing that seems insurmountable or even especially difficult given our current computer and material skills.

The engine I propose is this:

A Gas cored NTR using a silica lightbulb. The silica bulb is cooled and pressure-balanced against the thrust chamber by high pressure hydrogen gas. The cooling gas from the silica bulb is used to power three turbopumps "borrowed" from the Space Shuttle Main Engine. These pumps are run at a very relaxed 88 percent of rated power at their maximum setting. The three pumps move 178 kilos of liquid hydrogen per second combined. Most of this is sprayed into the thrust chamber. A portion of the liquid hydrogen is forced into cooling channels for the thrust chamber and expansion nozzle, where a portion of it is bled from micropores to form a cooling gas layer. The gaseous hydrogen that is not bled then flows down the silica lightbulb to cool it, and the cycle finally goes into powering the turbopumps.

This engine produces 1,200,000 pounds of thrust, with an exhaust velocity of 30,000 meters per second, from a thermal output of approximately 80 gigawatts. This equates to an Isp of 3060 seconds. Several sources state that a gas core NTR can exceed 5000 seconds Isp, so 3060 is well inside the overall performance envelope. The three turbopumps from the SSME are run at low power levels, and even losing a pump allows the engine to continue running as long as there is no damage to the nuclear core. Lets assume this design is able to achieve a thrust to weight ratio of ten to one, so the engine and all of its safety systems, off-line fuel storage, etc, weighs 120,000 pounds. I think we can build this engine easily for 60 tons.

We have the engine. Now to design the entire vehicle.

Since we are using the Saturn V as our template, we will make the new machine about the same weight, or six million pounds launch weight. With our engines giving 1.2 million pounds of thrust, we need at least five to get off the ground. But, since we have the power of nuclear on our side, we will use seven engines instead of five. Why seven? The most vulnerable moments of a rocket launch are the first fifteen seconds after launch. If we have to scram a motor in those fifteen seconds, having two extras is very comforting. Engine failures further along the flight profile are much easier to recover from, and having two spare engines allows us to be very "chicken" on our criteria for scramming a motor. We can shut one down even at one second after launch if we need to with no risk of crashing the entire vehicle. This further lowers the risk of nuclear power as a means of getting off the earth.
With seven engines, we have a thrust of 8.4 million pounds available. In addition, the turbopumps can "overthrottle" the engines easily in dire straits. This gets more thrust at the expense of less Isp.

Let's design the vehicle for a total DeltaV of 15 km per second. This is very high for a LEO booster, but the reason for it is to allow enough reaction mass to perform a powered descent. In other words, this is a true spaceship, that flies up and then can fly back down again.

The formula to calculate DeltaV from a rockets mass is:
DeltaV = c * ln(M0/M1).

'c' is exhaust velocity of the engines and equals 30,000 m/s.
'ln' is the natural log.
'M0' is the initial mass of the vehicle, and we have set this to be 6 million pounds.
'M1' is the mass of the vehicle when it runs dry of reaction mass.

The value of M1 is what we need to find, since we know we want a total DeltaV of 15,000 m/s.

Doing a little simple math, we find we need 2,400,000 pounds of reaction mass. Since we are using liquid hydrogen, we can now calculate the size of the hydrogen tank needed, which is 15,200 cubic meters. This works out to be a whopping 20 meters in diameter and 55 meters long!

We look at the Saturn V and find our new booster is going to be quite plump compared to the sleek Saturn V, but we have no choice if we want to use liquid hydrogen as reaction mass. Since hydrogen is the best reaction mass physics allows, and is cheap, plentiful, and we have decades of experience handling it, we will use it.

A design height of 105 meters seems reasonable. We assign 15 meters to the engines, 55 meters for the hydrogen tank, 5 meters for shielding and crew space, and a modular cargo area which is 30 meters high and 20 meters in diameter. This is enough cargo space for a good sized office building!

How heavy is the rest of the vehicle? Well, we already decided that the engines are going to weigh 120,000 pounds each, for a total of 840,000 pounds. (To make a comparison, the entire Saturn V, all three stages, engines and all, weighed a mere 414,000 pounds dry.)

Let's splurge here. With nuclear power, we have the power to splurge. Let's use 760,000 pounds to build all of the structure of the new booster. We use thicker and stronger metal, we use extra layers of redundancy, we make it strong and safe and reliable.

We have now used 2,400,000 pounds for reaction mass, 840,000 pounds for the engines, and 760,000 pounds for the rest of the ship's dry structure. This adds up to 4,000,000 pounds, fully built, fully fueled, ready to launch.

But we said at the beginning, the booster has a design launch weight of 6,000,000 pounds! If it only weighs 4 million pounds ready to launch, the rest must be cargo capacity.

This machine has a Low Earth Orbit cargo capacity of TWO MILLION POUNDS.

It is fully reusable. We gave it enough fuel to fly back safely from orbit.

It has MASSIVE redundancy and multiple levels of safety mechanisms.

Its exhaust is completely clean: It is very difficult to make hydrogen radioactive in a fission reactor. It basically can't happen.

It flies to space with a thousand tons of cargo, and flies back using some gentle aero-braking and its thrusters with another thousand tons of cargo.

This means it has eight times the cargo capacity of the Saturn V, which was not reusable at all. No longer will the Saturn V be the mightiest American rocket. No more resting on our laurels.

With this sort of performance potential, can anyone argue that NTR's are NOT the only sensible course for heavy lift boosters?

There are risks, of course, but careful design and the proper launch site can easily mitigate those risks so that the huge advantages of nuclear propulsion can be realized.

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

Well, no.

The sort of huge nuclear powered booster described above is pretty far out, but it is based on technology and ideas that are decades old. Modern technology just makes it possible to make the design more powerful while keeping it safe, but the concepts and design principals are all well established.

Indeed, NASA has been bringing up the scary 'nuclear' word more and more in the last few years, dipping a cautious toe to see what the mood of the country is. They seem to find that mood agreeable, as more and more nuclear projects get looked at.

As well, the government recently approved funds to construct a new facility in the US deserts to test nuclear rockets, the SAFE facility. In the 60's we could just test them out in the open. Enough of the unjustified nuclear paranoia of the last three decades has infected the government that they desire to be insanely cautious. But the important thing is, progress has begun.

Nuclear power is really the only viable option to open space to our use. Fortunately, the government finally realizes that fact and is slowly moving to capitalize on it.

Part 12: But isn't this just too big?

Well, a one thousand ton capable booster is certainly much too large to perform the sorts of commercial missions of today. Heck, a one hundred ton capable booster is probably too large.

But that is assuming the commercial missions won't change. I believe there is a huge pent-up demand for resources in space, and if we could put huge payloads into orbit, uses for those payloads would appear quickly.

Orbiting hotels could be made in thousand ton chunks and orbited high enough to provide truly spectacular views. Those hotels could be shielded by enormous masses of material from a captured asteroid. L5 would also be a useful place to place such structures, and with such a powerful booster system, getting to the L5 point would be trivial. When you can designate a few hundred tons to radiation shields, space is suddenly a much safer place for humans. Not to mention recent breakthroughs in radiation resistant clothing.

Or imagine the exploration capabilities. NASA and others have designed many, many deep space exploration missions, all of which have one fatal flaw: They are too heavy. The smallest feasible Mars expedition requires 150 or so tons in Earth orbit, which takes 5 trips on the most powerful rocket flying today, the Space Shuttle.

A large nuclear powered booster could put six times that mass in orbit in one flight! If we have a large capable booster, the Solar System is easy to get to. Others have designed spacecraft to carry men all the way to Jupiter and back, which requires a mere 750 tons in Earth orbit.

We could go to Jupiter in 20 years. We could really explore the asteroid belt. We could have the ability to stop a 'dinosaur-killer' asteroid before it hits us.

Imagine a permanent space station around Saturn. Think of the science we could do there, imagine the pictures, imagine the knowledge, if scientists could go there and look for themselves for years at a time, not simply get pictures trickled back with agonizing slowness.

So, yes, this booster is far too big for what we do in space now. It is just right for what we should be doing in space for the near future.

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

Ahhh, this is possibly the best part of the whole system. Yes, it does make nuclear waste, but unlike all Earth-based nuclear power, the disposal of this nuclear waste is built into the system. Indeed, with just a little work, this sort of a nuclear booster could get rid of more waste than it makes.

How does it do this? It shoots it into the Sun!

I can see you all out there rolling your eyes. For how many years have we been wishfully saying, 'why don't we just load it on a rocket and shoot it into the Sun.' Well guess what, when you build a nuclear powered rocket, it is positively easy to use that same rocket to do exactly that.

Here's how: Almost every rocket, as it gets into orbit, shuts off its motors for about 45 minutes, then fires them again one last time, halfway around the Earth. The reason for this is a little complicated, but can be summed up simply. Since the Earth is round, your orbit around the Earth had better be round too. And it's a lot easier to make round orbits if you do that small 'circularization' burn halfway through your first orbit. We will use this standard feature of rocket travel to get rid of our nuclear waste.

In a traditional chemical rocket, the circularization burn is used to add a tiny bit more speed to the spaceship, making the orbit nicely round. In this nuclear system, we have so much power to burn that we deliberately 'overshoot' on the way up, so the circularization burn is a lot larger than normal.

Now, if you will remember, up above I mentioned that the exhaust of this nuclear spaceship shoots out at a whopping fast 30 kilometers per second. If you add this 30 kilometers per second to the 8.5 kilometers per second the whole rocket is moving while in orbit, and you point your rocket in just the right direction, you can literally shoot the exhaust right away from the planet so fast that it never comes back. You can then aim it to drop into the Sun without too much trouble.

Now, the radioactive spent fuel of this rocket is gaseous, remember? So, if we only use one of the seven big rocket engines to perform the circularization burn, it is a trivial chore to pump the gaseous waste from the other six rocket engines into the rocket chamber, heat it super hot, and shoot it into space forever.

If we take a few hundred pounds of the worst waste from the ground up with us on each trip, stored in the fuel vaults for safety, of course, this system can easily get rid of more waste than it generates.

What's not to love?

Part 14: Conclusions

Decades ago, we thought of the way to get into space in a big way, but our technology then was only really able to build chemical rockets. Today, we have much better technology and much more experience with nuclear power and space travel. We can use the experiments and tests of the past to build powerful, safe nuclear rockets today that not only give us incredible access to space, but also gets rid of nuclear waste!

So, when you read about nuclear power in space, be excited by the idea. It is the key to a future so bright we can hardly even imagine it today, no more than the people who sent Columbus on his way could imagine what his three little ships would begin.
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