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Opening the Next Frontier
by Anthony Tate

 

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 sub variant. 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 hexafluoride 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.

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