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 heavy lift booster.
The most powerful booster America has built to date was the Saturn V. The size and weight of the Saturn V are easily accomodated by existing infrastructure.
So, 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 the engines. The Saturn V used five of the massively powerful F1, burning kerosene and liguid oxygen. The 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 Uranium 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.
First, 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 above 3000C, 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 well over 50,000C. 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. 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, flourine 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, 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. Such as a positive void co-efficient.) 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. 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.
So, 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 the MAXIMUM. The three pumps move 178 kilos of LH2 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 is then flowed 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 an thermal output of approximately 80 gigawatts. This equates to an Isp of 3060 seconds. Several sources I have seen state the gas core NTR can exceed 5000 seconds Isp, so 3060 is well inside the overall performance envelope. 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, offline fuel storage, etc, weighs 120,000 pounds. I think we can build this easily for 60 tons.
So, 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 6 million pounds launch weight. With our engines giving 1.2 million pounds of thrust, we need at least 5 to get off the ground. But, since we have the power of nuclear on our side, we will use seven engines instead of 5. Why? The most vulnerable moments of a rocket launch are the first 15 seconds after launch. If we have to scram a motor in those 15 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.
So, 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.
So, 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 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 we have set this to be 6 million pounds
M1 is the mass of the vehicle when it runs dry of reaction mass.
This value is what we need to find, since we know we want a total DeltaV or 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 hydogen, 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 to 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 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 again. 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.
So, 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 weight of 6,000,000 pounds!
This machine has a LEO cargo capacity of TWO MILLION POUNDS.
It is fully reusable.
It has MASSIVE redundancy and multiple levels of safety mechanisms.
Its exhaust is completely clean: It is impossible to make hydrogen radioactive in a fission reactor. It 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.
It has eight times the cargo capacity of the Saturn V, which was not reusable at all.
With this sort of performance potential, can anyone argue that NTR is 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.
