However, I was totally unaware that there was any active research in this very interesting field. I guess secret military programs are supposed to be kept secret, so that's probably the reason :-)
Below is a brief background on the history and construction rocket engines, before we get to the even cooler nuclear stuff:
All rocket engines operate on the principle of squirting expanding gas out through a nozzle. They are reaction devices, generating motion by taking advantage of Newton's Third Law of motion - "To every action, there is always an equal and opposed reaction". When you squirt a stream of gas out of a rocket nozzle, the reaction causes the engine to move in the opposite direction.
Chemically-powered rocket engines differ mostly in how the expanding gas is generated. There are several interesting combustion designs currently in use with liquid propellant engines, and I may at some point do a post about all the cool variations and their advantages and disadvantages.
Up until the 20th century, all rockets were solid-fueled, using gunpowder for the propellant. Solid fueled rockets are mainly used for military purposes, due to the non-volatile nature of the fuel, and the fact that it remains stable for years.
Robert Goddard pioneered the exploration of using liquid fueled rocket engines - the most commonly used type of engine currently in use. During World War II, Germany developed liquid-fueled rocket engines to the point where they became practical propulsion units. Liquid fuel rockets were used both for aircraft (Me-163 Komet), and of course the infamous V-2 rocket, the first weaponized ballistic missile.
Liquid fueled rockets come in three basic flavors:
- Fully cryogenic - Both the fuel and oxidizer are liquid gas (typically liquid hydrogen and liquid oxygen)
- Semi-cryogenic - Oxidizer is cryogenic liquid oxygen, while the fuel is usually a liquid hydrocarbon (kerosene, gasoline, alcohol)
- Hypergolic - Oxidizer and fuel combust spontaneously when mixed (frequently Nitric Acid mixed with Kerosene or Dinitrogen Tetroxide or Hydrazine)
Take a look below at a liquid-fueled rocket. The left tank holds fuel - a liquid hydrocarbon such as alcohol or kerosene. The blue oxidizer tank contains liquid oxygen. The fuel can be fed via pressurized tanks, or pumped via several different techniques.
Next we get to the combustion chamber. It is here that the fuel and oxidizer are mixed and burned to create the hot, expanding exhaust gas. Blending fuel/oxidizer and burning it smoothly is relatively easy in a small engine, and very difficult in a large engine. If a flame instability develops, it will destroy the engine in short order.
Lastly we get to the throat, which speeds up the exhaust, and the nozzle, which directs the exhaust gas outwards and tries to bring the exhaust pressure down close to ambient before releasing it.
The important limitation of a chemical powered rocket though, is how much chemical energy is contained in the fuel.
Now let's discuss Nuclear rocket engines.
Nuclear rocket engines were invented and tested almost immediately after World War II, when the power of nuclear chain reactions first began to be harnessed and investigated. A nuclear reaction will generate about six million (6,000,000) times more energy than any chemical reaction. A tiny reactor can generate enough energy to melt itself in spite of massive quantities of propellant/coolant flowing through it... So it's easy to understand why a nuclear powered rocket engine would be a desirable goal to pursue.
... Which brings us to the nature of a nuclear powered rocket engine. It is a barely-controlled nuclear reaction cooled by massive quantities of fuel, rapidly pumped in from an external fuel tank. Liquid hydrogen is the preferred propellant, due to the ability to accelerate lower mass molecules to higher velocities and thus achieve the most possible thrust.
The US has experimented with and developed such nuclear powered rocket engines - although there apparently was no intent to use them to launch weapons. Between 1955 and 1973 the US ran a program called Project Rover. What began as a USAF program eventually morphed into a joint NASA, AEC, and SNPO project. Project Rover became a part of the NERVA (Nuclear Engine for Rocket Vehicle Application) program. Under Project rover were several shorter development programs with different goals - KIWI, PHOEBUS, and PeeWee
The KIWI program was a proof-of-concept study to determine if the notion of a nuclear rocket motor was even viable: Whether or not a nuclear reactor could vaporize and accelerate liquid hydrogen to the temperatures required for spaceflight. KIWI was chosen as the project name, because the New Zealand bird of the same name is flightless, just as these first nuclear rocket engines were test platforms and never intended to be flown.
Phase A of the KIWI program was intended to develop instrumentation, fuel element design, control, and structural design parameters. This phase was considered a success. The highest power level achieved used the KIWI-A3 engine at 112 Megawatts.
Below, a Phase A KIWI rocket engine
Below: Cutaway of KIWI A rocket engine
The B phase of the KIWI program involved improved reactors with higher power levels and fuel flows. Early testing showed that when throttled up, the engines would vibrate badly, sometimes ejecting fuel elements - once the engines reached full power they were stable. However, even when tested to destruction, the rockets continued to be controllable and highly stable in output, whereas a chemical engine would have been destroyed. These were considered to be rugged and reliable engines, although they clearly needed additional research and engineering to address the vibration issue. The highest power level achieved in the B phase was KIWI B4D at 915 Megawatts.
Below: One of the KIWI B rocket engines
By Los Alamos National Laboratory - Los Alamos National Laboratory, Attribution, https://commons.wikimedia.org/w/index.php?curid=25422553
The next phase of Project Rover was called Phoebus.
Phoebus 1A (below) was tested June 1965 and ran at full power for 10.5 minutes at 1090 Megawatts. Unfortunately the harsh radiation caused one of the two hydrogen fuel tank level indicators to fail, and technicians chose to believe the faulty one that showed the tank 1/4 full. The tank ran empty, then the reactor melted down and exploded. The clean-up resulted in quite a bit of radiation exposure to personnel.
Phoebus 1B (Below) was tested in February 1967, and was very successful. The engine ran a total of 46 minutes, 30 minutes of which were over 1250 Megawatts, with a peak power of 1450 Megawatts.
The final nuclear rocket engine in the Phoebus series was Phoebus 2A (below). In June 1968 it was run at 2000 Megawatts. During its second and final run it operated for 32 minutes, 12 minutes of which was over 4000 Megawatts. Peak power was 4082 Megawatts - the highest sustained power output of any reactor, to my knowledge.
The Rover program ended with the PeeWee engine (below), a third-generation small-scale nuclear rocket engine. The lessons learned in the KIWI and Phoebus programs were engineered into PeeWee. In spite of the engine's small size, it generated the highest exhaust temperature, combustion chamber pressure, and specific impulse of any of the above larger rockets. PeeWee was measured delivering twice the specific thrust of any theoretically possible chemically-powered rocket engine.
These early nuclear rocket engine experiments were intended to gather enough knowledge to engineer and design a flight-approved reliable nuclear rocket engine, the NERVA. The NERVA program ran at the same time as, and alongside the Rover program, incorporating the lessons learned in the experimental Rover program.
The NERVA program was specifically interested in engine longevity, and determining the ability and reliability of the engine to re-start many times. Several several engines were tested and re-started many times, and the program was showing great promise. The project was cancelled in 1973. Environmental regulations had become strict enough that releasing a plume of radioactive gas was no longer acceptable.
This ended the nuclear rocket engine era for the US - as far as is known to the public.
Below, a cutaway view of a NERVA engine.
By NASA - https://www1.grc.nasa.gov/historic-facilities/rockets-systems-area/7911-2/#b1-test-of-an-axialflow-pump, Public Domain, https://commons.wikimedia.org/w/index.php?curid=80364957
Below, the NERVA NRX. Note the curved liquid-cooled radiation shield on one side of the reactor. I would assume that's to protect the instrumentation in the shack to the lower left. Obviously nobody could be anywhere near one of these things when it was in operation.
The construction of the reactors is a very interesting aspect of these engines. Obviously these cannot be built like a standard PWR or BWR core. The zircalloy cladding in a water-cooled reactor would be instantly vaporized at these kind of temperatures. So what does the reactor of a nuclear rocket engine look like?
Below is an artist's drawing of one of these engines.
As we know from previous posts, hydrogen is an excellent neutron moderator, and liquid hydrogen is about 3500 times more dense than gaseous hydrogen. Therefore the power density at the inlet would tend to be 3500 times higher at the cold end of the reactor where liquid hydrogen entered entered. The thermal stress across the reactor from -472 degrees F to 4200 degrees F would be insurmountable.
To make the above reactor work with liquid hydrogen entering the top would either require the uranium fuel to be very sparse near the top or the addition of neutron poison - to compensate for the added moderation of liquid hydrogen. A difficult task that would be made more difficult by varying fuel flows, and a mobile boiling region within the core.
Simply from a reactor control point of view, it's reasonably safe to assume the liquid hydrogen propellant was evaporated elsewhere in the engine before it entered the reactor. Modern rocket engines frequently perform this operation in cooling tubes that surround the engine nozzle. This boosts efficiency by preheating the fuel. At the same time, it cools the nozzle, allowing the nozzle to be made thinner and lighter. It's likely that these engines did the same.
Even so, cold, dense hydrogen gas entering the top of the engine would have a tendency to moderate neutrons more effectively than superheated hydrogen gas exiting the engine. It's still likely that the core was designed with much less reactivity where the cold gas entered. This would reduce overheating near the top of the core. Reducing reactivity could be accomplished by a few different means: Reducing the amount of U-235 present, adding small quantities of a neutron poison, reducing neutron reflection, or with control rods (adjustable neutron poison)
Below: The F1 rocket engine that was used to power the first stage of the Saturn V. Note that the liquid oxygen flows down the left side, then coils around the nozzle in a distribution header. It then flows upwards through tubes and is vaporized into gaseous oxygen before entering the combustion chamber. Nuclear rockets probably do the same thing with liquid hydrogen.
The KIWI A core was highly enriched Uranium Oxide (UO2) fuel pellets embedded in a graphite moderator matrix. During the high temperature graphite curing process, the Uranium Oxide converted to Uranium Carbide (UC2), a very hard refractory ceramic, perfect for the expected and required high temperatures. This dense fuel/moderator matrix was machined into plates, between which hydrogen would be rapidly superheated in a very short period of time, in a single pass through the core.
Control of the reactor was accomplished by using control rods that rotated, rather than sliding in and out of the reactor. These rods were cylinders that were made of a neutron absorber on one side, and a neutron moderator/reflector on the other. As the neutron moderator/reflector side rotated towards the core, the reactor would gain reactivity and would become critical.
EDIT: 26 August 2019
I found an awesome video on nuclear rocket engine research that I covered earlier in this post. Very informative, and with some excellent video footage.
Also I found a couple of other videos that explain why the US Air Force was initially interested in nuclear rocket propulsion - A very nasty doomsday weapon.
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