Nuclear Thermal Rocket Engines

[u235]

Quote:
"Not exactly - hydrogen is the fuel in a chemical rocket, used in conjunction with oxygen (oxidizer) to provide energy. The propellant is the reaction product water with some excess of hydrogen"
"In a Nuclear Thermal Rocket, hydrogen is simply the propellant and uranium (primarily U-235) is the fuel by virtue of the fact that the fission of U provides the thermal energy."

We are not discussing chemical rockets here. Hydrogen -is- the propellent which in turn is also the accelerent - giving it the classification as the primary fuel. Uranium inside a nuclear reactor only provides thermal and pressure properties to the Hydrogen flowing through the internal finned passages and thus causes it to accelerate, cause pressure inside the combustion chamber, and an actual higher velocity for H than possible with chemical reactions. You should check your information, heat loads for NERVA based NTR's are much higher than the SSME. The NTR also uses the cryogenic liquid Hydrogen to cool down the engine and reactor structure, however nuclear fuel being processed through a state of fission produces much high tempratures.

I refer to pressure properties of Uranium not for fuel pressurization, line feed or re-pressurization, but for additionally accelerating H velocity. Fission provides both thermal and heat pressure. Unless you've made a break through to stop fission from producing pressure, even at a micro-fissioning level then the breakup of U235 (92 nuetrons + 143 protons) = 2 lighter nuclei usually krypton - barium + [1 - 3] 'spare' nuetrons. This cause 188MeV of energy + about -- PSI - can't find the PSI for 1 atom? - Help? But this should be the pressure that i have stated at the 'added pressure properties' for light H acceleration.

(Something on the order of 200 MeV (million electron volts) is released by the decay of one U-235 atom (if you would like to convert that into something useful, consider that 1 eV is equal to 1.602 x 10-12 ergs, 1 x 107 ergs is equal to 1 joule, 1 joule equals 1 watt-second, and 1 BTU equals 1,055 joules).



Quote:
Mechanical pumps do not exist in space flight.
- I re-state my position.

Hydrogen is a bi-modal fuel that can sort of ''look after itself.''
In some parts of the SSME, the LH2 is boiled off to provide a gaseous pressure, which is returned to the ET to provide re-pressurization.
The term 'Mechanical Pump' specifies a turbine based suction unit that is driven mechanically by rotors, motors, fans and other solid structure entities.
I state that - space flight - does not use mechancal pumps because the pumps inside such systems as NTR's, inside the OMS, inside the RCS, inside the SSME use gas (either GN2 - He - or a Bi-modal appraoch) to pressurize / re-pressurize. The ''two duct powerhead'', and other designs, that you talk of, are all built around pressure systems that act out on traps, pipes, and other non-classified mechanical structures...Mechanical systems are not used in space because they incur a great weight burden on designs...

Almost 2 dozen different Nerva based NTR designs exist, most harnessing different approaches from gas core to solid core. My statement on nuclear reactors is clear, different designs are likely to variate for analysis. Yes fission reactors may work different to fusion reactors, however many variations for reactor based technology exist.

Quote:
"All nuclear reactors based on fission produce neutrons. In high temperature hydrogen gas the free mean path for neutron scattering is so large that the neutrons would contribute little to the velocity of the hydrogen atoms."

And no, why would someone build a hydrogen accelerating reactor and compell it to undergo the exact simalar processes as a fission based reactor. Obviously this would'nt work. But if reactors are changed, then direct process
for nuclear fuel - acceleration production must also be changed...

BTW, how do you quote: properly ?
sad,

 

[hitssquad]

how do you quote: properly?

Click the Quote icon on the lower right corner of the post you want to respond to. The BBCode tags it places in the message can also be manually entered.

 

[u235]

how does em+W+S meet G in one unified equation?

 

[luiscar]

Concerning what you say that electrical engines are heavy, I strongly disagree. In fact, the problem with the MHD and the ion propulsion is that it needs a high level of power; we are speaking from 15MW to 5GW (depending on the efficiency you want and the time to accelerate you have). That means that you need heavy nuclear reactors. The electrical part, ionisation/acceleration of propellant has an insignificant mass (less than 1%).
On the other hand, with the Nuclear thermal rocket, there is the problem of the maximum exhaust speed. Since the speed in the NTR is caused by temperature, there is the limit that you mention. Higher than that (9500m/s) would have no possible container and the exhaust pipe would burn.
In the ion and MHD, you can achieve exhaust speeds up to 150000m/s (which is under development) which is 15 times more (although, achieving near c would be possible, it would not be recommended due to the power needed and therefore, the weight of the power reactor needed, which would be problem)
On the other hand,in an NTR since the exhaust gases pass through a radioactive material, atoms of this radioactive material would also go out with the exhaust gases. You would have a radioactive gas!.
In the ion and MPHD approach, the radioactive core remains confined and the only thing you send out is the gas (maybe Xenon) de-ionised, as in the final part of the exhaust pipe, there is the de-ionization step.

 

[luiscar]

Mandrake, About youir reactor, for how long could it maintain 3000MWt?
Which was the total estimated reactor mass?
Was it possible to make it work to let's say 3MWt when not propelling the spacecraft ?

 

[Coolblueflame]

is it possible the last gentleman was saying the hydrogen was super heated to a nuclear reaction and directed for thrust ...thereby transforming a plain propellant into a propellant/fuel ?

 

[Astronuc]

NTR seems to be favored by the Griffin administration over NEP (Nuclear Electric Propulsion).

NTR and Lunar Surface Reactor Programs will likely get priority.

 

[~()]

Any re-development of previous programs which promote nuclear reactor based rocket engines will likely fail. It fundamentally won't work because the conversion of using nuclear fission to create heat and transfer this to a working fluid is quiet an in-effective design for maximum force and thrust output. Future development will probably need to opt into direct fission fragement propulsion (i.e using the fissioned products directly as expellant), or possibly fusion. Smaller systems need to be built which can undertake efficiently fissioning and fusing particles but that's likely a long way away. Just the amount of energy needed to operate something is enourmous (size of a powerplant).

 

[Astronuc]

Future development will probably need to opt into direct fission fragement propulsion (i.e using the fissioned products directly as expellant), or possibly fusion.

Not necessarily so with respect to fission, and fusion has yet to be perfected as a useful energy source.

 

[superczar]

Let's Scrap Old Technology.

NTRs, NEP, MPD, Orion, Gaseous Fission Reactors, etc., etc. all worked out disappointingly to anyone interested in a workhorse propulsion system designed to make interplanetary manned exploration more feasible. I, for one wouldn't want to be an astronaut on a flight to Mars driven solely by NTRs. I have nothing against a fission plant (such as a pebble-bed reactor) which can deliver 10GW in under 20 tons. I just don't think you're taking full advantage of your brain if all you want to do with that energy is double the ISP over a chemical rocket. Some of the lastest thinking today better exploits the technological possibilities, such as the VASIMR concept (although I think the actual VASIMR concept will fail).

VASIMR has been in development for decades and still dosen't seem to have produced a prototype which I think will be the workhorse of the future. I think ion propulsion (or plasma propulsion) is a great way to achieve high ISPs. My problem is that no one has been immediately successful in demonstrating a concept that achieves an ion propulsion system which can be scaled-up to the kind of thrusts we'll need to perform missions such as near-Earh asteroid deflection, manned interplanetary and near-interstellar probe missions. I think the main problem with VASIMR is that it tries to get too fancy with heating the ions instead on just using an electric field to accelerate them to the necessary energy. A helicon wave machine is an excellent way to make ions. In fact rough calculations show that if this method were used an engine of sufficient power for a 30 - 45 day trip to Mars could be constructed in approximately 5 tons. Also, current reasearchers in helicon wave technology seem certain ion densities could be increased above current levels with increase in magnetic field strength, thus reducing engine mass until it becomes a non-factor.

Apparently, after taking the existence of the pebble-bed reactor of 10 GW (and assuming problems with clogging due to frits can be worked out) and assuming the helicon wave engine can be constructed, all's we need is a converter (and a closed-cycle MHD system would appear to provide a low-mass answer). This appears to be the most efficient possible propulsion system likely to be constructed in the near future whose promise is far beyond what is currently available. Boasts have been made for superior systems employing technologies which I don't see as feasible yet, such as fusion, etc. With only a modest research & engineering effort a small start-up corporation might be able to demonstrate the feasibilty of the propulsion system in a short time. Any advance in ion-propulsion technology is likely to affect the current market for low-payload interplanetary probes like DS-1. So, there's money to be made in this field and ample opportunity of entrepreneurship. Question is, would this lead to a massive effort to privately fund a manned interplanetary mission which could be made profitable by sample return from say Mars or Europa, government funding of space exploration & asteroid deflection, a Mars colonization effort or an interstellar probe.

Although there are a number of other technological hurdles to work out with any particular mission configuration. It is almost a certainty that many missions will not be feasible without such a propulsion system. The types of missions which include the chance for a large and profitable economic expansion, the chance to protect civilization from devastating meteor impacts, the chance to gather data on our solar system and other solar systems, and a furtherance of man's push into space to provide for an expansion of population without overcrowding Earth.

 

[Astronuc]

Actually, NTR's have been and are the only nuclear propulsion technology successfully demonstrated!

NEP certainly have very high Isp, but low thrust, and they still need to be demonstrated over the lifetime of a trajectory (i.e. 1-many years). There are still issues with cathode and anode erosion, and fault tolerance.

VASIMR has yet to produce any significant thrust.

There is still a lot of work to be done in order to develop a reliable and safe nuclear propulsion system.

Forget Gas Core and Orion!

 

[~()]

Although nuclear energy is a viable and much more greater source of energy than chemical , the designs for NTR's need to be drastically refined in order to extract, transfer and control the energy into direct thrust. The general-design method of using a fission reactor which transfers heat to a working fluid (commoly hydrogen) just does not realistically produce any significant differences in thrust performance. The NERVA program failed because of low thrust performance, commonly only reaching values between 50,000 - 250,000 pounds of thrust. We must develop new ways in order to directly extract nuclear energy into thrust if we are to harness a larger potential of the energy source.

 

[Morbius]

Although nuclear energy is a viable and much more greater source of energy than chemical , the designs for NTR's need to be drastically refined in order to extract, transfer and control the energy into direct thrust. The general-design method of using a fission reactor which transfers heat to a working fluid (commoly hydrogen) just does not realistically produce any significant differences in thrust performance. The NERVA program failed because of low thrust performance, commonly only reaching values between 50,000 - 250,000 pounds of thrust. We must develop new ways in order to directly extract nuclear energy into thrust if we are to harness a larger potential of the energy source.

Short of creating a "Maxwell's demon" - how do you do that?

The nuclear energy released in a fission comes out principally as kinetic energy of the
byproducts - i.e. "heat".

Dr. Gregory Greenman
Physicist

 

[~()]

I've been working on, for a while, my own design for a NTR. I have tryed to stay away from the general approach of a fission reactor creating and transferring heat to a working fluid and rather tryed a new approach of directly creating larger initial and then sustaining fusion reactions. I have been looking into the generation of an initial fusion reaction confined in a combustion chamber, then a addition of a fuseable aneutronic fuel added to this reaction to create a sustaining reaction which mimics the way in which a chemical reaction in a standarnd rocket engine works. However there is many complex facets to this problem, such as creating accelerating structures which can initiate a initial reaction, confining the reaction inside a sufficient EM structure, establishing whether a smaller yet sufficient power source exists (much smaller than a power station) and whether it is generally theoretically possible to create and sustain a fusion thrust in a simalar way to a conventianal chemical reaction (burn).

Essentially the SSME (Space Shuttle Main Engine) is the most sophisticated working hybrid chemical engine to date and runs of an efficiency of about 80% in converting the H2 / LOX propelland and oxidizer into energy for direct thrust. It operates within the ranges of about 400,000 to 500,000 pounds of thrust which tells us that even in the next decade with little refinement, the standard chemical rocket will have reached its maximum performance ability. Unfortunately, these engines which rely on a chemical energy source are insufficient for space propulsion outside of LEO. This is why it is most probable that sustaining nuclear reactions which use direct fragments as exhaust thrust will naturally replace the chemical ones if they are theoretically possible. Although we know these reactions are possible inside suns and stars which initiate these reactions under their own gravitational fields, maybe we will be able to replicate this with man made engineering systems.