O.K., I get what you're saying. The VASIMR part at high Isp will not benefit from thrust augmentation from LOX.

In constant power throttling of VASIMR engines, to get the higher Isp, you heat a smaller amount of propellant to a higher temperature: about 1/10 the amount of propellant at 30,000 sec Isp as would be used when the engine operates at 3000 sec Isp. So there won't be very much hydrogen to burn. Also, the kinetic energy of the hydrogen ions will be so high that they cannot interact chemically with oxygen: remember chemical reactions depend on the presence of electron orbitalsinteracting around each atom. If there are no orbitals, then there can be no chemical reaction.

VASIMR in effect would actually act as a very high power, expensive ignition source for a conventional chemical engine...

Sorry, I completely misunderstood you then. Yeah, the orginal patent drawings do indeed indicate a downstream injection of molecular hydrogen.. Although, I am not sure if this was for thrust augmentation purposes. I think its was a way of ensuring 'plasma disconnect' from the space craft structure but I am not entirely sure. The plasma dynamics are quite complicated and some of the mathematics is quite challenging for me...

I know from writing Dr. Chang-Diaz that they intend to explore various possible propellants in the VASIMR, including deuterium, lithium, and argon.

I would imagine that thrust augmentation requires thorough mixing of the injected fluid with the plasma stream so that the total momentum of the plasma stream is distributed nearly homogenously in the downstream fluid. If momentum distribution is not homogenous then this ought to show up as a performance loss with an Isp penalty.





-- Edited by GoogleNaut at 16:52, 2007-03-29

I've been looking at mixing the two as I do have a specific application in mind. A rather large interplanetary vehicle utilizing VASIMR is going to need quite a bit of electrical power--some figures tossed around go from a relatively 'modest' 100KWe for a JIMO type drive, to a whopping 100-200MWe system proposed for the nuclear VASIMR 'HOPE" Mission.

Personally I've been toying with a paper conceptual design with about 500MWe--but this level of power is intended to provide main power to a large mining, processing, refining operation on a comet or asteroid.

NTR's do offer the advantage of high thrust, which is ideal in certain circumstances, especially where passing through the Van Allen belts quickly with a crew is needed, and also for relatively efficient large delta-v impulsive menuvers. Cruising with a VASIMR can build up velocity more slowly, but more importantly, can provide a constant control navigation which can make certain classes of menuvers easier, especially if 'anytime' access is desired.

However, I am at a loss to figure out how to use the same reactors that run the NTR's to run the VASIMR's because of the widely different power regimes they both need. The NTR's generally will produce an order of magnitude more thermal power than is needed by a power conversion system for VASIMR. A cluster of fairly large NTRs could produce 5000 MWt or more, but a Brayton conversion cycle might only tap 1% of that to produce electrical power, so 50MWt for electricity conversion which yields about 20MWe which is very substantial, but not necessarily enough for a VASIMR.

Also, NTR's tend to not 'like' to operate at high atomic burnups--they usually will only run for an hour or so at full power, and maybe can last for a year or two at a much lower 'auxilary' power level of a few tens to hundreds of kilowatts. But no where near megawatt levels.

So if one were to use NTR in conjunction with VASIMR then you'd have to use two sets of reactor power plants--and that is probably not acceptable for a single vehicle.

A neet little formula to estimate thrust levels, jet powers and exhaust velocities is:

Pjet=1/2*F*Ce where Pjet is the actual kinetic power of the plasma jet in Watts; F is the thrust level in Newtons; and Ce is the exhaust velocity in meters per second, found by multiplying the specific impulse in seconds with the Earth's acceleration due to gravity: 9.80665 m/s^2

Given a VASIMR engine operating with say 75% efficiency (conversion of electrical power to kinetic power of plasma jet,) a Pe = 100MWe, and a Isp 30,000 seconds; we can expect a thrust level of around: 510 N or 115 lbf. Dropping the Isp to 3000 seconds at the same power level implies that the thrust should increase by ten times: to about 5100 N or 1150 lbf. Dumping more hydrogen into the plasma flow to drop the overall Isp to 1000 seconds, implies a further thrust increase of about 5 times more again: 15,300 N or 3440 lbf.

I have looked at an NTR 'monster' engine that has the parameters: F=4.45 MN (1 million lbf) with an Isp = 960 seconds. Such an engine will gulp 472.5 kg/s of liquid hydrogen, and produce a jet power of about: 20.9 GW. If the reactor if about 85-90% efficient at converting its heat into jet power, this implies a reactor fission power of about 25 GW. This is a monster five times more powerful than the Phoebus NTR reactor tested in the late 1960's!

None of these engines is likely to develop the thrust to weight ratio necessary for a 'blast off' from planetary surface like Earth, although NTR's should be capable of liftoff from the surfaces of the Moon and possibly even Mars.

I think the biggest NTR tested was the Phoebus or Phoebus 2 which if memory serves was about 5000 MW. The "Monster" NTR I was toying with on paper had a jet power of 20,000 MW and a reactor power of about 24,000 MW thermal. This would have been capable of nearly 1 million lbf, at 960 seconds of specific impulse with hydrogen. However, currently I haven't delved far enough into it to even think about thrust to weight ratio. This is an important parameter in rocket engine design. However, for vehicles starting off in orbit, the need for really high thrust to weight ratio is lessened to more modest levels. Once in orbit, 0.9 g's is quite acceptable. This was close to the initial acceleration of the Apollo Saturn 5 when the first stage was jettisoned and the second stage powered up.

I suppose it could be possible to create a launch vehicle that can use NTR's at sea level for a flight to orbit. However, for other reasons relating to rocket vehicle design, the mass constraints for single stage vehicles becomes almost insanely obsessive. A detailed looked at the failed Lockheed Venture Star, the 'supposed' original SSTO replacement for the Shuttle, became a tug of war between the weight control engineers, and the engineers who wanted something that would work. The difficulties centered on the detailed, nested, multi-fold geometry of the cryogenic propellant tanks. The difficulty of fabricating the complex geometries that didn't leak ultimately killed the program. There was a real tug of war between the carbon composites crowd, and the crowd that wanted to try the then completely new Aluminum-Lithium Alloy.

Building a chemical SSTO is insanely difficult. Building one with NTR eases some of the weight contraints a bit, but the propulsion system will be heavier and so will be the shielding. It would be really close--the gains in performance by going to NTR may be completely overshadowed by the losses incurred by needed radiation shielding, and also having amuch heavier engine.

I don't know--detailed engineering analysis might tell the tail. Of course if I just happened to have the resources available, I'd say let's build one and find out!

Ty Moore

Yeah, I think almost all of the technical problems have since been solved. Now might not be such a bad time to restart the Venture Star project. However, I suspect that because of the prolonged delay, it's pretty much like starting from scratch. Which is very sad!

I suppose a single stage NTR could work. Again, even the problems of fission product contamination I think have been solved by using non-porous monolithic Tungsten Cermet configuration where the tungsten physically seperates the reactor fuel from the reaction mass. Also, tungsten metal will have far and away better heat conduction than any ceramic based nuclear fuel composition. And it can easily be used in a fast fission mode (highly enriched uranium--no moderator,) which is better for rocket reactors anyway (it makes them more compact and thus lighter!)

So maybe it is possible. Still, I'd be concerned about irradiating cargo and especially people--personally I would want plenty of shielding. And a NTR SSTO is still going to have a whole lot of reactor power, probably tens of Gigawatts.

That all depends upon how well the NTR core is designed. I have heard and read some things that a Tunsgten Cermet fuel design will trap all fission products. A carefully engineered core that is designed from the beginning with low fast neutron capture cross sections will have very little radioactive release--maybe even none at all. However, engineering special specific isotope materials will greatly increase the cost of the engine, to the point that it will cost as a whole a lot more than its weight in gold. This is no joke--look at the price of a space shuttle: $3.5 billion. If gold is $600 per troy ounce, this price equates to 5.83 million troy ounces of gold which is a gnat's whisker shy of 400,000 lbs! (about twice the mass of a space shuttle!)

Isotope tayloring of construction materials is very costly, but it's about the only way to ensure little or no contamination of the launch site and the air from a ground start.

I don't think this is very practical--which is why I endorse the use of LEO start for space operation only or using a 'lofting stage' to lift the vehicle up say 30-50 km before starting the nuclear engine.
Of the two, I would choose the first one. Better to start and operate in space--better performance anyway.

The termination of the Venture Star was a sad episode.  One thing should be noted is that the Venture Star project was started during a time period that the Skunk Works was an seperate company within Lockheed Martin.  Lockheed Martin got into some economic trouble when the stock price fell resulting in a reorganization that put the Skunk Works under the management of the aircraft company, i.e. the Fort Worth plant that made the F-16.  I don't think they were real space people.  So when it had problems and NASA wouldn't pay the cost of continuing the LM Aeronautics management was more than happy to ditch it.

It would have been worth flying just to test the aerospike engine.  It X-33 itself would never have achieved orbit but it would have been a useful X-vehicle.  I had the idea of putting two of them together like British MUSTARD concept with shared fueling.  It's amazing how many ideas I have independently rediscovered!

The idea of a nuclear single stage to orbit vehicle is interesting.   That would need to be one powerful reactor to do it.  Maybe it could be launched with strap on solids and the reactor could be activated in flight.  I think NERVA was originally to replace the S-IVB stage of the Saturn V.

What I'm trying to justify is some kind of craft using fission reactors to get into high altitude. Akin to the x-33, just more powerful. Perhaps even mixing it scram jets?
My line of thought was "hey the fission engine uses hydrogen as propellent, why not use that hydrogen cooling the reactor with the air's oxygen to make a scramjet? Use scramjets for general atmoshperic flight, use rocket engines to get into orbit and ion engines once in outer space".

That's actually some schemes I've seen around, although using more general kerosene jet engines.

By the way, wouldn't the x-33 be able for interplanetary flight if it would have been refuelled in orbit?

Sorry, I've got confused what jet engine was what. A quick seach on http://www.daviddarling.info/encyclopedia revealed me:

Scramjet: supersonic engine, usually useful only at Mach 5
Turbojet: standard jet engine
Ramjet: jet engine without moving parts, needs to speed up before it can be useful

What I meant is a ramjet. It is simple, reliable and it could be possible to use heated hydrogen as fuel.

EDIT: Does anybody know anything about the nuclear ramjets, such as those used at Project Pluto.

-- Edited by Andrew at 10:14, 2007-04-22

More details and some interesting photos can be found at http://www.vought.com/heritage/special/html/sslam3.html

Yes, the resemblance to the Navajo cruise missile is striking.

As for the class of engine -- ramjet versus scramjet -- the distinction is somewhat awkward for a nuclear-heated engine (there is no SC - Supersonic Combustion - as in scramjets).

But the basic distinction is between subsonic and supersonic flow inside the engine, the former being achieved by an inlet that slows down the airflow, as compared to vehicle air speed.

This works up to about Mach 4 or 5, but beyond that, too much energy is wasted in slowing down the airflow in the engine intake (and the resulting heating eventually ends up being greater than the highest feasible temperature of the solid reactor fuel elements....).

For a nuclear reactor, I would think that keeping the airflow through it subsonic would be much easier to design, than one where you are trying to heat a supersonic airstream (for flight at speeds above Mach 5).

In subsonic flow you can rely on convective heating of the air stream, if the fluid density is reasonably high.
But at very high altitude, low-density hypersonic flow, I suspect that convective heating wouldn't work, and designing for radiative heat transfer in such a system is probably impossible.

At that point its much easier to just close the air intakes and feed hydrogen propellant to the reactor, in rocket mode.

What about using liquid hydrogen without bothering to find a way to cool the system?
A small stage by getting up the air with a hydrogen-ramjet and once the highest altitude and speed is reached workable by the ramjet, then turn on the rocket(s)?

Do you know anything about MITEE systems? I've heard that they are improved NTR designs, have much lower mass yet still be able to get off.

Also, regarding magnetoplasmadynamic drives, I've read about high thrust, but Atomic Rocket gives its mass in in 1540 tons! I know this is the theorital maximum, but isn't this a bit too large for a electric drive?

I'm much more interested in how it can be used as a engine for take-off, and could they be scaled up enough to allow a manned vehicle to get into LEO and dock with a station that transfers cargo and personal into another much more space-worthy thing. Jet engines can be used to gain initial velocity, and MITEE system could be used as a rocket motor, then use atmospheric re-entry to land.

Jaro,

Just curious--you mentioned beamed power propulsion powered by a ground based powerplant. This is actually not a bad way to go for frequent launches of small cargo into low earth orbit. I wonder if using a large infra-red laser might do it. In the past large gas dynamic lasers have been mentioned--these would be lasers based on the far-infrared radiation emitted by hot CO2 flowing through a windowed de Levaal nozzle. If a nuclear reactor provided the 'heat' for such a laser, we could be talking about multi-hundred megawatts of beam power.

A little brain sweat suggests that this might be enough to put something like a ton or so into orbit.

Hmmm. Interesting....