ohiovr wrote:

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... Of course no NTR has ever been powerful enough for a ground booster. But maybe I can get away with this on mars where strenght of gravity is just over a third of the earth's...

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Errors by ground crew caused all NTRs so far to be mounted sideways or upside down. Spaceflight is complex! You have to get the details right!

NTRs that expel hot hydrogen get a lot more specific impulse and a lot less thrust than ones that expel, say, hot ammonia decomposition products, or hot carbon dioxide. If they don't have to accomplish as big a delta 'V' as a rocket lifting off Earth must, these high-thrust slow propellants might be good choices.


--- G.R.L. Cowan (How fire can be domesticated)

I think that one idea for the orginal Orion project was to dig a big hole in the ground and detonate one of the nuclear charges to get the think flying.  I don't now how they were going to land and take off from Mars.   I guess to land you would set off one last charge to slow to a stop and then use the shock absorbers to soften the touch down.  But how to take off?  Also would the area around the landing site be just a little hot...radioactively?

Hello ohiovr!

I've done some writing on Orion in the past--and I did some simple analysis of potential RCS(reaction control system) propellant consumptions--and I came to some startling conclusions: that the necessary RCS prop loading could eat away a substantial fraction of the Orion's payload. If the same monomethyl hydrazine/nitrogen tetroxide propellant combination is used for an equivalent total delta-v similar to that available for a space shuttle, then for even modest (5000 t) Orions, the required RCS prop loading could easily be 1500 t or more.

As far as landing the thing on a planet--that would involve some pretty tricky menuvering, and very precise control of the main propulsion system. I'm not sure it could be done. Not only that, the main reaction dish isn't exactly the best geometry for a tailfirst atmospheric entry. As long as the ground is flat, it would make a good resting pad, but otherwise, the dish is too narrow compared to the vehicle's moment of inertia--on any kind of hill or incline, the vehicle is likely to topple over. Dropping landing legs just before touch down wouldn't helpt either--the blasts from the pulse units would likely either blow the legs off, vaporize them or both.

It is much easier to launch an orion into orbit, followed by interplanetary travel. Orbital insertion, followed by deployment of a lander and habitat modules, with an ascent vehicle in there. Its really the only practical way to go. Landing an Orion on Mars is a very tall order and could be technically insermountable in my humble opinion.

But damn--it would be spectacular!

Ty Moore
"GoogleNaut"

I looked at a couple of systems: the gyro attitude control system is one way to go; but it requires humongous gyros: probably something around a dozen tons or more each. The other thing is you still need an RCS system for fine translation control; small orbital or trajectory changes; and as a way to 'desaturate' your roll control gyros: you have to be able to dump excess roll momentum someway. The gryos can only spin so fast, eventually you have to fire a thruster and spin the gyro down in a controlled manner...

I also looked at a cool idea for boost attitute and tajectory control--while the Orion is accelerating under main pulse drive: by seperating the main habitable and command areas of the ship from the propulsion module by a flat bearing and displacing one section slightly orthoginally, one can alter the center of gravity of the forward section from the propulsion section. With a controlled displacement, it is possible to make Orion 'lean' into a turn, just as a motorcycle rider 'leans' to steer the bike. It is just about the simplest way of steering such a monster as a Nuclear Pulse Orion. Gimbaling the main reaction plate is very difficult because it is too hard to control lateral deflections of the plate. Also, the sharp lateral impulses exerted by that method would likely shake the crew to pieces....Same goes for fireing pulse units slightly off center--bad news.

Think of trying to steer an RV down the highway by fireing a 105mm howitzer out the side: there has just got to be a better way!

-- Edited by GoogleNaut at 19:44, 2009-01-26

The mechanical steering bit I envisioned would use maybe 20-25 Boeing 747 nose gear bogies mounted on shock absorbers with the tires on the plate at the forward end of the propulsion section. Using a network of steel cables and large pulleys it becomes possible to 'tether' the two sections together for limtied slip. Then using about four really large hydraulic rams to displace one section with respect to the other, it becomes possible to adjust the center of the gravity of the forward section relative the rear. In prinicple this will cause the vehicle to 'lean' into a turn. The 747 nose gear ought to be able to take a beating, but the vacuum and the cold would be a problem. Keeping them in an insulated box radiatively heated with steam from an auxiliary power nuke plant should keep them nice and pliable. Vacuum could probably be fairly easily taken care of with silicone based elastomers and inflation with nitrogen.

Not really sure how big the RCS thrusters would have to be, but again using the Space Shuttle as a conceptual template to scale the thrusters: the space shuttle primary RCS thruster is 870 lbs in vacuum, for an orbiter that 'weighs' just about 200,000 lb. So by example, a 5000 t Orion ship will probably 'need' an RCS primary thruster that scales approximately: 5000t/100t * 870 lb= 44,000 lb. Such a thruster will burn just about 200lb per second of propellant per thruster. That's a pretty big engine! You'd still have to position about 30 of these babies around the vehicle, and you'd probably have to fire them in groups too. And this is for 'fine' trajectory control: we haven't even looked at the Orion's equivalent of an OMS engine.
Again, looking to the space shuttle as a good guess: 2 STS OMS engines each generate 6000 lb of thrust in vacuum=12,000 lb thrust total, for a 200,000 lb orbiter. So an Orion OMS engine might look like a pair of engines developing: 50*6,000 lb =300,000 lbs of thrust. An Orion Ship may very well use something like the Aerojet LR-87 twin chamber engine used for the first stage of a Titan II missile; again, you'd probably need two of them...

A realistic assessment would require detailed engineering analysis to find probable 'conventional' propulsion delta-v requirements for a typical mission. There may be other propulsion alternatives, but for vacuum propulsion the high thrust of rockets is a tough nut to crack. Shoving around a 5000t ship in space is no trivial task by any means.

-- Edited by GoogleNaut at 07:55, 2009-01-28

Actually it doesn't work that way. Some material does ablate off the pusher plate, but a properlay designed pulse unit supplies both the energy source and the propellant. This is where the smart people at Los Alamos realized that by creating a different use for tamper materials, materials could be induced to 'ablate' in certain directions: thus you can get directional momentum impulses.

It's a little hard to describe, but I can visualize it. A properly design pulse unit will use something like a manhole cover sized plate of Tungsten (a meter wide by 4 cm thick) behind which is tucked a 1-5 kiloton nuclear device inside a radiation bottle. The radiation bottle is composed of a relatively thin layer of uranium-238 sheet (several absorbption lengths thick;) just thick enough to allow the radiation (here electromagnetic radiation: x-rays) to quickly reach a thermal equilibrium inside. The x-rays striking the tungsten plate cause a portion of the inside of that plate to ablate away: this induces a powerful reaction-driven shock in the tungsten as it is propelled --uniformly-- away from the nuclear device. The shockwave induced in the plate vaporizes the remainder of the plate creating a relatively cool plasma jet--composed of tungsten vapor--which then strikes the momentum plate at the base of the Orion. The plate stops and reflects this jet of gas--the absorbed momentum is transfered to the Orion in stages through momentum conditioners so that the impulses are smooth and as close to uniformly as possible.

It is an extemely violent and dynamic process.

Using multiple pusher plates is possible, but you still have to contend with lateral forces induced by differential timing of detonations (akin to differential throttling.) The analogy with steering an RV by fiering a howitzer out the sides still holds. It is difficult to control such impulses and moderate them without tearing your vehicle apart or causing unacceptable levels of metal fatigue in the structure. It is easier to precisely position a smaller ballast mass (say the command section) to cause the whole structure to gently shift its center of gravity which will induce a turning moment. The key is controlling what you can: precise timing of blasts is possible, but difficult. Precise blast locations is more difficult due to uncertainties in the speed and timing of the ejection mechanism. The only way to steer this thing that is tolerant to errors in both, is the CG manipulation method.

Incidently, I can find almost nothing in the literature describing how the original Orion engineers intended to steer the thing. I think George Dyson's book briefly mentions 'off center' detonations as a possible mechanism. As far as I can tell, the CG manipulation method was original...

-- Edited by GoogleNaut at 19:20, 2009-01-28

"Like most propulsion systems" --

But unlike kerosene-lox rocket propulsion. Recall Apollo 13's reaching orbit on only four of its five engines, and Captain Tom Cruise saying, I guess that's our glitch for this mission.

"-- everything needs to be perfect for it to work."

I might like to ride a continuous-flow NTR, if it were big enough to be no-doubt-about-it shielded. I think no-one here is eager to ride the complete, ready-to-go Area 51 Orion prototype that has been awaiting a test pilot ever since they got the captured alien tech to wor-- wait, there's someone at the door.