Common Near-Future Sci-Fi Spacecraft Design

tori

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The infrared radiation coming from the five F-1 engines of the Saturn V flash ignited the grass around the launch pad during most lift-offs. And that was just a measly 263 s of Isp. A fusion drive would really be slice of hell. An ion drive is ludicrously hot as well, but the actual power flowing into it is low, and therefore so is the heat coming out.

If the engines are canted outwards, then the reduction in efficiency has to be taken into account when deciding between a puller/pusher configuration - I'd imagine the net delta-v would go down with the cosine of the angle the engines are canted out at. At some point it would be probably better to invest some mass into a truss and parallel engines (α=0°) instead of losing a third of your delta-v to angling the drive (α=45°) in an attempt to avoid the plume.
 
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vejiita

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This kind of intelligent discussion is the main reason why I stalk... err... read this board.
 

jedidia

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If the angle allows the actual exhaust stream to avoid the rest of the spacecraft, then that should not be a problem.

The problem then would be the light emitted by the exhaust stream as it passes the rest of the spacecraft structure. Calculations would have to be done to see if this poses any real threat.

I was actually thinking about the em radiation. if the actual particle stream would hit any part of the ship, no shield in the world could protect it at that short distance.
 

T.Neo

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Well, of course. Having the stream hit the ship would just be stupid.

Kind of like sawing off the branch you're sitting on.
 

Izack

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You'd end up with a bending load on the base plate of the pyramid - as the four engines in the corners push forward, the main mass pushes on the plate, inducing torques that try to fold the pyramid.

It'd be better to flip it upside down - one engine on its point and the pyramid would accelerate base-first. That way all the load would be divided between a compressive load on the side edges (going from corners to the point) and a tensile load on lines going from the corners to the center of the baseplate. Trusses for compression, cables for tension, add some tankage and what not, and off you go. :)
That's what I was originally thinking, but then you get back into the balancing dilemma. Maybe a diamond shape? Although, I guess the single long truss is still the way to go; diamond's just unnecessarily complex.

As for pullers, I have to agree with the previous posts (Low thrust, electric Hall/ VASIMR/ Ion drives). I can see a 'tug' configuration on automated/radio controlled cargo haulers (say, an independent engine module tugging a 'dumb' cargo pod) where there is no crew to be protected, and thus shielding can be minimal.

I haven't seen Avatar and therefore cannot comment on the Venture Star. As far as I know it was an old Lockheed Martin concept spaceplane.
 

Andy44

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The problem with any design that requires more than one engine to work, such as a tractor-cable configuration, is that if you have an engine fail the system doesn't work anymore. With all engines at the tail end of a long rigid fuselage, an engine out just means you move the gimbals and keep flying.

This is also one of the problem with a pyramid design. One engine out means a very steep gimbal angle for the others. Also, a pyramid is not the most mass-efficient method of placing the radioactive engines at a safe distance from the rest of the vessel. Instead of one long truss, now you need three or four long trusses, with braces in between the legs. Extra mass for no reason.
 

Izack

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Well, looks like all the evidence points to a single-truss design as the best after all.

However, I have one more postulation: if a ship did require plate-shielding (military reasons, maybe?), would a cone be an effective shape? Forgo the truss entirely, and simply put the load on the hull?
To be specific, put the engine at the point of a narrow (10-15 degree or so) cone, with a hemisphere at the bow?

Also, I have to bring up the bulletlike design of the Apollo spacecraft. I know at least the CM was designed to be aerodynamic, but the SM wasn't exposed to an atmosphere under normal circumstances was covered in plating.
 

Andy44

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Well, Apollo is not the best example for this discussion because it was not a very large vehicle, nor did it have a radioactive powerplant, and it wasn't assembled in orbit by multiple launches, rather it was designed to act as the nose of the Saturn rockets. The service module was thus cylindrical, and the internal structure consisted of bays and bulkheads, with an out skin that served as both an aerodynamic skin during launch and as a thermal control device for the propellant tanks in space.

A better example of a large moonship might be von Braun's early ideas for a moonship, which was much larger and had a bigger crew:



Notice the use of trusses for structural strength, with a crew module at the bow, the engines at the stern, and propellant tanks attached to the inside of the truss structure.

 

T.Neo

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The problem with any design that requires more than one engine to work, such as a tractor-cable configuration, is that if you have an engine fail the system doesn't work anymore. With all engines at the tail end of a long rigid fuselage, an engine out just means you move the gimbals and keep flying.

Then you could either:

A. Accept the fact that you are screwed (not desirable).
B. Have more than two engines, so that if one fails others can take up the slack.
C. Make the systems within the engines highly redundant as to prevent failure.

All in all though I think the benefits outweigh the issues. Compare a dry mass in the order of 100 tons for Project Valkyrie to a mass of many hundreds of even thousands of tons in structure for other interplanetary spacecraft concepts.
 

tori

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You sure a quadruple-redundant twin fusion drive canted outward would be better mass-wise than a little bit of structure and say two parallel drives?
 

T.Neo

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Yes. The savings from having a structure primarily in tension would be large. It must be remembered that these are often kilometer long spacecraft.

Redundancy, on the other hand, isn't going to add that much mass. And you should have it anyway.

Canting the drives (or at least the thrust streams) slightly should not add much mass either. It detracts more from a thrust perspective. If you do it right, you might even be able to have a single drive that redirects thrust away from the craft in an equal manner.
 

Sky Captain

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Is there any actual advantages of having a puller design when your engines put out very low thrust like electric propulsion. If maximum acceleration is only like 0.01 G then there would be very little loads on the structure and the structural truss would be only tiny fraction of overall mass.
 

T.Neo

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This is a good point.

Then again, even if it is a small advantage, it might be enough to warrant a tensional design.
 
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