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litup
litup is offline
#5
Jan24-13, 01:05 PM
P: 127
Someone correct me if I am wrong, but I did some back of the envelope calculations that seems to show if you have everything at 100% efficiency, it takes 32 horsepower to accelerate 550 pounds at one G, 9.8 m/s^2 or 32 ft/s^2 if you will. Or 32 HP to give 880 Kg 1 G of acceleration, pick your units:)

So I imagine something like this: a metal runway with grooves perpendicular to the length, like a linear gear and a dragster with metal gear matching wheels so you get 100% coupling of power. So assuming the gear combo has zero friction, it still connects the motor to the wheels. So the whole thing weighs 880 kg or 550 pounds and has a 32 hp electric motor, also running at 100% efficiency.

That is my working assumption. Is that scenario wrong?

So you look at a major rocket like the shuttle or the Saturn V, fueled by LOX and liquid hydrogen. So you get the most bang for the buck so to speak, the best you can do with chemical rocket fuel and you clock in at 450 specific thrust.

It seems to me that kind of rocket is only getting about 1000th of the energy actually turning into usable thrust but the rest just being 'wasted' as heat.

Does that sound about right?

So if you are in space and have a very long light weight cable say hooked to the surface of the moon, and you have a spacecraft say 100,000 km away from the moon so you can ignore its gravity, and you have a reel-in motor that just winds up the cable, wouldn't that be about the same thing as a motor driven acceleration on earth but now you don't even have gravity or atmosphere to consider.

Under those conditions, wouldn't you get close to that number I mentioned, 32 HP=1 G of accel for 550 pounds (880 Kg)?


I am trying to visualize how much energy is wasted in chemical rockets or fusion rockets for that matter VS how much thrust you actually get for the energy expended.