Engines, Thrust, and Specific Impulse.

[Flixxbeatz]

Simple question that suddenly popped out of my mind after reading the NEXT thread. We got normal high-thrust, low Isp engines (typical rocket engines) then there's the low thrust, high Isp engines. And then we got the switchabl type like VASIMR. But is it possible for a rocket engine/ion thruster/nuclear engines/anything that can propel things into deep space and other planets to have a consistently high thrust and specific impulse? If it isn't then why? What are the limiting factors?

[rryy]

Nuclear pulse propulsion.

[metaphor]

Isp is based on the exhaust velocity of whatever you're using as propellant. Higher exhaust velocities require higher energies to accelerate those particles, and energy goes up as the square of velocity (E=1/2*mv²). Thrust is based on the force given by those particles, or momentum per given time (mv/t).

So if you want higher Isp, the energy required per particle goes up really fast. If you want high thrust, you have to accelerate a lot of those particles in a given time. So an engine with both high Isp and high thrust requires both a lot of energy and a way to give all that energy to the particles of propellant in a short time. So the power needed (energy/time) goes up very fast. Also, the temperature is proportional to energy, so you need very high temperatures to give high Isp (and a lot of material at very high temperatures to also give high thrust at the same time).

Realistically, you would need at least a nuclear reactor to generate high power like that. Even then, with a nuclear thermal rocket having a high TWR, the Isp would only be about 1000s (This is mainly limited by the temperatures of ~3000K needed, which are at the limit of what materials can withstand without melting). Nuclear pulse propulsion has an Isp of 10,000-100,000s because of the millions of degrees at the center of detonating nuclear bombs. Antimatter rockets can get up to maybe 1,000,000s Isp theoretically, but are not really practical.

Another problem is the fact that the more powerful a rocket becomes, the more energy can be unleashed by it, either purposefully or in case of an accident. An antimatter rocket that could bring 1000 tons to 10% of the speed of light packs almost the same energy as the asteroid that wiped out the dinosaurs.

[K^2]

In short, the limiting factor is power consumption. Increasing either thrust or ISP requires more power. And when your conventional rocket is already basically a controlled explosion, going higher is a bit problemantic.

[Fuzzy Dunlop]

In short, the limiting factor is power consumption. Increasing either thrust or ISP requires more power. And when your conventional rocket is already basically a controlled explosion, going higher is a bit problemantic.

Which is why most of the most promissing future propulsion technologies rely of a controlled nuclear explosion

[K^2]

And yet, the limitation is much the same. The biggest problem is that we haven't figured out how to get the necessary power output without melting everything. If it weren't for that, you could just run NERVA supercritical and get ISP comparable to nuclear pulse with a signifincantly higher thrust. Nuclear pulse, while much better than ion drive, still has quite modest average thrust.

[Flixxbeatz]

In short, the limiting factor is power consumption. Increasing either thrust or ISP requires more power. And when your conventional rocket is already basically a controlled explosion, going higher is a bit problemantic.

So it's possible to make it, but impractical for use?

Nuclear pulse propulsion.

-snip-

Millions of tons of thrust and 6,000s Isp for short bursts are pretty much promising already. I think this is what I'm asking for...

Edited July 6, 2013 by Flixxbeatz