Which type of rocket engine offers the best performance for spaceflight?

[easyrider]

Ok, I am going to start this out by saying I know little about rocket engines, but I sure wish I knew more.

When looking at things like specific impulse, thrust to weight ratio, actual exhaust velocity, etc; what type of rockets are the best choice/most used? Ie, solid, liquid bipropellant, hybrid, tripropellant? How would an air-augmented rocket compare to these in specific impulse, thrust to weight, and actual and effective velcity? I LOX/LH2 probably has highest specific impulse, or atleast the highest Isp that's stable or economically practical?

Thanks for any help.

 

[Astronuc]

IIRC, HF has a slightly higher Isp than H₂0, but HF is rather nasty, so it's not used. The Space Shuttle Main Engines use a H-rich mixture of H2 + O2.

Basically to get high Isp, one wants the smallest molecule H2 or H at the highest possible temperature, or specific enthalpy/energy, or exhaust velocity.

The practical considerations are then the heat source, e.g., combustion or nuclear, and pressure, which are determined by the mechanical limits of the structural system, e.g., combustion chamber and nozzle.

Magnetic confinement offers very high temperatures, well beyond chemical combustion, but it comes with a trade off on pressure and propellant density, as well as the need for massive superconducting magnets and their cooling system. In theory, one could use liquid hydrogen propellant to cool the superconducting magnets.

 

[easyrider]

With jet propelled vehicles, specifically space borne vessels; does exhaust velocity pretty much determine the ultimate velocity that can be reached by the vehicle on a given amount of fuel?

Out of all the classic chemical combustion rockets, so H2 has the highest actual exhaust velocity? Does it matter what oxidizer is used?

 

[Mech_Engineer]

Actually there is a very important fundamental difference between "rockets" and "jets". A jet would be an air-breathing engine which utilizes atmospheric oxygen with carried fuel for combustion, and its maximum speed is limited by its exhaust velocity and of course drag. A rocket carries both the fuel and oxidizer making it a very simply a momentum transfer engine. Therefore a rocket's "theoretical" limit is the speed of light (!); practically though it is limited by drag (in the atmosphere) and once in space by the amount of fuel being carried.

Basically for a rocket in space things like the weight of the fuel, spacecraft , etc limit the final speed. I think there might be a trade off rule-of-thumb based on a rocket fuels energy density but I don't know it off the top of my head.

 

[easyrider]

Yes I know the difference between a "jet" and a rocket. I should have specified but I was trying to encompass everything that's propelled by a jet that doesn't need air that may not exactly fit in as a rocket. Rockets are propelled by a "jet" or stream of high speed high temperature gases along with gas turbines. Jet can encompass anything that accelerates using thrust really. But my bad on that because most people know a jet as a turbofan/turbofan/turboshaft/turboprop and you probably thought that's what I meant.

Anyway, with x amount of propellant, is exhaust velocity the main factor in determining peak velocity? Ie, as the vessel gets to a point where its traveling at the same speed and eventually faster than the exhaust velocity, won't it use more and more propellant for the same change in velocity the farther away the vehicles velocity gets from the exhaust velocity? Like the more it accelerates away from the exhaust velocity, the less efficient it is? Or does the rate of acceleration stay the same up until drag starts to take effect?

Also, what's the difference in exhaust velocity between: solid, liquid bipropellant, tripropellant, hybrid, etc?

 

[Mech_Engineer]

Exhaust velocity and mass flow determines a rocket's acceleration, but has no limitation w.r.t. its maximum velocity in space.

As I mentioned before, a rocket's maximum speed is only limited by how much fuel its carrying, in other words how long it can maintain thrust and therefore acceleration.

 

[Pkruse]

It is mostly a question of how much the fuel weighs, its energy content, and how far you have to carry it. That is why solids are only used on the first stage and all upper stages are hyperbolic. All others are used in between. Apollo first stage was kerosene, then H2, then hyper. Only hyper went to the moon. Today, probably first stage would be solid.