Jet powered stage?

[mikegarrison]

I think some of these answers are a bit biased by thinking of jets only in terms of airplane engines. Jets are also used as cruise missile engines, and there are some significant differences between a jet engine designed for many thousands of hours of use between overhauls versus a jet engine designed to have a complete operational lifetime measured in hours. The missile engines can have higher thrust-to-weight ratios and lower costs because they don't have to worry about things like whether their turbine nozzles will survive 10,000 takeoffs but rather whether they will survive a 12-hour flight.

When you consider that a turbojet is just a compressor, a combustion chamber, and a turbine with an inlet and an exhaust, you realize that most rocket engines have many of the same components.

I think that the real limitation is the inlet. The advantage of using a jet is that you can carry only the fuel while getting the oxygen for free. But you can only get just so much oxygen in through a reasonable-sized inlet. There is also drag associated with the inlet. In a rocket, you can pump much more oxygen through the system without any associated drag penalty. So you can also burn more fuel and therefore get more thrust.

Edited August 22, 2017 by mikegarrison

[sevenperforce]

7 minutes ago, mikegarrison said:

I think some of these answers are a bit biased by thinking of jets only in terms of airplane engines. Jets are also used as cruise missile engines, and there are some significant differences between a jet engine designed for many thousands of hours of use between overhauls versus a jet engine designed to have a complete operational lifetime measured in hours. The missile engines can have higher thrust-to-weight ratios and lower costs because they don't have to worry about things like whether their turbine nozzles will survive 10,000 takeoffs but rather whether they will survive a 12-hour flight.

When you consider that a turbojet is just a compressor, a combustion chamber, and a turbine with an inlet and an exhaust, you realize that most rocket engines have many of the same components.

I think that the real limitation is the inlet. The advantage of using a jet is that you can carry only the fuel while getting the oxygen for free. But you can only get just so much oxygen in through a reasonable-sized inlet. There is also drag associated with the inlet. In a rocket, you can pump much more oxygen through the system without any associated drag penalty. So you can also burn more fuel and therefore get more thrust.

Lifetime considerations drive up the cost of commercial jet engines, but the performance is still a factor of raw physics (with a little chemistry thrown in for gits and shiggles). Allowing a lower lifetime doesn't make the rocket equation any less punishing.

Thinking about the inlet is the right track. The problem with airbreathing launch is known as "the air-breather's burden". If you want to continue accelerating (which is pretty darn important if you want to go to space today), you have to have net-positive thrust. If you want to have net-positive thrust, you want your exhaust to have more momentum per second than your intake. And here's the problem. Even if you had NO parasitic drag or compression drag from your intake (not to mention your vehicle body), your thrust is only the difference between your intake momentum and your exhaust momentum. If you're gulping one ton of air per second to burn with your fuel, your net thrust is your actual exhaust velocity times one ton per second plus your fuel consumption, minus your airspeed times one ton per second.

The closer your airspeed gets to your exhaust velocity, the closer your net thrust gets to zero. That's the problem with trying to get up to a significant fraction of orbital speeds with an airbreather.

[wumpus]

15 hours ago, sevenperforce said:

The closer your airspeed gets to your exhaust velocity, the closer your net thrust gets to zero. That's the problem with trying to get up to a significant fraction of orbital speeds with an airbreather.

NASA's X-43 data (the fastest airbreather so far) hits net thrust~=0 at mach 10.  At mach 7,  net thrust should be higher than error (but apparently actual data is classified) but remember the catches in the X-43:
It is a tiny little aircraft launched by a giant SRB (in pictures it is that tiny shiny blob on top the SRB).
It is optimized for a single speed. 
It only carries about 10 seconds of fuel and no "higher stage".  The drag and mass of all that could easily drag mach 7 flight down to zero
The actual acceleration is completely obscured (just "more than error") and axises are unlabeled.

https://hapb-www.larc.nasa.gov/Public/Documents/AIAA-2006-1-317.pdf

Depending on the leftover fraction of delta-v needed, you might end up needing three stages.  Expect the economics of this to ruin any benefit of using airbreathers (exception: recovering two stages out of three stages could easily have advantages over recovery one or two out of two).

[kerbiloid]

(Not sure if this is on topic, but just have seen.)

http://www.pmview.com/spaceodysseytwo/spacelvs/sld007.htm

The right one, Martin Marietta's "Renova" was going to use air inlets to make additiona thrust.

Also,

http://www.pmview.com/spaceodysseytwo/spacelvs/sld014.htm

Edited August 27, 2017 by kerbiloid