4 ms·

Some background: This company's engine is based on its previous design, the SABRE engine. That engine is designed to reach an altitude at 28km, flying at Mach 5

by pwnna 12y ago

Some background: This company's engine is based on its previous design, the SABRE engine. That engine is designed to reach an altitude at 28km, flying at Mach 5 (which works out to be about 1.5km/s at that altitude, ~15% of orbital velocity at LEO). After that the engine is designed to switch to a rocket mode, where it burns fuel and oxygen it brought, rather than taking it from the atmosphere. Since the an air breathing engine is much more efficient (10x) than a traditional rocket engine, it allows the launch vehicle to conserve mass that would otherwise be used to overcome the high drag encountered in lower altitudes. You can roughly think of this as a rocket with all of its mass being launched at 28km, with a velocity of Mach 5 already. "Getting out of the atmosphere" is a pretty vague statement. Atmospheric pressure and density is about 1-2% that of the ground. So drag at these altitudes will be less. However, some quick and dirty calculations by me makes this seem like a relatively small number compared to the amount of energy it takes to actually get to orbit. Quick search indicates the same as well[1]. However, a "thick" atmosphere is something that has to be dealt with. For example, rockets also need to overcome pressures that exists at lower altitudes. If you take a look at any rocket launch videos, you will notice that the exhaust at higher altitudes (typically about 2-3 minutes after launch) will expand outwards with respect to the rocket itself. This means that the flow out of the rocket engine is "underexpanded", making the engine less efficient than it would be otherwise. By starting your rocket engine at a higher altitude, you can improve the efficiencies by designing your rockets to lower atmospheric pressures (I don't know if this is done for the SABRE engine, but this is theoretically possible). So based on these, the main motivation for this is probably for the mass savings and efficiency. Additionally, this engine presents itself with potentials for complete reusability, which is very attractive as most current launch cost is from the unrecoverable launch vehicle itself. [1]: forum.nasaspaceflight.com/index.php?topic=9959.msg189860#msg189860

deleted 12y ago
[deleted]
iwwr 12y ago
>So based on these, the main motivation for this is probably for the mass savings and efficiency. They plan to invest all those savings into a single-stage-to-orbit launcher. It's appealing in a similar manner that SpaceX's stage-return strategy, but in a one-spacecraft package. High-mach aircraft will find other uses too besides orbital launchers. As for saving a 'mere' 15% at launch that can be crucial from a payload perspective in the rocket equation.
- m_mueller 12y ago
  Starting the rocket at 15% velocity is not the same as saving 15% of the energy though. * you have slightly reduced gravity field * you have a big win in reduced drag energy loss * these wins have an exponential effect on your fuel requirements at sea level, since less weight in additional fuel again means less fuel to transport that fuel up there etc. * On the other hand, energy is proportionate to velocity squared, so that gives you less than 15% savings if you assume no atmosphere. Looking at the wiki[1] these SABRE engines (according to manufacturer specs) will have a 7-8 times higher fuel efficiency than typical rocket engines while in atmosphere (3500 seconds vs. 450 seconds specific impulse). Since this has been peer reviewed by ESA, I'm trusting these numbers. The ascend profile of these machines will look quite a bit different than rockets, so it's hard to compare the efficiencies, but the 'rocket starting at mach 5 horicontal velocity and out of atmosphere' comparison seems to be a good intuition to me, although we still have to reduce it by the amount of fuel that has been used in order to get to that point (which will not be insignificant, even though it's 7.7 lower). We also shouldn't forget that the hybrid system has a thrust-to-weight ratio of 14 compared to 150 of SpaceX' merlin engine, which means that it will take longer for such a system to reach the same speeds, reducing its efficiency gains. The reduced weight of the whole orbiter will make up some of that though, so unless we get more publicised informations on the whole ascend path along with simulated data it's hard to tell. All I know is: When using the simplified versions in KSP these hybrid engines make a lot of sense, i.e. it's the easiest way of getting an SSTO there ;-). [1] https://en.wikipedia.org/wiki/SABRE_(rocket_engine) https://en.wikipedia.org/wiki/SABRE_(rocket_engine)

by pwnna 12y ago

deleted 12y ago

[deleted]

iwwr 12y ago

>So based on these, the main motivation for this is probably for the mass savings and efficiency. They plan to invest all those savings into a single-stage-to-orbit launcher. It's appealing in a similar manner that SpaceX's stage-return strategy, but in a one-spacecraft package. High-mach aircraft will find other uses too besides orbital launchers. As for saving a 'mere' 15% at launch that can be crucial from a payload perspective in the rocket equation.

m_mueller 12y ago
Starting the rocket at 15% velocity is not the same as saving 15% of the energy though. * you have slightly reduced gravity field * you have a big win in reduced drag energy loss * these wins have an exponential effect on your fuel requirements at sea level, since less weight in additional fuel again means less fuel to transport that fuel up there etc. * On the other hand, energy is proportionate to velocity squared, so that gives you less than 15% savings if you assume no atmosphere. Looking at the wiki[1] these SABRE engines (according to manufacturer specs) will have a 7-8 times higher fuel efficiency than typical rocket engines while in atmosphere (3500 seconds vs. 450 seconds specific impulse). Since this has been peer reviewed by ESA, I'm trusting these numbers. The ascend profile of these machines will look quite a bit different than rockets, so it's hard to compare the efficiencies, but the 'rocket starting at mach 5 horicontal velocity and out of atmosphere' comparison seems to be a good intuition to me, although we still have to reduce it by the amount of fuel that has been used in order to get to that point (which will not be insignificant, even though it's 7.7 lower). We also shouldn't forget that the hybrid system has a thrust-to-weight ratio of 14 compared to 150 of SpaceX' merlin engine, which means that it will take longer for such a system to reach the same speeds, reducing its efficiency gains. The reduced weight of the whole orbiter will make up some of that though, so unless we get more publicised informations on the whole ascend path along with simulated data it's hard to tell. All I know is: When using the simplified versions in KSP these hybrid engines make a lot of sense, i.e. it's the easiest way of getting an SSTO there ;-). [1] https://en.wikipedia.org/wiki/SABRE_(rocket_engine) https://en.wikipedia.org/wiki/SABRE_(rocket_engine)