Which orbit is more efficient to get to?

[DunaRocketeer]

I'm away from my pc for the time being so can't test this out. I'm wondering what is the most efficient way of getting into orbit for a spaceplane.

Say my target orbit is 150kmx150km. During the initial climb into space, if I aim for an apoapsis of 71km, then I'm minimising the amount of time my plane is unpowered and slowing down. However, the lower your altitude, the higher your orbital velocity, so I might have a longer circularisation burn to perform.

On the other hand if I shoot for an initial 150km apoapsis on the initial climb, I'm losing a lot of speed coasting up to that height, but it might be partly offset by the lower orbital speeds at the higher altitude.

My instinct is to shoot up to an initial 71km apo, then do an orbital burn that results in an apoapsis of 150km and circularise from there.

Thoughts?

[worir4]

I think the 71km one is best. Assuming you are using air jets first, it is easier to get a 71km apoapsis than 150km. Then you can use your rocket engines out of the atmosphere which means they have a higher ISP so they are more fuel efficient. Also you will have less loss due to gravity. The orbital velocity is lower the higher up you go but it requires more fuel to reach that high first.

[Laie]

It's always better (more energy-efficient) to go for the high apoapsis right away, rather than circularising at a lower altitude first.

In the case of jet-driven vessels: if you can produce useful thrust at very high altitudes, you can use that to raise your apo to well above 100km basically for free -- go for it. If your jets no longer work at high altitudes, you want to get out of the atmosphere quickly; going for a 150km apo will lead to a steeper ascent and gets you out of drag sooner.

[DunaRocketeer]

But wouldn't a steep ascent mean that I don't accumulate much speed on the jets, and have to rely on the less efficient rockets more? I get what you're saying about achieving a high apoapsis on the jets 'for free', but my spaceplane doesn't have that capability - it flames out with an apoapsis still in the upper atmosphere.

[EtherDragon]

But wouldn't a steep ascent mean that I don't accumulate much speed on the jets, and have to rely on the less efficient rockets more? I get what you're saying about achieving a high apoapsis on the jets 'for free', but my spaceplane doesn't have that capability - it flames out with an apoapsis still in the upper atmosphere.

This is true to an extent. But there is a balance between the two extremes - the peak efficiency of which is dependent on the craft design.

[r_rolo1]

@OPSupposing you are using a jet/rocket hybrid, it depends on what you mean with "efficient", but in general you want a plane that can go to as high as it can while on jets ( you'll most likely will not get to 150 km AP though, due to the power output curve of jet engines vs speed in stock added to the availability of intake air vs altitude ( basically jets give zero thrust above 2400 m/s ), but it is not that hard to get a 100 km Ap from jets only ) for obvious reasons... but there might be situations where you might want to not go so high in jets ...

Edited February 24, 2015 by r_rolo1

[Laie]

A (very basic) primer on how to fly jet-driven craft: you basically make two ascents.

• The first only takes you to an altitude where the jets work well; here you go into nearly level flight and try to build as much horizontal velocity as possible. There's a fine balance between drag holding you back and still having enough air; you should be climbing all the way, but while you're building speed you climb only comparatively slowly. This speed-building amounts to raising the periapsis, by the way.
  
• When you've milked the jets for all they can give and have to switch to rockets, you start climbing again. The plan is to leave the atmosphere before drag eats all your previously built velocity. Depending on how high you already are, it may be advisable to agressively pitch up to as much as 40-50 degrees.
  

Whatever apoapsis you eventually want to achieve, this plays no role during the first phase of the flight. During the second phase, you should directly aim for your target altitude.

[eddiew]

As to whether to gun for 71 or 150 first, I suspect that's irrelevant, or at least it would be on a planet without atmosphere... Either way, you have to raise your periapsis to 71 before you can raise it to 150. The question is likely about optimising the ascent to get to vacuum as quickly as possible without pitching up so hard that your forward velocity falls.

Generally, with FAR, I pitch up to about a 200-250m/s ascent once I've reached maximum speed and altitude on jets and cut the rockets in (usually 30km). Seems to clear the worst of the drag (below 50km) pretty readily and results in a periapsis that is often positive by the time my apoapsis is 100km.

Pitching up harder for a faster ascent tends to cost you forward momentum, since your putting the broad side of your wings against your prograde.