Physics Question Regarding Lowest Point in an Orbit and Highest Point of 'Burning'

[Piggius_Max]

Earlier today while debating with a friend the theoretical possibility of reaching a stable orbit outside of atmosphere with jet engines; I came to the conclusion that the lowest point in an orbit around a single body may not be higher than the highest point at which you have changed your velocity, thus proving that getting into an orbit outside of atmosphere all the way around kerbin with jet engines alone is impossible.

Am I talking bull or have I argued this correctly? I cannot think of any instance where this isn't true

(to simplify; I'm theorising that the periapse will never be higher than the highest point at which you have contributed to creating a successful orbit)

Sorry if this is hard to understand; but I'd like to know whether this is true or not

[Nibb31]

No, you're right. You can't have an apoapsis above the atmosphere if you only burn in the atmosphere.

[Deadpangod3]

Jet engines I think only work in the atmosphere, and your have to burn (preferably at apoapsis) to get the orbit entirly out of the atmosphere, otherwise its orbit will decay due to atmospheric drag

[Chruschtschow]

No, you're right. You can't have an apoapsis above the atmosphere if you only burn in the atmosphere.

I'm pretty sure you wanted to say periapsis. An apoapsis outside of the atmosphere is no problem (at least theoretically).

There MIGHT be a way to get into a Kerban orbit, but it involves other bodies, too. If you get enough speed within the atmosphere to leave it, if you have enough speed after drag to still reach the SoI of the Mun or Minmus, if you get there in the right angle, if you don't hit the Mun or Minmus and not get thrown into deep space, if you fall back to Kerban in the right way, you can get a stable orbit. Theoretically. With lots of ifs.

I don't have the slightest idea whether you can fit enough engines and fuel on your flier to achieve it practically to reach this speed in the first place...

[Spatzimaus]

There MIGHT be a way to get into a Kerban orbit, but it involves other bodies, too.

Correct. There are actually three ways to get a circular orbit on a jet-only vessel:

1> Slingshot off Mun or Minmus, resulting in an orbit outside the atmosphere. Note that this new orbit will intersect the orbit of whatever you slingshotted around, so sooner or later you'll encounter that body again.

2> Reach Kerbin escape velocity. The next time you encounter Kerbin, it's possible that you'll slingshot around Kerbin/Mun/Minmus into a stable orbit (but see #1 for the problem). Note that if you aerobrake off Kerbin, you'll be left in an orbit that intersects the atmosphere.

3> Get your jet into a circular orbit above the "kill altitude" (25km, I think), and then never play it again. The ship will continue to orbit at that altitude, since the game doesn't model atmosphere on anything other than the active vessel.

[EndlessWaves]

Jet engines I think only work in the atmosphere

That is incorrect, jet engines work anywhere.

Intakes only work in atmosphere, but you can store up IntakeAir and burn when you're outside the atmosphere.

Admittedly with the only stock parts with IntakeAir reservoirs holding just under 1% fuel at best you're not going to get much DeltaV out of them.

[Banbite]

Not possible, the theoretical maximum speed of a turbojet is 2400 m/s. All jets lose thrust as the velocity increase with the point of hitting zero being 2.4 km/s for the turbojet. You won't be able to go to the mun, let alone leave kerbin's SOI with that.

[Noticeably FAT]

That is incorrect, jet engines work anywhere.

Intakes only work in atmosphere, but you can store up IntakeAir and burn when you're outside the atmosphere.

Admittedly with the only stock parts with IntakeAir reservoirs holding just under 1% fuel at best you're not going to get much DeltaV out of them.

Hmm. Would it be possible to store just enough to raise your periapsis out of the atmosphere?

[Sir Nahme]

banbite, that's maximum speed not maximum delta V

and that speed is enough for a low kerbin orbit. you could achieve a slingshot with a suitable enough craft theoretically with the current drag and physics model

[Nao]

banbite, that's maximum speed not maximum delta V

and that speed is enough for a low kerbin orbit. you could achieve a slingshot with a suitable enough craft theoretically with the current drag and physics model

You can't slingshot out of Kerbin alone as you are in orbit with it .Even at 69km altitude maximum orbital velocity is ~2600m/s (Jet top speed + kerbin surface rotation), and that gives less than one milion meters of Ap (mun is at 12mln m)

The most Ap i got on stock parts with infinite fuel but only thrusting inside atmosphere was 550km - that simulates ideal scenario of infinite zero mass intakes.

Hmm. Would it be possible to store just enough to raise your periapsis out of the atmosphere?

The amount of intakes required for storing enough air (even using radial intakes as they can hold more) is reeally big, i doubt anyone can do it on stock part.

There is also a problem of auto intake ventilation that empties the intake semi quickly (even closed) when you unblock air to use it.

I wonder about Laythe thou, since it has smaller orbital velocities.

[Banbite]

banbite, that's maximum speed not maximum delta V

and that speed is enough for a low kerbin orbit. you could achieve a slingshot with a suitable enough craft theoretically with the current drag and physics model

That wasn't really the point, if you theoretically is sitting at 2400 m/s at 69km altitude you will have an ap of 203 km, and that won't put you anywhere near the mun or any other potentional slingshot candidate, and you will still need to do a small puff with a rocket to raise your Pe above the atmosphere.

But it would be interesting to know if you could store intake air for later use, then you would theoretically be able to orbit with jets only.

[HoY]

if you are storing the oxidizer with your craft, it isnt considered an air breather, its a rocket.. albeit not a conventional rocket, but your storing and burning both its fuel and oxidizer... if you can get a periapsis of 30km and an apoapsis over 100km (ive had this several times testing a build that just tumbled every time I switched to rockets due to drag up front) you could fire some sepratrons at apoapsis to circularize, or an ant engine with an oscar-b tank.

[drakesdoom]

If you can manage an 80km+ apoapsis on the jets a little rcs could get you a stable orbit fairly easily.