Space planes

[krblman]

I like playing with space planes, but what is the actual goal with them? I keep sailing to other continents on Kerbin but I wonder, are we supposed to go to space with them? Strap rockets to them and launch the rockets high up?

[Dientus]

You will get many views on this I wager. From doing temperature contracts, to reusable SSTO's, the only real limit to space planes is your imagination.

And the Kraken....

I myself use them to get into space and back very cheaply.

[KerikBalm]

2 hours ago, krblman said:

I like playing with space planes, but what is the actual goal with them? I keep sailing to other continents on Kerbin but I wonder, are we supposed to go to space with them?

Generally,  yes, space planes do go to space... otherwise they are just planes.

2 hours ago, krblman said:

Strap rockets to them and launch the rockets high up?

Well, you could use a rapier, and just change the engine mode... but yes, you would carry some sort of liquid fuel+oxygen burning engine to get to orbit. Although in stock KSP, you can also get to orbit with just jets and the LV-N (thus no oxidizer needed).

The point is that they get you high and fast in the atmosphere for minimal fuel use. Using them, you can do single stage to a moon of Jool.

Speaking of moons of Jool, jet engines work on Laythe, and its even easier to use jets to get to orbit from that moon.

For myself, in stock KSP, I would make heavy cargo carrying SSTOs. Are you aware that SSTOs can have payload fractions over 50%?

[krblman]

48 minutes ago, KerikBalm said:

Generally,  yes, space planes do go to space... otherwise they are just planes.

Well, you could use a rapier, and just change the engine mode... but yes, you would carry some sort of liquid fuel+oxygen burning engine to get to orbit. Although in stock KSP, you can also get to orbit with just jets and the LV-N (thus no oxidizer needed).

The point is that they get you high and fast in the atmosphere for minimal fuel use. Using them, you can do single stage to a moon of Jool.

Speaking of moons of Jool, jet engines work on Laythe, and its even easier to use jets to get to orbit from that moon.

For myself, in stock KSP, I would make heavy cargo carrying SSTOs. Are you aware that SSTOs can have payload fractions over 50%?

That last bit is cool. However, I have tried to build an SSTO in Sandbox mode for testing 3 times. SSTOs definitely seem difficult to create. The furthest one got an apoapsis around Kerbin of 154km and was only sub-orbital, the orbit only went half way around kerbin before crashing.

[KerikBalm]

17 hours ago, krblman said:

SSTOs definitely seem difficult to create.

Just a nitpick, SSTOs are easy to create. You are probably referring to SSTO spaceplanes, or at least recoverable SSTOs. Disposable SSTO rockets are rather easy to create in the stock game.

17 hours ago, krblman said:

The furthest one got an apoapsis around Kerbin of 154km and was only sub-orbital, the orbit only went half way around kerbin before crashing.

Yea, functional air breather SSTO spaceplanes are harder to create than disposable rockets. Sadly getting to orbit isn't the whole battle. You will also have to deal with getting down again.

With a disposable lifter, any shift of CoM is irrelevant for reentry, and likely makes the rocket more stable on ascent (provided that you have the payload on top of the booster). With a SSTO spaceplane, just placing the engines and fuel tanks (and any payload) in such a way that it is aerodynamically stable when fully loaded and when empty is a chore.

Furthermore, airbreathing engines act very differently from rocket engines, and need a very different ascent profile. Rocket engine's don't have their thrust vary depending upon their speed, and their thrust increases with altitude.

Jets have a "power band", this is most noticable for the Whiplash and Rapier, but also the panther. 

Panther: 

 

However, thrust drops off as you get higher. You need to stay in thick enough air for the jets to work, and get going faster enough down low to hit the power band of the engine, or your spaceplane will just languish with insufficient thrust, and not really get the high speed and high altitude situation when you engage rocket engines that is so critical for a well functioning SSTO spaceplane,