Returning a spaceplane, to Earth

[Laie]

Hello,

I'm having trouble returning my winged lander (think DynaSoar) to the runway. Entry appears to be pretty ballistic most of the time; I can't affect the direction by much until I've slowed down to <1500m/s @ 30km or so. At that time, my cross-range capability is limited to perhaps 500km. So, in order to return to KSC, I need a reentry that at least hits Florida.

Even though I can't affect direction by much, I'm pretty sure that I have a lot of freedom regarding the length of the trajectory. That is, starting from the same PE/AP I can fall short and and come down at the longitude of the Mississippi, or overshoot to Bermuda. I figure that I should be capable of making it to any point in between, but I don't really know how.

Could anyone advise me on what to do (and which metrics to watch) in order to make my landing in the designated landing area?

[ARS]

Aerobrake several times. Don't commence reentry directly from orbit if you cannot slow your craft enough for precision landing. Aerobrake (lowest point in orbit is inside atmosphere but not getting into the ground) allows you to slow your craft down for a more controlled landing. If you want an extra help, installing Trajectories mod helps a lot since it gives you in-atmosphere rough trajectory estimation where you touchdown

[Foxster]

No air-breathing engines on the craft?

[Laie]

1 hour ago, Foxster said:

No air-breathing engines on the craft?

Nope. Think DynaSoar, or Space Shuttle. More generally, this question isn't about how I could work around the issue by adding more stuff, but about how to use the stuff I have: namely, quite a bit of wing which should give me a lot of cross-range capability... if I only knew how to use it well.

Things I figured out in the mean time (should've been obvious, but ...well):

• pitch to brake. Increasing AoA is *the* way to increase drag and effect a braking.
• bank to control descent. Pitching alone also affects climb rate -> altitude -> air density, and frankly I don't know how shooting up and coming down later ultimately affects the trajectory.  But: banking allows for AoA without climbing, hence maintaining deceleration.

On a well-balanced craft with a lot of wing, one can virtually pull the brakes when approaching the landing area, then circle down to the runway.

Would anyone happen to know, how does pitching up and climbing into higher atmo affect the length of the trajectory? I recall having heard of planes that were supposed to skip along the upper atmo, so i guess it allows for a longer range... at least for a while. But every time you come back down, you do so at a steeper angle than before. When is it no longer worthwhile?

Edited March 30, 2018 by Laie

[ARS]

By pitching while inside the atmosphere, you expose more surface area of your craft into atmospheric drag, slowing your craft further, however, it also makes your reentry angle steeper. On the other hand, controlling how much surface area of your craft being exposed with atmospheric drag is crucial in determining whether you can pull off successful aerobrake or your craft getting spinning out of control. Exposing too small surface area would result overshooting your landing site, and exposing too large surface area would result your craft stalling

[Bornholio]

If you are using trajectories it is important to know that it assumes the l/d and drag will remain stable, that is the orientation you have at each point it is calculating.  If you are using FAR it has VAB tools for seeing the stability and drag. Isolate the craft in VAB and determine the stable angle you re-enter at and then using either Trajectories or MJ landing estimate you can get the target area visual. If neither of those, place several nodes in front of your descent, each one should halve the arc left. place them at critical altitudes, for instance, at 65km, 35km, 15km, 5km on Kerbin, 90km, 65km, 35km and 15km on Earth. This will allow you to estimate the descent path. Use side roll (Yaw) to vary your descent rate not pitch, slowly rock back and forth  to decrease lift, stay centered to keep lift high. That way your drag, stabity and burny bits don't change excessively.