SSTO space plane stabilizing orbit at Ap

[Blue5]

Hi all -

I think have a decent SSTO space plane - I am trying to determine when I should cut my engines to coast to my desired apoapsis (Ap).

When I observe that I reach my desired Ap I cut off the engines to coast to the Ap point.

When I cut off engines and coast the Ap drops (many times significantly).

I believe this is due to drag created by my space plane design.

Is their a way to determine the optimal coasting point so I can minimize delta-v (dV) need to stabilize my orbit?

Thanks!

[bewing]

Generally, if you click your SAS to prograde, your drag should drop to nearly zero. So try that once you have the Ap you want. If your Ap still drops more than you want, it's going to be an amount specific to your design. So you just need to add the loss into your initial Ap.

[pincushionman]

Pictures always help.

[mk1980]

i think you're overestimating the problem. you will lose a few hundred meters AP altitude (probably even a few kilometers) when you cut your engines while you're still somewhere at 30-40km current altitude, but the loss looks much worse than it really is. in my experience, when the plane (or rocket) gets near orbital velocity, the AP increases very very quickly. at first, it takes a long time to get it to 70km+, but once you're moving around 2300 m/s, a few seconds of thrust can raise the AP from 70 to 100+ km. the reverse is also true - lose a few m/s to drag and you lose hundreds of meters of altitude.

i don't really bother trying to get an optimal cutoff point. i usually cut off when the AP is around 80km and if it drops dangerously close to 70km during the "coasting" phase, i just fire up the engines for a second or two to correct the problem.

[AeroGav]

I'm having the opposite problem, Apoapsis continues to go UP after engine cutoff

These days I start to cut power a few km early knowing i'll make gains from wing lift

 

[Blue5]

Hey guys -

Thanks for the advice and input.

I will be more diligent with my coasting and SAS prograde pointing. If I witness significant changes I'll let the team know.

My next personal challenge is building a successful small SSTO that can get past 100km with significant margins for several orbital maneuvers and dockings. Most of my designs have been failures - im leveraging off designs Ive seen on the forum, Reddit, and actual vehicles.

[bewing]

If you want a few more designs to leverage from, here is a little bit of AeroGav and me collaborating on some SSTO spaceplanes: