SSTO plane flight profile suggestions

[rogerawong]

Hey all!

I designed a methane/hydrogen space plane with fairly decent seeming flight characteristics. Take off speed around 60 m/s, climbs very quickly to high alt.

My current profile is:

1.  Climb to 15 000 m ASAP near Mach 1
2. Lower AOA to about 5 degrees over horizon to gain speed.
3. Progressively lower AOA to 0.5 degrees over horizon as altitude rises.
4. At around 19,500 m when speed begins to drop off from peak of ~1050 m/s, start hydrogen engine and raise AOA back to 4-5 degrees over horizon
5. MEC when AP is at 105 000 m,  drop AOA to just follow prograde.
6. Glide above 70 000 m and then circularize.
7. Orbit achieved!

My question is, would there be a benefit to having a more ballistic flight profile to sacrifice top speed for altitude before starting the vacuum engines?

For example, if I tried to maintain a higher AOA during air-breathing flight, and got up to 24 000 m, but my velocity was only 600 m/s slower,  would that potentially save me more dV in the vacuum engines by not burning them at all in the thicker 19 000 m - 24 000 m atmosphere?

Youtube video of current flight profile:

 

Edited February 14 by rogerawong

[Mikki]

I use to keep full throttle and 15° degrees above horizon and let it roll. If the plane is well balanced and doesn`t need any pitch it is the fastest and most straightforward trajectory, at least in my opinion. With Rapier engines only...

It all depends on the drag and TWR of the craft. Mixing jets and vacuum engines is another can of worms, nothing is impossible.

Mixing jets and vacuum engines maybe more fun than clustering Rapiers, which has strange downsides due the size and weight of them.

Hauling dead weight to orbit (first vacuum engines then jets) has downsides aswell, again it comes all down to the payload ratio or any other desired job for the craft, there are no limits in this game.

Keeping the craft stable in reentry and fit for a nice landing has its own constraints, SSTO is a neverending task of improving and scrapping.

Nice Vid!

[Dakitess]

I did not try SSTO Ascent Path in KSP2, but what you describe is a bit different of what we have as an optimal in KSP1.

In KSP1, using Rapiers Only, most of my SSTO has a very simple optimal path to orbit : SAS ON, proper take off, nose at 1° above the horizon and... That's it. It will accelerate very strongly at low altitude and the rotondity of Kerbin will naturally and very progressively raise it up without any input, thus without sacrificing any sudden AoA change. I tend to Lock Prograde past the 8-10° above the horizon to avoid the nose to get too high and it even improves the last minutes aerodynamic, when all what matters is to get as fast as possible in closed cycle. 

That's all.

Sometimes I have to "Power Dive" : take off, climb gently at 1-2°C above the horizon until I reach 2-4km, Mach Aerodynamic barrier would prevent me to go faster : I lock prograde, thus falling nose down my velocity vector, so that aerodynamic is as good as possible, to help the Rapiers kicking past the 400 m/s. Past that point, it will continue to accelerate and you're good to go, without adding another(s) Rapiers which would represent drymass, just because of the Mach barrier.