How much dV for rocket stage of a SSTO Spaceplane?

[guitarxe]

If you have to hit your rockets at about 20-20km altitude, how much dV do you need to reach orbit from that point on?

I've found I have finally managed to design and learn to fly my spaceplane consistently to the height where the jets cut off, but my rockets still lack the required dV to put me in orbit.

[EtherDragon]

Part of it might be your flight profile, too. Just FYI - I'm not sure we've figured out how much DV is actually required to get from a 20km trajectory to 75x75km orbit, yet.

I've noticed, if I don't pitch up enough for Rocket mode, I lose way too much speed in atmosphere, and at 20km it's hard to pull up hard enough to affect your trajectory much. So I've taken to starting my hard-pull hack at 17.5km and trying to establish a nice 30 degree (or steeper) climb angle before my turbojet cuts out. Then it's rocket mode to get Apoapsis to 75km - then circularization. Not sure how much Dv is required, for that...

[arq]

Just going by reckoning and assuming 1200m/s velocity when changing to rocket mode, figure on the order of 1500-1700m/s. I haven't actually worked it out or tracked it very closely, though.

As the previous poster suggested, you want to have a decent ascent angle when you switch to rockets. 30degrees is my target, also. Spending too long below 40km (and especially below 30km) can result in considerable drag losses that are very expensive in terms of dV.

[Olterin]

As the two posters above me said, you should aim for a (25-)30 degree climb by 20 km altitude. My latest ascent profile was somewhat like: go up to 10km while going at roughly mach 2, begin final ascent, go full throttle at 12-13km, switch to rockets when jets cut out. This leads to some overheating issues if the spaceplane isn't carefully designed though, but it worked for me. Final jet velocity was about 1350 m/s, from there I think I used around 1200 m/s to get a final circular orbit at 80km altitude.

[Chris_2]

Part of it might be your flight profile, too. Just FYI - I'm not sure we've figured out how much DV is actually required to get from a 20km trajectory to 75x75km orbit, yet.

I've noticed, if I don't pitch up enough for Rocket mode, I lose way too much speed in atmosphere, and at 20km it's hard to pull up hard enough to affect your trajectory much. So I've taken to starting my hard-pull hack at 17.5km and trying to establish a nice 30 degree (or steeper) climb angle before my turbojet cuts out. Then it's rocket mode to get Apoapsis to 75km - then circularization. Not sure how much Dv is required, for that...

Ohh, I do exactly the opposite... I'll go for a 5° climb until 20km when the rapiers cut out, then I'll pitch up to ~20°, and at 35km altitude I'll pitch down again (to 5°). Then I'll reduce throttle on the way so the apogee is never more than 1 minute in front of me. That's much more efficient...

Basically I'll pitch up never more than 20° (except for the sub-sonic part of the lower atmosphere but that doesn't really matter). And I'll make a very long low power burn 'towards the horizon' until apogee is 70km. Then circularisation is hardly required.

Edited May 13, 2015 by Chris_2

[guitarxe]

Oh ...., I did it! I finally did it! I got my little space plane into 100km orbit!

I kept tweaking and retweaking my design and never understood what I was doing wrong, but the answer was all along in how I was flying it, not my design. I kept flying at a very shallow angle, no more than 10 degrees, and pushing the jets out until they completely cut out at about 25km, then hitting my rockets and that would never work.

This time I climbed at 25-30 degrees from the beginning, and hit the rockets as soon as my velocity from the jets dipped below 1,000m/s, and it worked! \o/

Now to see if this thing can land...

Edited May 14, 2015 by guitarxe

[guitarxe]

... no, it couldn't land because the air intakes melted off during what I thought was a very shallow re-entry.

Time to put some airbrakes on.

[Temeter]

... no, it couldn't land because the air intakes melted off during what I thought was a very shallow re-entry.

Time to put some airbrakes on.

20km periapsis tends to be a failsafe reentry height. Keep your plane at a high angle to get slowed down by air resistance.

[guitarxe]

Can't really pitch myself much away from my velocity vector. My center of mass is way back from the actual aircraft's centre, so it flips out at very low angles of attack.

[Gerandar]

if you have to use a second stage to get into orbit its not an SSTO O.o

[SkyRender]

In my experience, you don't need much dV once you reach the cutoff altitude. If you build up a solid 1000 to 1200m/s before the cutoff, you'll need around 1700m/s for your burn to over 70km AP and subsequent circularization (accounting for gravity losses, which are pretty minor if you do it right).

[Vanamonde]

This question about how to play the game has been moved to Gameplay Questions.