SSTO runs out of fuel.

[Barrrrry]

Hello all!

I am new to the Kerbal forums, but have been crashing rockets since April 2015. I have recently been working on flying a SSTO, and have been having problems. I generally will get to a 70-80km orbit with no fuel left over. I use either RAPIER engines or a mixture of Ramjets and aerospikes. I generally fly a flight profile with a 30-40 degree climb to 10km, then I throttle up to full and go at about a 2 degree AoA. Once I hit about 1km/s speeds I pitch up to about 30 degrees, raising apoapsis. Can you guys/gals help? I don't know what I can do to save more fuel. 

Thank you in advance!

Edited December 10, 2015 by Barrrrry

[Gooru]

Try to reach 1400m/s while on 10+ km before going for your apoapsis.

A picture of your craft would be helpfull to tell more

[Barrrrry]

6 minutes ago, Gooru said:

Try to reach 1400m/s while on 10+ km before going for your apoapsis.

A picture of your craft would be helpfull to tell more

I will give a picture when I get the chance (I'm away from my home computer right now).

[cephalo]

Get those rapiers up to 450 at sea level before your ascent to 10k. The thrust increases with speed on those, and at 450 you'll enter a feedback loop of increasing thrust. Once you pitch up, you'll still be accelerating even at a steep angle and by 10k you'll be around 700ms. You have to be a bit careful because it's easy to overheat stuff in 1.0.5. Everything with under 2400 temperature tolerance should be inside a service bay or cargo bay.

[Val]

3 hours ago, Barrrrry said:

Once I hit about 1km/s speeds I pitch up to about 30 degrees, raising apoapsis. Can you guys/gals help? I don't know what I can do to save more fuel. 

30° is too much. Climbing that steeply spends more fuel fighting gravity.

Try 15° instead. A shallower climb will let your wings do the fighting of gravity and allow you more horizontal speed from your fuel.

A circular orbit consists only of horizontal movement, so spend as little fuel as possible going up.

But in general ascent profile depends on how your craft is designed.

If you have high TWR (< 18 t per RAPIER) then you'll want to climb 10° pretty much right off the runway. Maybe a short build up of speed to 200-250 m/s and then around 8-12 km (1000 m/s speed) increase that to 15° climb.

If you have medium TWR (18-22 t per RAPIER) then you'll want to build up speed to 400 m/s before starting a climb of 10°-15°. Level out at 10 km, build speed to 1000 m/s, then gently pitch up to 10°-15° again. Lower or higher proportionally to your TWR.

If you have low TWR (> 22 t per RAPIER)  then you'll want to build up speed to 450 m/s before starting a 10° climb. Again level out at 10 km, build speed to 1000 m/s, then gently pitch up to 10° again. Though you may need to pitch up a little above 15 km, to avoid overheating.

For all of the above you, should end up at around 1300-1400 m/s around 22 km, and that's where you switch to rocket mode. Keep holding the pitch set at 10 km, until AP is above 45 km. Then turn prograde.

Now for some examples. First there are 2 almost identical ships in outward appearance.

The Mk.1 has very low TWR. 30 t for a single RAPIER. It has a very shallow 10° ascent.

The Mk.2 has very high TWR. 36 t for 2 RAPIERs. It has a steeper 15° ascent.

And if your craft is insanely heat resistant you can do it like this.

Hope this helps.

[Barrrrry]

1 hour ago, Val said:

30° is too much. Climbing that steeply spends more fuel fighting gravity.

Try 15° instead. A shallower climb will let your wings do the fighting of gravity and allow you more horizontal speed from your fuel.

A circular orbit consists only of horizontal movement, so spend as little fuel as possible going up.

But in general ascent profile depends on how your craft is designed.

If you have high TWR (< 18 t per RAPIER) then you'll want to climb 10° pretty much right off the runway. Maybe a short build up of speed to 200-250 m/s and then around 8-12 km (1000 m/s speed) increase that to 15° climb.

If you have medium TWR (18-22 t per RAPIER) then you'll want to build up speed to 400 m/s before starting a climb of 10°-15°. Level out at 10 km, build speed to 1000 m/s, then gently pitch up to 10°-15° again. Lower or higher proportionally to your TWR.

If you have low TWR (> 22 t per RAPIER)  then you'll want to build up speed to 450 m/s before starting a 10° climb. Again level out at 10 km, build speed to 1000 m/s, then gently pitch up to 10° again. Though you may need to pitch up a little above 15 km, to avoid overheating.

For all of the above you, should end up at around 1300-1400 m/s around 22 km, and that's where you switch to rocket mode. Keep holding the pitch set at 10 km, until AP is above 45 km. Then turn prograde.

Now for some examples. First there are 2 almost identical ships in outward appearance.

The Mk.1 has very low TWR. 30 t for a single RAPIER. It has a very shallow 10° ascent.

The Mk.2 has very high TWR. 36 t for 2 RAPIERs. It has a steeper 15° ascent.

And if your craft is insanely heat resistant you can do it like this.

Hope this helps.

It helps a lot, thank you so much!