SSTO to Mun

[CrazyPenguin0111]

So I have an SSTO and I'm trying to land on the mun and reture. I heard that you need about 7,000 m/s of DV to do it. My SSTO has 13,000 m/s of DV in the hangar, checked with Kerbal Engineer Redux, but everytime  I launch I always end up in low orbit around kerbin with no fuel left. I just want to know what I did wrong.
Image of SSTO

How I get to orbit:
1. Fly flat until I get to 450 m/s to maximize  thrust for the rapier engines

2. Ascend at a 15 deg angle of attack untill I get to the dark blue bit in the altitude meter. (I forgot the exact altitude)

3. Change angle of attack to 20 deg

4. Switch rapier engines to closed cycle mode and turn off the intakes

5.  Wait for apoapsis to get to 70,000 km.

6. Circularize

7.  Run out of fuel 

Edited September 16, 2021 by CrazyPenguin0111

[Majk]

The delta-v reading you see is based on the ISP of air-breathing mode of Rapier. The closed cycle has a vacuum ISP more than 10 times lower, which also means 10 times lower delta-v.

[CrazyPenguin0111]

 

3 hours ago, Majk said:

The delta-v reading you see is based on the ISP of air-breathing mode of Rapier. The closed cycle has a vacuum ISP more than 10 times lower, which also means 10 times lower delta-v.

so what do I do to get 7x more delta v, adding that much fuel tanks seems impractical

 

[Majk]

The air-breathing mode doesn't use oxidiser. This means that you carry some dead weight. You should drain some of it, or better yet use some lighter tanks that only carry fuel. 

This way, you can add more fuel while maintaining reasonable TWR.

Still, this will likely be insufficient. You can try adding even more fuel and engines, but only up to a point. I don't know whether single-stage-to-Mun-surface-and-back is possible with just rapiers. Perhaps you need to add some more efficient engines to the mix.

I'd suggest aiming for Minmus first.

UPD: I just made a quick assembly with 9 RAPIER engines, 2x Mk3 Rocket Fuel Long, 1x Mk3 Liquid Fuel (Medium) and managed to get to LKO with 1700 m/s left, which was sufficient for Minmus sample return. I suspect single-stage-to-Mun-and-back with just RAPIER is possible with better ratios and/or piloting.

Edited September 17, 2021 by Majk

[IronMaiden]

On 9/16/2021 at 10:15 AM, CrazyPenguin0111 said:

So I have an SSTO and I'm trying to land on the mun and reture. I heard that you need about 7,000 m/s of DV to do it. My SSTO has 13,000 m/s of DV in the hangar, checked with Kerbal Engineer Redux, but everytime  I launch I always end up in low orbit around kerbin with no fuel left. I just want to know what I did wrong.
Image of SSTO

How I get to orbit:
1. Fly flat until I get to 450 m/s to maximize  thrust for the rapier engines

2. Ascend at a 15 deg angle of attack untill I get to the dark blue bit in the altitude meter. (I forgot the exact altitude)

3. Change angle of attack to 20 deg

4. Switch rapier engines to closed cycle mode and turn off the intakes

5.  Wait for apoapsis to get to 70,000 km.

6. Circularize

7.  Run out of fuel 

As far as design goes, you have too many engines. 2 RAPIERs is more than enough for a plane this size. You've also got way too much oxidizer for a RAPIER/Nerv plane. A good starting point is about 10:1 liquid fuel to oxidizer. Also, angle your wings up at least a few degrees to minimize body drag and swap radial monoprop tanks for an inline one to decrease drag.

As far as the ascent goes, fly at sea level til super sonic then slowly allow the nose to rise and the plane will climb. Your pitch shouldn't go much above 15°. Just to be clear, angle of attack is not the same as pitch angle. Angle of attack is how far your prograde marker is from your pitch angle, your pitch angle is how far your control point is angled up from parallel to the surface. Your angle of attack shouldn't ever be much more than a few degrees off of zero and ideally less than 0.01° from zero once past 15km or so (this requires wing incidence as explained in the design part above.)

Once your plane has climbed past 10km start leveling out. You want your pitch and angle of attack to be about 0° by the time you reach 15km or so. Here you want to accelerate on air breathing mode as you very slowly climb. A good target is 1700m/s at 22km. NEVER PITCH ABOVE 5° ONCE PAST 15KM!

Once you've reached mach 5 and your acceleration in airbreathing mode is starting to peter out, activate your Nerv and continue accelerating up to mach 6 (should be close to target above). Once there switch to closed cycle for the final acceleration to orbit. DO NOT PITCH UP. KEEP PITCH BELOW 5° ALL THE WAY TO ORBIT!

Wait for your apoapsis to get to 70km or so (by this point your periapsis should be close to if not already above sea level).

Circularize (should take less than 100m/s).

If you do this right you should easily have 5000m/s+ left once in LKO, probably closer to 10,000m/s.