Lightest double SSTO

[camacju]

The challenge here is to build a craft that can get to orbit twice without refueling, staging, or docking.

Score is based on mass - the lighter the craft, the better.

Bonus cookie points if you make an all rocket craft.

More rules:
You must come to a stop at KSC or the ground nearby between orbits. (Screenshot at 0 m/s on the ground).

 

Scoreboard:

camacju - 9.2 tons

Edited October 4, 2020 by camacju
j

[swjr-swis]

32 minutes ago, camacju said:

Landings can be touch and go; all the proof that's needed is a screenshot of your approach to KSC. (Even if you cheat on this, the fuel required to get you back up to speed is negligible.)

No, most definitely not negligible. There is a significantly higher fuel expenditure at low altitude and speed. A good bit of fuel will be expended in an actual effort to fully break and accelerate again and/or to touch down gear safely, which can easily make or break a craft's ability to get back into orbit a second time.

My opinion: I think in this type of challenges touch and go shouldn't be allowed - it favors spaceplane entries over rockets, as rockets still need to lose/reverse all their speed while spaceplanes get to keep a good chunk of momentum.

If you still want to allow it *and* accept still screenshots as proof, a bit of allowance is inevitable - gears/wheels being in the state they are it's very easy for a craft to bounce before you can hit the screenshot key, whether in horizontal or vertical landing. But to be fair to all, the screenshot should be close enough to show an actual touch down was at least attempted.

[camacju]

Okay, that's fine. I didn't realize. I assumed that since comparatively little fuel is used for a jet to accelerate back up to ~100 m/s, it wouldn't matter much.

@swjr-swis Is this better?

Edited October 3, 2020 by camacju

[swjr-swis]

7 hours ago, camacju said:

You must come to a stop at KSC or the ground nearby between orbits. (Screenshot at 0 m/s on the ground).

Ok clear. That equalizes things a bit more between spaceplanes and rockets, at least for that part.

If I remember correctly, the last time this specific objective was called for in a challenge, they added categories for trying to pick up the same payload and bring it back again, also without refueling (as a separate leaderboard). Might be interesting to compare how the game progression influences the results - back then it was done in 1.0.5, with significant differences in aero and engine performance.

[camacju]

https://imgur.com/a/ux0zUFu

Well, here's my submission to start. 9.2 tons double SSTO, landed with 84 liquid fuel remaining.

I used two intakes because:
Diverterless supersonic intake + mini ram intake is less mass than a half fueled normal tank and a normal ram intake
Diverterless intake dies at about 1400m/s, mini ram intake takes me to over 1700m/s
Precooler doesn't have enough fuel in it, I'd have to add another tank

Possible improvements:
Use a precooler to cut out the extra intake, and have some small liquid fuel tanks stuffed in the service bay (not sure if that would even save any mass)
Replace the nuclear engine and one of the fuel tanks with xenon, batteries, and ions - but ion TWR is very low and speed on rapier is limited. I couldn't get this one to work sadly
 

[moar ssto]

Should be doable with a sub-4t craft using dlc propellers and ion engines. Someone has tested successfully his 3.6t single stage to eve and back unrefueld vehiclle, getting rid some of the xenon gas and rtg for just kerbin ascend twice might push the mass below 3t.

Edited October 5, 2020 by moar ssto

[camacju]

I don't have dlc so unfortunately I can't try that. Do you have a link to that craft?

Edited October 5, 2020 by camacju

[18Watt]

I have an entry, which at 99.3T is going to be instantly in last place.   However, it gets 16 Kerbals to LKO and returns.  Twice.   That comes out to about 6.21T per Kerbal, gets them all to orbit and back twice.   It's not really optimized for this challenge, so on a per-Kerbal basis someone should be able to do better.

The Ship:

Spoiler

Here's the ship, a spaceplane SSTO.   Uses 6 RAPIER engines, and holds 16 Kerbals.   Has a docking port, and full RCS control, so docking is pretty easy.

Also, I can get it to orbit with plenty of fuel to get to Minmus, and either dock in orbit or land on the surface.   To return to Kerbin from Minmus requires refueling.   The Fuel/Oxidizer ratio is set up to get to orbit once.   However, there's plenty of fuel left in orbit.   Doing this challenge, getting to orbit twice, I ended up with a lot of oxidizer left over.

As you can see, I actually loaded 16 Kerbals on board for the trial, because in 1.11 their mass is accounted for (I think..).

The First Orbit:

Spoiler

Sitting on the runway, ready to launch.

Made orbit, about 75X70km.   Profile is climb at about 10 degrees, try to hit 1500+m/s before engines lose steam (at about 24km high).   Switch to closed-cycle.

Shot of airplane in first orbit.  Plenty of oxidizer left, should be enough LF to go again.   Once I get close to orbital speed, I shut off the center four engines, for better thrust control- that's why they are not glowing like the outboard engines.

Re-entry.   I normally enter much steeper, which is better for thermal control.   To save fuel for this challenge, I entered very shallow, which did cause a little more heat.  But it was manageable.

Back on the runway, about 40 minutes after launch.   As you see, I'm facing west, because I always end up overshooting the runway.   I keep a couple of the PAWs for fuel tanks open during flight to balance the CG as needed.

Showing my total mass after landing from first orbit:  now down to 66.85T.

The Second Orbit:

Spoiler

The second orbit, roughly 78X75km.   Still a lot of oxidizer left, but not much LF.   Plenty to get home though.

Moving fuel forward before second re-entry.   It's much more stable with a forward COM.   I'll move some of it back again just before landing to increase pitch response, but for the high-speed entry its better with all the weight forward.

Second re-entry.  Looking good.

I over-shot the runway.   Again.  Of course.   No problem, make a U-turn.   I have engines if I need them, but almost never do.

Back on the runway, the second time.   Still a little LF left, and lots of Oxidizer.

One of the passengers on EVA at KSC, after two trip to orbit and back.   In-game time it took 1 hour 22 minutes.