100 Kerbals to Duna, Full Reuse, Rocket Only

sevenperforce · June 7, 2017

This mission report is in pursuit of this challenge:

as well as:

This was my first time ever going interplanetary.

I knew I could do this easily enough with ISRU, but I wanted to try something different, since ISRU on Mars isn't nearly so simple as extending a drill into one spot in the ground. I decided I would use chemical engines only with a booster+tanker+ship arrangement. In lieu of Dunian ISRU, I decided I would send one of the tankers along for the ride, aerobraking both in turn but leaving it in orbit around Duna so it could refuel the crew ship for the homeward journey.

However, after building my ship, putting it in LKO, and painstakingly refueling it, I realized that my not-very-balanced TSTO design and KSP's smaller scale meant that I had enough dV in my crew ship to get to Duna and back in one piece, so I dropped the tanker-along-for-the-ride plan.

The only mod was Tweakscale, and I only used it to decrease part count and streamline aerodynamics. All crew-carrying parts are fully stock.

With no further ado:

Spoiler

My crewed vehicle/lander was a cross between Dragon 2, Chrysler SERV, and the SpaceX ITS. I used stock cargo ramps to enclose and shield my six tweakscaled Dart engines, with an action group to simultaneously toggle the engines and open the cargo ramps. This design allowed me to use a large heat shield for the base and keep engines safe from re-entry heat without cosine losses.

The ship uses twelve scaled-up landing legs and oversized vernier thrusters. There are no reaction wheels other than the ones in the command modules and it does not carry any monopropellant, a nod to Musk's plan for hot-gas thrusters on the ITS. During ascent, the canards are locked and the airbrakes are fixed; during EDL, the canards control pitch, roll, and yaw and the airbrakes deploy to keep the center of pressure as far back as possible. Thanks to this, and thanks to the high mass of the six engines, I didn't need to worry about keeping fuel low. I would learn that it is very passively stable but is still controllable enough to squeeze out some lift.

I would find out that my distribution of vernier thrusters was insufficient, not because they didn't have enough control authority, but because they weren't oriented properly for effective translation and docking. Docking would prove...challenging.

I have 16 Kerbals in each of the six passenger modules and one Kerbal in each of the four cupolas mounted on top, bringing the total to exactly 100.

I used a fun trick to get the tweakscaled Darts inside the cargo ramps. I would have preferred the ability to gimbal my main propulsion, but oh well. No real problem relying on Verniers instead.

In this view, you can also see the scaled-up airbrakes on the top of the 10-meter booster, as well as the fuel cell to ensure my booster command core didn't ever lose power. There are a few Verniers wrapped around the decoupler as well, to kickstart the boostback flip.

I tried to make a cluster of Mainsails, but at the 10-meter scale the structural parts kept breaking, so I just went with a single scaled-up Mammoth. You don't even want to know how much fuel this thing drinks. The landing legs were tricky; I tried scaling them up but they kept breaking. I had to ultimately use ten landing struts at 400% to land it successfully.

The lower tank is reserved solely for the boostback burn.

I had to use a ton of launch clamps or it always ripped itself apart on the pad if I warped even a little.

It strikes an imposing appearance on the pad! Its size is hard to get a grasp of until you realize that the Mammoth covers the entire flame trench and those tiny little passenger modules up top are stock Mk3 parts.

In case you hadn't guessed, the aerospike-in-ramp design allows me to use the thrust of the six Dart engines to get off the pad rapidly, with a high TWR, then close them up to decrease drag until MECO. Since the upper stage has a LOT of fuel capacity, this also burns some of the fuel from the upper stage, helping it get into orbit more rapidly.

Liftoff! She's a beauty. And all 100 passengers are so excited!

If I had wanted to make it a little more interesting, I could have given each of the six pods an independent LES pretty easily, adding decouplers and Sepratrons. They could have even used their Dart engines. Each of the six pods could then be able to re-enter independently, if needed. But it wasn't required for the purposes of this challenge, so...

Shutting down the upper-stage engines and closing the cargo ramps helps streamline the craft for the transonic regime. The ramps remain closed until MECO. After some experimentation, a lofted trajectory seemed to be the best tradeoff.

First view of the top of the launch vehicle, showing in particular the position of the cupolas. I know these are draggy, but with a lofted trajectory this doesn't matter much.

In hindsight I probably should have gone with a much, much taller booster to increase staging velocity and reach orbit with more fuel, but I was already taller than the VAB could manage, and as previously stated I was having trouble trying to fit a cluster on the first stage.

MECO with just a few residuals in the upper tank segment of the booster, while the lower segment remains fueled. This is definitely a low staging velocity. Like I said, I should have gone with an even larger booster.

Even though I'm up above 22 km, there's still enough drag that I lose airspeed rapidly during the separation and flip initiation. Like SpaceX did in its last mission, I used the thrusters to start the flip but used main engine gimbal to level out.

Switching back, and I'm kicking the upper stage engines on immediately to avoid losses. This also helps push the booster back toward the launch site.

Back to the booster; using gimbal to straighten out for the immediate boostback. Thanks to the low staging speed, I didn't actually have to boost all the way back to the site; I could simply kill a portion of my horizontal velocity and let Kerbin rotate under me as I fell.

Boostback complete; on my way back! Using an action group to open my airbrakes and activate the fuel cells.

Switching to the crew vessel, now. I set to prograde and I'm letting it burn at full throttle until my apoapse is where I want it.

The booster descends right on target. I had to try this step quite a few times; kept either hitting water, or overshooting and coming down on a slope in the mountains. It's important to remember that this is a BIG stage. 10 meters is huge.

Coming in fast, but not hot.

Extending landing legs to help me slow down even more; thanks to the Verniers and the airbrakes I don't have any trouble controlling it on the way down. Center of mass is way, way ahead of COP.

And the landing burn begins!

Correcting for the water and throttling way way down. Do I have too much fuel? Too little?

This is KSP, not real life, so I can hover pretty easily if I want to.

Easing my way down over the beach.

Killing off the last bits of velocity, and...

Touchdown!

Meanwhile, my upper stage is on its way to an efficient ascent:

Lowering thrust and easing my way into orbit.

I have to say, she does look good in space.

Nice neat circularization node.

Burning to circularize.

LKO achieved!

Closing up the engine ramps! Quite a bit of fuel remaining, really. Hopefully this will mean not too many refueling trips. Now it's just time to wait! As you can see, the Kerbals have dimmed the cabin lights for some inflight entertainment.

That's the first part! More to come, very soon........

Edited June 7, 2017 by sevenperforce

CatastrophicFailure · June 8, 2017

KerbalJoinReinforcement is your friend. or at least, it should be...

kerbinorbiter · June 8, 2017

Hello, Elon

sevenperforce · June 9, 2017

Alright, next installment!

Time to refuel! It's essentially the same, but with a probe core and battery, added solar panels, and tankage in place of command modules.

The refueling ship looks about the same on the pad.

Lovely liftoff

Closing down the upper stage engines at about the same spot.

Keeping a close eye on fuel.

MECO!

Separation and restart.

After reaching orbit, I had to correct my inclination and line up for an intercept.

Easing into the intercept.

This is where things got hairy. I didn't have any reaction wheels and I didn't have properly-distributed RCS, so I had very little translation capability, and I kept getting misaligned.