sevenperforce

VTOL SSTO using only Demo parts

8 posts in this topic

I dug through the forums and couldn't find anything quite like this. It promises to be a pretty kerbal sort of challenge, so hopefully we will get some interesting entries.

Building an SSTO seems to be pretty popular for early players. Airbreathing spaceplanes are the most common, though pure-rocket and VTOL SSTOs are typical too.

Building a reusable SSTO in Demo Mode, on the other hand...that's a bit of a challenge. It's easy enough to stack stages in Demo to achieve pretty much anything you want, but with only a single liquid-fueled engine, what you can do in a single stage is limited. The LV-T30 (at least, the one in the Demo) has a sea level TWR of 14.8, which isn't exactly ideal for an SSTO. With an ISP maxing out at 305 s and the demo's maximum tankage ratio of 9:1, the theoretical maximum delta-v of an LV-T30-based SSTO is 5.47 km/s with a launch TWR of exactly 1:1. This is more than enough to reach orbit, of course, but when you factor in decreased atmospheric specific impulse, aerodynamic drag, a reasonable launch TWR, and a host of other considerations, you end up cutting it pretty close. And getting back down to the atmosphere, surviving re-entry without heat shields, and still having enough fuel to land without parachutes...that's tough.

So that's the challenge! Build a single-stage rocket using only Demo parts that can take off, reach a stable orbit, re-enter, and land propulsively without a parachute.

A list of demo parts is available here. However, this list includes the TVR-1180C Mk1 Stack Tri-Coupler, the AV-R8 Winglet, and the Telus Mobility Enhancer, none of which are present in the demo I have, so they aren't allowed for the purposes of this challenge. Anything else on that list is fine.

To qualify as a single-stage rocket, the entire rocket (minus launch clamps) must reach orbit and return to land without any loss of parts. A stable orbit is an orbit greater than 70x70 km. Water landings are permitted as long as you touch down under rocket propulsion and no parts break.

No autopilot mods, but anything else is fine.

Control is going to be a challenge; without a vectorable engine or control surfaces (and minimal mass budget for reaction wheels or RCS), a lot is going to depend on aerodynamics and your center of mass. For that reason, some part clipping is fine.  

Scoring of qualified entries is based on extra points, available as follows:

  • Playing in the actual demo: +200 points
  • Reaching an orbit of at least 100x100 km: +500 points
  • Retrograde orbit rather than prograde orbit: +300 points
  • Using a single re-entry rather than multiple aerobraking passes: +100 points
  • Landing on the daylight side of Kerbin: +50 points
  • Landing on land rather than on water: +75 points
  • Landing in sight of the VAB: +200 points
  • Landing on the actual launch pad: +100 points
  • Landing on deployed landing legs: +25 points

Ties will be decided by whichever entry has a lower launch mass.

Good luck to all!

Edited by sevenperforce
added points

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6 hours ago, sevenperforce said:

re-enter, and land propulsively without a parachute.

aaaaand you lost me there.

5470 m/s to reach orbit is enough, but no way you will have enough fuel left to land by rocket propulsion alone.

Edited by wibou7
you <--> me stoopid typo

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39 minutes ago, wibou7 said:

aaaaand you lost you there.

5470 m/s to reach orbit is enough, but no way you will have enough fuel left to land by rocket propulsion alone.

Well, maybe the standard game is a lot harder than the demo, because I've come awfully close numerous times. Terminal velocity for my SSTO after a tail-first re-entry is around 320 m/s, which really isn't much dV. The times that I've judged the suicide burn properly, I've usually been within 50 m/s of success. My biggest trouble is a landing target... somehow I always end up landing in the dark in the middle of the ocean. 

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3 hours ago, sevenperforce said:

Well, maybe the standard game is a lot harder than the demo, because I've come awfully close numerous times. Terminal velocity for my SSTO after a tail-first re-entry is around 320 m/s, which really isn't much dV. The times that I've judged the suicide burn properly, I've usually been within 50 m/s of success. My biggest trouble is a landing target... somehow I always end up landing in the dark in the middle of the ocean. 

Hmm... right the drag is going to slow you quite a lot, I didn't think about that.
Then it's true that a perfectly timed suicide burn could do it, ideally over the ocean (water forgive more than grass).
But the margin are going to be so tight there won't have any room for error, you litterally need the perfect suicide burn there.

I guess it is doable but it will most likely be a one-short thing, hardly something you will be able to reproduce.

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4 hours ago, wibou7 said:

Hmm... right the drag is going to slow you quite a lot, I didn't think about that.
Then it's true that a perfectly timed suicide burn could do it, ideally over the ocean (water forgive more than grass).
But the margin are going to be so tight there won't have any room for error, you litterally need the perfect suicide burn there.

I guess it is doable but it will most likely be a one-short thing, hardly something you will be able to reproduce.

I come out of orbit with about 800 m/s of dV remaining...that's with two liquid engines and a single solid booster. Should definitely be enough for a hoverslam with corrections if a better pilot than I was at the wheel, so to speak.

EDIT: And now that I recall, terminal velocity is closer to 250 m/s anyway. So yeah, that should definitely be enough. I've air-launched several times while trying to hoverslam so I know I have enough fuel. 

Given the usefulness of SRBs for their high TWR and relatively low dry mass, I think there could be numerous ways to squeeze a little extra fuel onboard and really maximize delta-v. I'm also hoping to see some unique designs, for ships that maintain a forward COM on the way up and then a rearward COM on the way down. The aerodynamic fins in the Demo are very heat-sensitive and fixed, but giving the ship a bit of crossrange (to get it back to the launch pad) or using aerodynamics to slow it down more before the suicide burn would be neat to see.   

Edited by sevenperforce

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Bumping this with my entry.

Two LV-T30s parallel to a single RT-10, with a small inline reaction wheel just above each engine. I've set up the fuel to flow up into the central stack of fuel tanks in order to keep the COM as far forward as possible during ascent. I've used just two struts to tie the parallel tanks to the SRB. I even drained the command module of monopropellant in order to save weight. Going to rely on the engines to keep the command module's battery charged to run its reaction wheel. The fuel tank clipping at the top and bottom is an attempt to help maintain aerodynamic stability.

SSTO_1.png

Sunrise...and...launch!

SSTO_2.png

GLOW of 38.2 tonnes and a SL thrust of 556.8 kN gives it a spry T/W ratio of 1.49. My initial attempts focused on stacking as much fuel as I possibly could, but I've gotten better performance using high initial thrust to get the LV-T30s up into the atmosphere as fast as possible to improve their Isp. The dry mass of the RT-10 is low enough that there isn't much of a loss from bringing it along for the ride.

SSTO_3.png

At SRB burnout, I've just broken 100 m/s and I've gained about 3% in specific impulse on the two remaining engines. It's enough to overcome drag and maintain a T/W ratio of around 1.15, enough to start edging into an early gravity turn.

SSTO_4.png

Just past Mach 1, thirteen kilometers high, and well into the gravity turn. As with virtually any SSTO, I have ample T/W, so I'm taking advantage of that by not using a lofted trajectory. Apoapse is just under thirty kilometers at this point. Notice that I've already almost maxed out the specific impulse of my engines.

SSTO_6.png

You can see how flat the ascent trajectory is. Outside the bulk of the atmosphere. I'm aiming for a 70x70 orbit, so I've throttled way down and trying to build up to it gradually rather than worrying with a restart at apogee.

SSTO_7.png

Orbit achieved! 70x72 km, with 2.73 tonnes of fuel remaining. Given the specific impulse of my engines, that gives me up to 881 m/s of dV. My electric charge is lower than I'd like, but I should still be able to pull off the deorbit burn, and aerodynamics will take care of the rest.

The trick here is going to be selecting the right deorbit point so I end up coming down in the water or on land near sea level, during the daytime.

SSTO_8.png

A 40 m/s deorbit burn, and I'm all set. Taking a shallow re-entry that hopefully will put me down in the ocean; I'm still a little shaky on planning deorbit trajectories. By my calculations, I'll have about 770 m/s of dV at sea level.

SSTO_9.png

Re-entry! With a trajectory this shallow, heating is minor and comes after I've already dropped pretty far, since the upper atmosphere slowed me down a bit already without heating. Aerodynamics holding me close to retrograde with a little residual wobble. Unfortunately, it looks like I'll be coming down over land close to sunset, which makes the suicide burn doubly dicey since I don't know my true altitude and shadows will be deceptive.

SSTO_10.png

Made it through re-entry and I'm already nearing the thickest part of the atmosphere, so my terminal velocity should be low. Just going to have to wing the suicide burn; that 770 m/s will go fast and I'm coming down in the mountains. My max T/W ratio is right around 3.5 which is good for a hoverslam.

SSTO_11.png

Acceleratometer is down to just about 1 gee, so I know I'm close to terminal velocity. I've got more than twice the dV I need, though, so I'll break it into two burns. A back-of-the-envelope estimate tells me I'll need to start my preliminary suicide burn around 1100 meters above the ground, but I'm not sure where the ground is. These mountain ranges average around 1 km so I'll start a preliminary burn around 2.2 km.

SSTO_12.png

Uh oh. I hate mountains.

SSTO_13.png

2,043 meters? Are you kidding me!? Still, if I had started that burn just three seconds earlier, I would have zeroed out with almost 400 m/s of dV remaining, which would have been more than enough for a feather-light landing. 

Catastrophic failure, yes...but it's clearly possible. So surely someone can do it.

 

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Hmmm, I'd try it, but I can't, looks interesting though, I wonder what you guys will come up with.

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ahh, i remember the good old demo days, building rcs upper stages and using landing legs as docking arms. oh what fun it was, having to build rockets larger than the vab just to put a lander on the mun.

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