Wimpy SSTO problems

[sevenperforce]

Trying to build a HTHL SSTO powered only by Sparks and Junos as part of my campaign to win this challenge:

I assembled a semi-Grand Tour ship in LKO and it's already on its way around the Kerbol system, but I wanted to build an SSTO spaceplane to pick them up my four explorers when they return, since they have no heat shields and only a single drogue. My payload, then, is merely a probe core, a battery, and four empty command seats. If I can get into LKO, they can take care of the rendezvous and then the return should be easy enough.

I've gone through about four different design iterations so far, all to no avail. The best I've gotten is the following:

In case you can't tell, the eight Junos are attached to the rear node of the Sparks and then translated forward, to reduce drag. Yes, it looks like a Skylon. No, that was not intentional; this shape was the result of a LOT of iteration.

I've tried a lot of different ascent profiles but I'm consistently burning out at around 40 km and roughly 500 m/s short of orbit. That's if I use the Junos for takeoff and then stage in the Sparks almost immediately, pitch up rapidly, and slowly lower the nose to keep my acceleration positive.

Some of my early iterations had a Terrier on the back end or other adjustments, but this particular iteration has been the closest so far.

Any tips?

[Streetwind]

If you almost immediately stage the Sparks after takeoff... have you questioned why you even have Junos in the first place? Jet engines only make sense if they can run for a while on their own.

Just for the sake of iteration, maybe try a plain rocket SSTO? You can toss all air intakes, save the mass of the engines themselves, can replace the LF tank with an LFO one your rockets can make use of...

Edited February 1, 2018 by Streetwind

[sevenperforce]

5 minutes ago, Streetwind said:

If you almost immediately stage the Sparks after takeoff... have you questioned why you even have Junos in the first place? Jet engines only make sense if they can run for a while on their own.

Just for the sake of iteration, maybe try a plain rocket SSTO?

I'll give it a shot.

The TWR with Sparks alone is rather low, too low to maintain positive climb and positive acceleration simultaneously.

[foamyesque]

The Spark is a SSTO capable engine on its own, if I recall correctly, so the Junos might just be dead weight. The problem is that there's going to be a fair amount of overhead mass needed to keep the drag low. I'll do some poking about.

[Geonovast]

Get rid of the Mk2 parts.  That drag is killing you.

[sevenperforce]

12 minutes ago, foamyesque said:

The Spark is a SSTO capable engine on its own, if I recall correctly, so the Junos might just be dead weight. The problem is that there's going to be a fair amount of overhead mass needed to keep the drag low. I'll do some poking about.

I've done little single-Spark vertical-takeoff SSTOs before, but not with the dry mass to pick up 4 Kerbals, get back down, and land deadstick on the runway.

[bewing]

Totally agree with Geonovast. Use interstage fairings for holding your payload, not a MK2 cargo bay. MK1 FTW.

 

[Spricigo]

+1 for mk1 form factor.

[foamyesque]

1 hour ago, sevenperforce said:

I've done little single-Spark vertical-takeoff SSTOs before, but not with the dry mass to pick up 4 Kerbals, get back down, and land deadstick on the runway.

 

Dry mass is just a question of adding more engines -- if you can SSTO any amount, you can SSTO any amount, unless you get burned on structural overheads. A 4-cmd-seat payload isn't particularly large, though.

 

Anyhow, some proof of concept:

 

Horizontal takeoff, pulling into a 70 degree zoom climb as you accelerate (should get to 70 at around 150m/s and TWR 1), then once you get up to 280-300m/s swap to following prograde. Reliably puts you into a ~100x100 orbit with ~200m/s of spare dV, for orbital manouvering or deorbit burns. Re-entry is a breeze; put your periapsis down at 40-45km, set SAS to hold radial out, and it'll fly itself until you're through the heat problems. Then just point your nose at the ground (I find 15 degrees works well) and glide in to your landing.

The narrow wheelbase complicates things a little, but it's not a severe problem. Two small solar panels make sure the battery to drive the probe core and reaction wheel inside the service bay stays charged.

 

[sevenperforce]

13 hours ago, bewing said:

Totally agree with Geonovast. Use interstage fairings for holding your payload, not a MK2 cargo bay. MK1 FTW.

I wasn't using a cargo bay -- those were all fuel tanks. I had figured that Mk2 rocket-propellant fuselages would save me on wing mass since they have appreciable body lift of their own. But, after iteration, I can see that they were really draggy.

12 hours ago, foamyesque said:

Dry mass is just a question of adding more engines -- if you can SSTO any amount, you can SSTO any amount, unless you get burned on structural overheads. A 4-cmd-seat payload isn't particularly large, though.

Anyhow, some proof of concept:

 

  Reveal hidden contents

 

 

 

Horizontal takeoff, pulling into a 70 degree zoom climb as you accelerate (should get to 70 at around 150m/s and TWR 1), then once you get up to 280-300m/s swap to following prograde. Reliably puts you into a ~100x100 orbit with ~200m/s of spare dV, for orbital manouvering or deorbit burns. Re-entry is a breeze; put your periapsis down at 40-45km, set SAS to hold radial out, and it'll fly itself until you're through the heat problems. Then just point your nose at the ground (I find 15 degrees works well) and glide in to your landing.

The narrow wheelbase complicates things a little, but it's not a severe problem. Two small solar panels make sure the battery to drive the probe core and reaction wheel inside the service bay stays charged.

 

 

Structural overhead is what I was worried about. If a Spark can do a vertical-takeoff SSTO with an 5% payload fraction, but you need at least 10% of the vehicle dry mass to have a winged EDL onto landing gear, then adding more engines and more fuel is a losing game.

But obviously you've demonstrated that it can be done! I still want to try to make one with Junos, just for the heck of it, but thanks!

[sevenperforce]

Tried again with a Mk1 form factor and just 4 Sparks and 4 Junos, and I'm just 93 m/s short of orbit.

Going to ditch the Junos and see what happens.

[bewing]

I suspect that optimizing your launch profile will easily get you 100 m/s.

[sevenperforce]

51 minutes ago, bewing said:

I suspect that optimizing your launch profile will easily get you 100 m/s.

That was after optimizing the ascent profile.

Anyhow, I ended up dropping the Junos, dropping the wing engines, and just clustering six Sparks on the tail inside a clipped fairing.

Aaaaaand it flies itself into orbit with nothing but trim adjustments.