How can I improve upon this SSTO design?

[Destroyer713]

I'm trying to design a 20 Kerbal SSTO that's meant to rendezvous and dock with a station in a 100 km orbit. I get the feeling this craft has the fuel to do that, but it's so excruciatingly difficult to fly efficiently that I haven't even been able to get it out of the atmosphere (And the few times I've been able to get the Apoapsis above 70 km, I'll do a Physics warp to get out of the atmosphere, and the drone core will spontaneously explode), even when I do fly the proper ascent profile.

Here's the SSTO in question:

Craft File: https://www.dropbox.com/s/nq58lvkhwysad81/Starchaser%20Mk2.craft?dl=0

The biggest thing that's driving me crazy is how hard it is to break 400 m/s with this craft. I know I need to do so in order to get the massive amounts of thrust Rapiers are capable of, but I'm finding it difficult to do this efficiently or without splashing into the ocean. I'm honestly not quite sure where I can cut down on drag, and I really don't want to add another engine or two. If I could figure this out, I might be able to save enough Jet Fuel that I don't risk running out of it before I'm ready to switch to rocket mode.

Another thing I've noticed is that under 400 m/s, this craft can fly directly into the prograde marker. Yet as soon as I break 400 m/s, its natural tendency is to fly below the prograde marker. Pressing F12 seems to indicate there's negative lift coming from the fuselage itself. What's up with that?

So obviously I suck at designing and flying SSTOs, but I really do want to get this to work in order to minimize costs to take Kerbals to a station in LKO. Think you guys can give advice on how to improve this design?

Edited October 6, 2015 by Destroyer713

[Nothalogh]

First question, do you really need to move 24 kerbals at a whack?

Try increasing your TWR by using only two passenger cabins

[Destroyer713]

First question, do you really need to move 24 kerbals at a whack?

Try increasing your TWR by using only two passenger cabins

First of all, it's only 20 Kerbals (5 crew cabins that hold 4 Kerbals each). Second, yes, I'd like to be able to launch 20 Kerbals at a time. The station I have in orbit holds that many, and it would be nice to fill it up all in one launch.

[Nothalogh]

First of all, it's only 20 Kerbals (5 crew cabins that hold 4 Kerbals each). Second, yes, I'd like to be able to launch 20 Kerbals at a time. The station I have in orbit holds that many, and it would be nice to fill it up all in one launch.

Ok, so the challenge is 20 kerbals to orbit on an SSTO with 500 dv to spare at 70x70?

I'll grab your .craft, and see what I can make do, and failing that, what I can make it into

-----

First off, you've only got 1400m/s of rocket dv, that ain't going to space

Edited October 6, 2015 by Nothalogh

[SpaceplaneAddict]

Now now, don't be hasty! 1400 m/s is plenty, for LKO...

More engines please. I know you don't want that, but sometimes that's the best way to go. And dang, a little more lift might help( a lot).

[Destroyer713]

And dang, a little more lift might help( a lot).

How exactly would more lift help in this case? Even with the pitiful TWR this craft starts out with, it can still fly off the ground just fine. I feel like adding more lift would increase drag, which would only make things worse. Or am I missing something here?

On another note, I've noticed the front of this craft likes to wobble quite a bit. I'm not sure if that's because of excessive reaction wheel control or weak joints, but I have to wonder if that's causing some of the problems with this craft.

[DeMatt]

My gut reaction (being as I'm nowhere near a KSP-capable computer) is that you're under-winged and under-engined for the amount of cargo.

The shortage of wing surface means you'll have to hold a higher Angle of Attack to maintain trajectory, which translates into more drag. The shortage of engine means less thrust to overcome that drag.

I'd look into adding two more Rapiers - should be simple enough, given the existing configuration. Doubling your wing surface would be a good start, as well.

It might be worth considering the Mk3 spaceplane parts - then you only need the Mk3 cockpit and Mk3 passenger cabin for your 20 kerbonauts.

Edited October 6, 2015 by DeMatt
No, there are no aerospikes there.

[Val]

...

So obviously I suck at designing and flying SSTOs, but I really do want to get this to work in order to minimize costs to take Kerbals to a station in LKO. Think you guys can give advice on how to improve this design?

No, you don't. It is a very fine and capable spaceplane SSTO you created. You just need the right ascent profile.

Unedited I managed to get it to an 80 km circular orbit with 400 m/s dV spare in the second try. First try I exploded when doing physics warp, while coasting to space.

It does have a few minor issues.

• It's a little wobbly when loaded on the runway, but that's nothing to worry about.
  
• CoL can be moved closer to or even right on top of CoM to improve handling, especially dropping the nose after steering input.
  
• The cargo bay explodes when doing max physics warp. Is there some clipping going on?
  

It also has a some good features.

• It has plenty of lift, making it easy to take-off.
  
• Good control authority, making it easy to fly.
  
• CoM and Dry CoM are very close, for less chance of ending up with unstable craft.
  
• Wing Incidence and other good low drag features.
  
• Main gear right below CoM, to avoid buckling landing gear and veering off the runway.
  

This is how I launched it:

1. Full throttle, SAS on, Stage.
   
2. Just before reaching runway end lift nose to 5° pitch.
   
3. Pitch down as needed to keep below 300 m until speed is 450 m/s.
   
4. Carefully pitch up to 10°.
   
5. At 7 km start pitching down to reach 5° pitch at 8 km.
   
6. When speed is above 1000 m/s start pitching up slowly.
   
7. Aim for 15° pitch at 15 km and hold that pitch.
   
8. At 22 km switch RAPIERs to closed cycle.
   
9. Keep going at 15° pitch.
   
10. When AP is 50 km pitch down and follow the prograde marker.
    
11. Throttle down when AP is 82 km.
    
12. Coast to space while following prograde marker.
    

I didn't try docking, deorbiting or landing with it. So can't comment on those aspects. I had to transfer fuel to the rear tanks to get MechJeb to display dV correctly.

Here are some pictures of my ascent.

Javascript is disabled. View full album

[Destroyer713]

3. Pitch down as needed to keep below 300 m until speed is 450 m/s.

I must admit, I didn't try to keep altitude low while building speed. I didn't even think it mattered, so while I kept pitch low, I was climbing to around 1500 m before even thinking about pitching down. Is that what makes all the difference?

The cargo bay explodes when doing max physics warp. Is there some clipping going on?

The only clipping I can think of is from the front Canard or the RCS Thrusters, and that wasn't even intentional. I'll have to take another look and maybe relocate the cargo bay if necessary.

But hey, glad to know my design wasn't as bad as I thought! I assume 400 m/s is plenty of dV to rendezvous and dock in LKO then land back at KSC. Thanks a lot to everyone for all the input!

[Val]

...

I assume 400 m/s is plenty of dV to rendezvous and dock in LKO then land back at KSC. Thanks a lot to everyone for all the input!

It's should be enough for rendezvous and docking up to at least 250 km orbit, I'd guess.

[ajburges]

Another thing I've noticed is that under 400 m/s, this craft can fly directly into the prograde marker. Yet as soon as I break 400 m/s, its natural tendency is to fly below the prograde marker. Pressing F12 seems to indicate there's negative lift coming from the fuselage itself. What's up with that?

So obviously I suck at designing and flying SSTOs, but I really do want to get this to work in order to minimize costs to take Kerbals to a station in LKO. Think you guys can give advice on how to improve this design?

If you suck at SSTO design then I'm a rank amatuer! I'm copying that bi-coupler use. Should be less drag than Mk1 radial stacks.

From the picture, this is a slow ascent plane. 50 t on 2 RAPIERs is a large load, but others have done more. I don't have the patience for such slow ascents so I'll leave it to others to help you with that.

The way the atmo currently works, the most practical place to get transonic speed on RAPIER engines is ASL. The power vs altitude curve is too harsh unless you are already at Mach 1. Get to about 330 m/s ASL and then climb as fast as possible while slowly accelerating slowly. Level off at angels 7-10 to push past 500 m/s and hit the power band.

You get negative lift with negative AoA. That Mk2 fuselage is also a wing. When you point below prograde that body will produce negative lift. If it has a different CoL than your inclined wings, you can also get complex lift vector behavior.

[GoSlash27]

Destroyer,

First thing that springs to mind for me is this:

You're sending 20 Kerbals in 1 go. A Mk3 spaceplane would be much better for this job. It's got way less mass and drag for the payload. It would also greatly simplify your design, thus further reducing drag.

*edit*

I knocked this together as a quick proof- of- concept.

Still uses 2 RAPIERs and carries the same payload. It exceeds Mach 4.4 in air breathing mode with no fuss.

You'd probably want your spaceplane to look a little prettier, though.

Best,

-Slashy

Edited October 7, 2015 by GoSlash27