Help me with SSTO design

[Foxster]

I have been banging my head against the wall with an SSTO design problem for a while. 

This craft is designed to have at least 5000dV left by Kerbin orbit...

It does this fine now. Problem is that it's a pig to get back down to the ground. 

The red circle indicates the Dry Centre of Mass and you can see that it is ahead of the centre of lift. That means it's unflyable on its return to Kerbin with all but a drop of fuel gone. 

But if I shift the lift back behind the DCoM then I can't get it off the ground in the first place, it's too nose-heavy. 

So, I guess I need the DCom and CoM closer to each other. But this sleek design is needed to make orbit with a minimal of drag to keep the remaining dV high. Every time I mess with the design to redistribute the fuel/engines it means a drag compromise and a big loss in dV. 

Any suggestions from the SSTO/spaceplane gurus?

Craft file: https://www.dropbox.com/s/5n19iok95w8qrhc/Mun SSTO 8.craft?dl=0

[AeroGav]

2 hours ago, Foxster said:

I have been banging my head against the wall with an SSTO design problem for a while. 

This craft is designed to have at least 5000dV left by Kerbin orbit...

It does this fine now. Problem is that it's a pig to get back down to the ground. 

The red circle indicates the Dry Centre of Mass and you can see that it is ahead of the centre of lift. That means it's unflyable on its return to Kerbin with all but a drop of fuel gone. 

But if I shift the lift back behind the DCoM then I can't get it off the ground in the first place, it's too nose-heavy. 

So, I guess I need the DCom and CoM closer to each other. But this sleek design is needed to make orbit with a minimal of drag to keep the remaining dV high. Every time I mess with the design to redistribute the fuel/engines it means a drag compromise and a big loss in dV. 

Any suggestions from the SSTO/spaceplane gurus?

Craft file: https://www.dropbox.com/s/5n19iok95w8qrhc/Mun SSTO 8.craft?dl=0

The only solution is to have pod mounted engines either side of the core, fuel containing fuselage.  Perhaps the rapier pods can be slightly ahead of CG to offset the mass of the nuke which will have to go at the back , since it only draws fuel from its own stack.  That does imply a drag/frontal area penalty sadly.

[Boris-Barboris]

Why not fly tail-first on descent? Implying you are sure it's retrograde-stable. The only diffirence will be that you won't be able to use engines, I guess.

Edited June 17, 2016 by Boris-Barboris

[kcs123]

You should realy have CoL behind dry CoM. At least just slightly behind dry CoM.

There is few things to avoid "heavy nose" on take off issue. Put rear landing gears in a way that legs are just behind dry CoM. Just enough that craft don't fall on engine on launch when you empty out all tanks. If you still don't have enough pitching authority on take off, you can also add canards on the nose to help you out with that.

[CSE]

As the CoM moves back, it's reducing the lever arm of the engine-mounted surfaces  but leaving you a lot of potential leverage in front. Would the front wings (with leading edge control surfaces?) go any further forward? Could you mount canards right at the front and use active stability?

Afterthoughts: if you move wet CoM forward, can you use a taller nose gear to push it into the sky based on runway AoA?

Also, RATO is still SSTO. You know, morally.

Edited June 17, 2016 by CSE

[bewing]

I mostly agree with CSE. Four little canards will help keep the nose pointed the way you want, move the front wings back a little to put your CoL closer to your DCoM, and save a little fuel to use as a counterweight. Just before reentry, manually transfer all the remaining fuel to those two FLT200 tanks at the very front.

Or maybe add another pair of wings at the very back end to move your CoL back. The extra lift will counterbalance any drag losses.

Interesting design, though -- I'm tempted to play with it.

Edited June 17, 2016 by bewing

[Foxster]

Ok. So I played around with some of the ideas from you guys and have something workable now...

A combo of things helped: Canards out front gave some more pitch control. Using one central pair of wings gave a pivot point. And a biggy was shifting a couple of LF tanks from the front of the craft to the back behind the CoM and rearranging the Rapiers to accommodate that. 

Craft makes orbit now with 5100-5200 dV to spare and flies much nicer. Not lovely but good enough. Still need to shift the fuel around a little but not much. 

Craft file if anyone wants to take it for a spin around the solar system and maybe improve it further: https://www.dropbox.com/s/xp4dnky6623jgve/Mun SSTO 9.craft?dl=0 

Thanks for the input, guys. 

 

 

[mk1980]

one small addition: if you build craft with very thight specifications, i find the BigS wings/wing strakes quite useful. they have the same weight ratio as the structural wings (0.1 ton weight per m² wing surface) but can also be used as fuel tanks. ie. their fuel capacity is essentially "free" - 4 BigS wing strakes hold the same amount of fuel as 1 mk1 tank, so you save the dry mass of the tank (0.25 tons). not a big deal for craft with simple requirements, but 5km/s in orbit is a tall order, so every little bit can help

[Foxster]

52 minutes ago, mk1980 said:

one small addition: if you build craft with very thight specifications, i find the BigS wings/wing strakes quite useful. they have the same weight ratio as the structural wings (0.1 ton weight per m² wing surface) but can also be used as fuel tanks. ie. their fuel capacity is essentially "free" - 4 BigS wing strakes hold the same amount of fuel as 1 mk1 tank, so you save the dry mass of the tank (0.25 tons). not a big deal for craft with simple requirements, but 5km/s in orbit is a tall order, so every little bit can help

Thanks for that. Even though I must have looked at those wing stats a hundred times I still had stuck in my mind that the wings with fuel capacity had poor dry weight. 

[Foxster]

Just in case anyone is interested, I've tweaked things a bit and it's looking more like a shuttle now. On-orbit dV is now 5600...

Lost one of the rapiers, used some more wing, reduced the stack count, got some angle on the wings. Pretty much fit for purpose now.  

[OrbitalBuzzsaw]

Less oxidizer maybe?