SSTO - problem, and my solution, whats yours?

[scarybeard]

I spent the past month making SSTO space planes. I have made designes which get a single kerbal into orbit, onto a space station if desired, and then bring the plane home with remote control. But I have had trouble getting a passenger section up, so I can use one trip to get a handful of kerbals up there at once.

I finally worked out a good ratio of jet engines to rocket power on these larger craft, but I find on re-entry the center of gravity on the craft has massively shifted, due to the lack of fuel on the way home. So the center of lift is now way in front of my center of gravity, causing me to flip out of control once I hit the atmosphere.

My solution, which has had only limited success, (its still pretty unstable in general, but better) is to have a wing section on the front of the craft detach on re-entry. Sense I don't need that much lift in front of the craft on the way home, I just get rid of it. somewhat wasteful, but I am having a really hard time balancing the center of gravity/lift on a full/empty craft.

Any tips on how to keep a craft stable once its way lighter than when it went up?

(I'd post images of my plane, but the forum wont allow it?)

EDIT: Ah, its because I'm new. I'll post a pic once I get approved to do so

Edited April 14, 2015 by scarybeard

[Alshain]

Build with smaller fuel tanks in greater quantity so you can shift fuel around. Normally with multiple tanks, the fuel drains front to back. You can move the fuel forward before re-entry, but not within the same tank. It is possibly to build a plane where the CoL will shift, but not too far. Speaking of FAR, are you using it?

It doesn't take a very big plane to carry crew, but I do use FAR so if you aren't, mine will not behave quite the same as yours. This is my most efficient crew carrier. It's small size is attributed to it's "reverse" docking style, which was inspired by the Dream Chaser. Those inline docking ports are huge.

Edited April 14, 2015 by Alshain

[cantab]

Design your spaceplane to have a small CoM shift between being full and empty. To do this means putting the CoM in the middle of the fuel tanks. Typically your heavy parts are your cockpit up front and your engines at the back, and you can use the lever principle to help. CoM too far back? Use a lightweight structural part to move the cockpit further forward. Too far forward? Use some bits to move the engines further back.

This isn't a spaceplane but it shows the idea: https://flic.kr/p/r6r5r5 The cockpit is heavy while the tail is light but because the tail stretches quite far back it balances to put the CoM in the Rockomax 16 tank between the wings. (Fuel lines run from that to the wing-mounted engines).

Once it's fine full and empty, you might add a fuel line setup to keep it fine as the fuel drains, or balance fuel using a mod such as TAC Fuel Balancer, or simply do manual fuel transfer as required.

[GoSlash27]

#1 placing the fuel tank in the middle of the CoM will keep your CoM from shifting.

#2 active control surfaces should be balanced around the aircraft to keep it from "infinigliding" into reverse.

#3 keep intakes behind the CoL

Tiny SSTO

Big SSTOs

Best,

-Slashy

[Tweety]

You could always bring your main lifting surface forward (or back) but the easiest way to save a lot of hassle is illustrated in alshain's picture. Put a couple fuel tanks/engine nacelles radially with jets and intakes and move them forward or back and build your wings around that. It's more about experimentation essentially. Slashys designs are excellent too but he is pushing boundaries with weight per jet but has in essence the same principle. He has the heavy fuel in the middle of his craft which reduces COM change when full and empty. There is an SSTO thread around here which can inspire and help a lot if you check out all the weird and wonderful designs people come up with.

Tweety

[InterCity]

My spaceplanes have all the payload exactly at CoM (calculated ) and they're usually a bit of overkill, so there is fuel left to transfer to the front, not to mention RCS that is usually mostly full as well.

[funk]

I'm using FAR for a long time so I cannot say how it's working in stock...

A small shift between COM and DCOM would be nice, but isn't reachable all time. My solution is: One big tank at the COM, smaller ones at the front and rear. Put all the non-fueled stuff to the front and all parts which produce drag to the rear. You want to reentry like a dart or shuttle, it will stabilise itself. COM to the front, Drag to the rear. I've heard of a rumor that some are using rcs tanks around the COM for pumping, but tbh if well designed, there is no need for them. If I need Monopropellant for docking maneuvers the tanks are placed in front of the COM. Be sure COL is always behind COM. If COM is to far in front of COL sometimes you will need to redsign your ship. Use FAR stability calculation for different ascent profiles as well as for hypersonic flight around mach 6-8 for an empty plane at high altitudes (simulating reentry).

In flight: Assuming COL is way behind COM especially in thin air, you will need to shift COM back. Use fuel pumping to the back or you'll need an high AoA for flight around 25-35km. When switching to rocket mode close all intakes.

For reentry open intakes, toggle air breaks and pump the rest of the fuel to the front.

Use TAC fuelbalancer and or Goodspeed fuel pump RCS BuildAid.

My universal CrewCarrier:

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[el_coyoto]

#1 placing the fuel tank in the middle of the CoM will keep your CoM from shifting.

#2 active control surfaces should be balanced around the aircraft to keep it from "infinigliding" into reverse.

#3 keep intakes behind the CoL

^ this

I find that keeping the intakes behind the CoL helps A LOT. They add drag to your aircraft, so it's best to put them at the back of your craft for stability (especially during re-entry).

I have a stupid SSTO design method to reduce CoM shifting that I will try to explain clearly (don't have access to my KSP right now, so no pictures, sorry). It works in stock aero, not sure about FAR...

1. I start with everything which weight doesn't change : cockpit, sas, cargo bay, crew bay, etc. and build what I'd call a "naked fuselage"
   
2. If I use a cargo bay to lift a payload into orbit, I rearrange the above pieces until the CoM is right in the middle of the bay : that way, CoM shifting will be minimized after releasing my payload.
   
3. Now, time to add fuel : I abuse the stock aero system and add fuel (and rcs) tanks on the sides of the craft while making sure that their center match the CoM of the naked fuselage, like this : (CoM in red, naked fuselage in green, tanks in orange)
   

   The goal is trying to minimize CoM shifting due to fuel consumption
   

4. I then deal with wing surfaces, basically using Slashy's advices, checking the CoL / CoM placement, etc.
   
5. I try to make sure that I have more intakes behind the CoL than in front in hopes that the back of the craft has more drag than the front, helping with stability during reentry.
   

It's a cheap way to deal with the CoM shifting problems, has some big engineering constraint but works fine for me, especially when building huge SSTOs...

Only thing is, after a while, I tend to get bored with building SSTOs that way (they tend to look the same), and sometimes go for the good old "tanks in front, tanks in the back, move fuel around to keep it balanced" solution.

Cheers!

[capi3101]

el_coyoto's method of fuselage construction works in FAR, incidentally. You just don't need quite as much junk out on the sides (you still need some as a rule - but it's mostly fuel, and that's because of the way KSP handles fuel drain).

Which aero model are you using, anyway?

Here's my general advice for stock aero spaceplanes. I'm good at those, and if you're using stock DocMoriarty's guide will be of great help to you. Plan for a 25% payload fraction - for a Mk3 Cockpit and a Mk3 Passenger Module (i.e. 19 Kerbal capacity craft), your paywad is just shy of 10 tonnes, so plan on building a 40 tonne plane. You'll need 4 RAPIER engines or 3 Turbojets (and you can couple each Turbojet with three or four 24-77s), 2080 units of LF, 1300 of Oxidizer, 12 Ram Intakes, 13 pairs of Delta Wings/Wing Connector A/Wing Connector B, plus all that other extra spaceplaney goodness...

And here's my advice for FAR spaceplane design. I suck at those, so take the advice there with a grain of salt...(you can read about my FAR plane woes here; start with page 4).

I can't get away without recommending a couple of mods for spaceplane jockeys; these are useful regardless of aero model - RCS Build Aid, and Intake Build Aid. RCS Build Aid, among other things, shows you how far and in what direction your mass will shift as you drain fuel. It also shows you if your engine placement relative to your CoM is going to generate any torque and how much it will add, which becomes important as you get higher up in the atmosphere and your control surfaces start to lose their effectiveness - I've had a couple of spaceplanes fail because of that particular phenomenon. Intake Build Aid corrects for a known issue regarding how Intake Air resources are allocated to the jets, which you can correct manually by placing each intake and jet on your plane individually - that mod lets you build as you will and then fix everything with the push of a single button - your Turbojets and RAPIERs can be operated at full power higher into the atmosphere as a result. Both mods are extraordinarily useful.

Edited April 14, 2015 by capi3101

[EtherDragon]

@GoSlash27 Tiny SSTO is my new favorite!

[FlyingPete]

I try to keep the radial docking port or cargo bay close to the COM if possible, but it's simple enough to position fuel tanks forward and aft of this. This approach actually is quite useful as you can manually transfer fuel between tanks during the ascent to improve control.

When you're building, it helps to check the 'dry' COM by emptying all the fuel tanks in the hangar. Just remember to put it back in before launch

[FlipNascar]

But don't forget to check CoM with your typical landing fuel (if applicable). I too have gone through a recent phase where my SSTOs enter the spin of death. And it cost several Kerbal lives before I realised that while wet CoM and dry CoM line up perfectly, typical re-entry and landing CoM have been ignored. RIP unintentional Kerbal sacrificial test pilots.

[panzer1b]

I always seem to have this problem in every one of my craft, owing to the type of designs i like to fly, rear cockpit rer engines and long but slender. basically those things ALWAYS have fuel shifting issues, as even if i elect to clip fuel into the engine section at the back, its still going to be ahead of the center of gravity of the craft with no fuel.

This is one of my newer craft that fixed this issue by adding ion engines inside the forward fuselage (its a sci-fi inspired craft, i dont know how the ions work when clipped inside a craft). But yeah, ive found adding weapons, RTGs, and anything that isnt going to drain in the nose of rear heavy craft helps.

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I know this style of design is 100% unpractical in teh real world, but rear mounted cockpits look so badass. Thgis particular one also happens to have 6000 dV once in orbit, and can do a laythe roundtrip and land on the mun (just barely enough TWR with the ions). Ohh and it has over 200 massless batteries clipped inside it .