Another spaceplane question: dealing with change in CoM height in mid-flight

[Cirocco]

Hi guys,

moar spaceplane questions!

I'm designing a heavy SSTO refueler spaceplane that I would like to be able to carry at least an orange tank's worth of fuel and oxidizer into orbit to refuel possible future fuel stations (I really need to get my priorities in order. I'm desiging logisitics for stuff that doesn't even exist yet...) but I have a bit of a problem: the center of mass goes up during flight. This means that while everything is fine and dandy in atmosphere where CoM and CoT are aligned and the flaps compensate for any small misalignments, in orbit everything does to hell because the flaps don't work and the difference between the center of mass and center of thrust means the plane flips like a coin.

Now, I know why the CoM moves: I use an overhead wing design with engines hanging from the wing (think C-130 hercules), engines fueled from the center tanks. So quite a lot of structural weight hangs above the CoM of the plane, a lot of fuel lies below it. Fuel runs out-->CoM shifts upwards-->misalignement, plane somersaults. I could redesign the plane to have the wings lower, but I like the whole overhanging wing design.

So rather than redesigning: anyone have any ideas on how to deal with a CoM that shifts up/down in flight? Current test designs didn't have a lot (read: none whatsoever) of extra SAS units so that might help already. Will test that tonight.

thanks in advance,

Cirocco

[MaverickSawyer]

Aha. You've hit the point where it's time to teach the "less is more" concept.

I NEVER run a full load of fuel. I actually run about half full. Why?

Because I can shunt the fuel and oxidizer around the airframe to adjust CoM.

I've been using the Spaceplane Plus pack, and typically use three Mk 2 fuselages in series. I'm carrying about half my maximum fuel load, and about a third to half of my oxidizer capacity. I balance the fuel in the SPH for an easy and responsive takeoff, lock the MechJeb Smart A.S.S. in surface mode to a pitch of 17* nose up, then pump fuel around until I get neutral trim (indicated by the little pitch slider in the lower left corner). Repeat the process as often as you need during the ascent.

Bonus of having more capacity than you actually use during the ascent: You can fully fuel it and run on rockets to whatever your destination is. Single stage to Minmus is well within reason.

Just remember, carrying fuel up takes even more fuel to get it there. That's why most of my tankers are rockets with parachutes and heatshields on the second stage.

[Cirocco]

I don't run mechjeb, so that's not really an option for me but I do think that you're on to something with the whole don't-take-off-with-a-full-fuel-load idea. Less fuel to lift means less fuel needed overall. Which in turn means less CoM shift.

In the meantime I've also figured I could angle the wings a bit downwards. This makes the outer engines hang a little lower and will put more structural weight closer to (below?) the CoM and will stabilise the plane a little more. Wings angled downwards also slightly increase stability in atmosphere if I recall my aerodynamics research correctly.

So that's two things I need to try, in addition to adding some additional SAS units. Any more suggestions are very much welcomed.

[Heagar]

You have two possebilitys, if you want to stick near your original design:

• To mount a engine in the back of your fuselage and restrict the max power (can be done in the SPH by rightclicking the engines) of your upper - wingmounted - engines. The downside is, if they are RAPIER-s, they have a limited thrust within the athmosphere too.
  
• Use a mod called infernal robotics, there is a rail (also used for small elevertors e.g) on with witch you can mount your engines and extend the rail downwards.
  
• 
  

@MaverickSawyer: You miss his point. He is building a refueler, therefore he needs every pint in LKO.

[Cirocco]

@MaverickSawyer: You miss his point. He is building a refueler, therefore he needs every pint in LKO.

Not really. I need a lot of fuel and oxidizer when I arrive, and preferrably in -almost- correct proportions. The refueler's goal is to refuel the station with both oxidizer and fuel. If I have a ton of fuel to spare but no oxidizer left, I'm going to have to make another run anyway so it would have been better to have less wieght to push up into orbit.

I really wish that smaller, oxidizer-only tanks were a thing so I could adjust the ratio without having to mess around with bulky half-empty fuel tanks...

[Pondafarr]

You could also try Sarbian's AutoGimbal plugin, it trims the Center of Thrust to align with Center of Mass. Your craft may fly at a slight nose-down attitiude, but it will fly straighter.

[the_bT]

Ok, here's what I do:

Throttle down to 10%-ish (or limit main engine thrust) and use RCS and SAS to keep you on course. I find that the easiest option with big cargo spaceplanes.

This design is comparable in configuration to yours I think, note the extra RCS thrusters in the front and back (well, hard to see but they're there).

[Claw]

You can also shift your engines up slightly to reduce the difference of the CoM/CoT shift. Then and add torque or a bit more aero controls to compensate for the smaller shift.

Or maybe add a high mounted engine that you can tweak the thrust on as you fly.

Also, angling the wings down makes a plane less stable. But a high wing makes it more stable. So as long as your CoL isn't lower than your CoM it should be fine.

[MarkyJ279]

I can think of three options off the top of my head:

1. Put long/thin fuel tanks alont the bottom corners of your fuselage (start them empty or with just oxidiser) and then pump fuel into them as balast while in flight. You might even be able to rig up fuel lines so this happens automatically. However, this might need a substantial redesign of your landing gear .

2. Additional engines on the top of the wings, the further up the better. The tiny rockmax engine weighs very little, produces a decent amount of thrust and can be surface mounted. Stick some on the upper tip of your tail fin pointing back and it'll shift your centre of thrust a bit higher to compensate for the CoM moving up. If you're worried about it throwing you flight off at launch then put it in an action group and only turn it on when you notice yourself losing control

3. Or the brute force method of a boatload of SAS and RCS to try and force the craft to fly straight. Crude but effective and makes attitude changes in orbit much quicker

[Heagar]

Ok then. I had build a CTV/refueler myself a while ago (took me some time to find it in an old install).

It was my first heavy STTO, therefore the intake spam. It consisted of the cockpit MK3, a hitchhiker container and an orange fuel tank. Now i would increase the jets (6-8 for a higher TWR) with three instakes each, decrease the wingspace and so on.

Four Jetengines with two fueltanks MK1-jet for each provides the thrust in the atmosphere and a single poodle engine for orbital injection and circularisation. As well as four ion-drives for obital operations.

Reaching 33 km altitude at an speed of 2100 m/s the switch between jet to rocket mode was done. This monster was able to deliver about 75% of the orange tank (liquid fuel and oxedizer) as well as RCS-fuel and xenon-gas to my space-station in an orbit of 150 km altitude.

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Maybe these informations helps you to find a soloution for your design. If not, look for example at this thread to get some inspirations for a heavy lift STTO.

Edited May 27, 2014 by Heagar

[LordFjord]

The folks around here also helped me to get my big bird into orbit, here is the thread.

Reposting it cause (I think) it has many invaluable tips for heavy SSTO building and ascent.

[Hodo]

The other option is to build your space plane with the cargo mounted centerline of your craft. This will remove or at least reduce the com shift issues. This is of course assuming you choose to stay stock. If not I can suggest two mods, TAC fuel balancer and B9 aerospace for the cargo bays. And always FAR. Good luck.

[Laie]

Coming late to the party, I have one more suggestion: drop tanks.

Forget about less is more, fill you tanker to the brim, and slap on a small amount of external tanks with decouplers and fuel lines. Remember to drain the oxidizer from said tanks, and most important: mount the tanks in line with the CoM.

A 5-8% increase in weight ought to get you past the point where you start the rockets. You will still experience CoM shift, but a) not much and vertical, which is much less problematic than fore-and-aft. A few tanks under the wings are no eyesore, either. Especially not on heavy craft.

[Laie]

Scrap that -- I should have re-read the original post first. The Wings are on top of the fuselage, the engines mounted to the wings but not in line with CoM. Works in atmosphere but not in space.

I see no other solution but rearranging the engines:

-1) this only works if you have all tanks in one string (or at least on the same level)

0) Turn her on the back (much easier that way)

1) stack pylons as needed

2) drain fuel to bring the CoM fore or aft, so it will be a little behind the engine you want to place

3) attach engine to the pylons while keeping an eye on CoM/CoT

In this example, the third pylon wasn't necessary: the engine is attached to the side of the second pylon. For aestethic reasons, I suggest to place a second engine on the other side. Then just clone the first pylon, complete with engines and all, and place the duplicates along the wing as you see fit. If the wing is inclined, you have to repeat this for every single engine. Btw, 100% accuracy isn't necessary: between reaction wheels and gimbals, near-perfect is enough. Don't sweat over two pixels or three.

Also remember to turn off the gimbals on all engines that are mounted ahead of the CoM.

The result sure looks neat, and familiar:

However, with so much length behind the CoM, it can't take off from the runway. It also lacks control at high velocities/altitudes. Canards would probably fix both, but just have a look at her: canards on that plane would be a crime.

Edited June 6, 2014 by Laie
aileron<>canard

[Hodo]

However, with so much length behind the CoM, it can't take off from the runway. It also lacks control at high velocities/altitudes. Ailerons would probably fix both, but just have a look at her: ailerons would be a crime.

Ailerons are the control surfaces that control the roll rate of the craft, usually on the ends of the wings. I think you are thinking about canards which are elevator/aileron or "elerons" that are placed near the front of the aircraft in place of the conventional elevator setup at the rear of the aircraft.

[O-Doc]

Design your craft trim around the fuel-load that you have when flying high altitude prior to getting into space. That means achieving level flight without SAS engaged or your hand on the stick. You can be nose heavy in atmo and your control surfaces will be able to compensate. If they can't then consider carrying some drop tanks on the shoulder and ditch them when the air starts to thin.

It's hard to get a true SSTO(no drop tanks) without your engines being aligned dead center while still being light on RCS or SAS for stabilisation. I have designs that will carry 500+ units of RCS to stabilise through the upper section of the ascent. This can be damaging to payload capacity though. However, it helps increase the weight range of payloads they can take.

One other thing to consider is getting fancy with your wing arrangement. In my heavy lift SSTOs I need more natural upwards pitch so, I stick upwards deflected wings on the front and back to produce that pitch moment in the craft. You could do the opposite and have wings at the back deflected downwards 10-15 degrees so induce a natural forward pitching moment in the aircraft dynamics. You will get more lift out of the aircraft as a result but, you will need to compensate in lower atmo with more control surfaces to trim the early part of your ascent.

[Laie]

@Hodo: thanks. fixed that.

@O-Doc: according to the OP, atmospheric flight is not (much) of a problem, but the off-center engines make his craft useless in space. Missed that myself (ahem).