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Yet another kerbal question here.

Background: I've built many SSTOs before, mainly small, light-payload or crew delivery ones (either boosted by a rocket or by itself). The reason why I've been shying away from heavy payload lifters is because of the center of mass shift.

Large payload-optimized SSTOs will experience significant center-of-mass shift during the ascent, causing issues with the placement of the wings, center of mass and lift (CoM, CoL) - certain payloads would end up shifting the CoM so far back that the CoL would be in front of the CoM, causing loss of vehicle/crew. Other cases would make the CoM too far forward so the plane is barely maneuverable.

My initial thoughts were either movable or swing wings to change the CoL to adapt to the changing CoM, putting the payload directly on the CoM instead of the front/back of the SSTO, or attaching a counterweight (which sounds dumb because that just wastes mass, unless there is another payload). I've already tried these methods on a craft and seem to work except for edge cases (payload mass near maximum capacity or payload is almost as large as the payload bay).

Any thoughts?

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  • $\begingroup$ what is "LO(C)V"? $\endgroup$ Jan 24, 2022 at 19:06
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    $\begingroup$ @BrendanLuke15 LOV is Loss Of Vehicle. The (C) is included because C stands for Crew. Some vehicles are unmanned while others are manned, hence the C in parentheses in the middle. $\endgroup$
    – WarpPrime
    Jan 25, 2022 at 2:29
  • $\begingroup$ You have simulated many SSTOs, but you never built any SSTO hardware? $\endgroup$
    – Uwe
    Jan 25, 2022 at 18:21
  • $\begingroup$ To be pedantic, you're asking about issues with winged launch vehicles of any staging, not SSTOs per se, which needn't have wings. $\endgroup$ Jan 26, 2022 at 20:31

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I'm not sure if this is really an appropriate question here on Space Exploration, or if it should be migrated to Gaming. That said, I'll answer it here and let the mods worry about whether it's on-topic or not.


The easy solution, at least in KSP, is to split your fuel & oxidizer tanks into two sections — one at the front and one at the rear — and put the payload bay in the middle, roughly where the vessel's center of mass will be. Done carefully, this will result in a craft that:

  • will experience little or no CoM shift during flight, since KSP will drain the the front and rear tanks evenly, and
  • will also have more of less the same CoM loaded and unloaded, at least for most payloads.

Having fuel tanks at the front and at the rear will also allow you to tune your craft's CoM in flight, if needed, by transferring fuel between tanks.


A handy way to verify that your CoM won't shift during flight is to turn on the CoM display in the craft editor (spaceplane hangar / VAB), note where your craft's CoM is when fully fueled (or even take a screenshot), and then drain all the tanks of fuel and check whether the CoM has moved. If it has, you will need to move some of your fuel tanks in the same direction — basically you're trying to get the fuel's CoM to match your craft's dry CoM. Keep filling and draining your tanks and comparing the CoM until it barely moves. For fine tuning, you can also leave some of your tanks only partially filled.

Also note that, if your SSTO is using air-breathing engines (which don't consume oxidizer) for part of its flight, you'll probably want to use some fuel-only tanks (or drain some oxidizer from your LF+Ox tanks) to avoid wasting mass on excess oxidizer. In that case, you will need to do the same balancing act both for fuel and for oxidizer. You can do this in (at least) two equivalent ways:

  • either check that the CoM of your LF-only tanks matches the CoM of your LF+Ox tanks (and that both match your craft's dry CoM), or
  • start with an unfueled craft, first check that adding the oxidizer won't move your CoM too much (moving your LF+Ox tanks if it does), and then check that adding the fuel won't move the CoM either (moving only your LF-only tanks if it does).

Finally, don't forget that a small amount of CoM shift during ascent isn't necessarily a bad thing. Generally you'll want your CoM to be fairly close to your center of lift on takeoff for easy maneuvering, but once you're on your ascent trajectory and gaining speed, a moderate CoM shift forward will help keep your craft stable.


Ps. In fact, real (or at least planned) SSTO craft, such as the Skylon,* use a very similar design. For example, here's an illustration of the Skylon vehicle from Wikipedia, showing the placement of the liquid hydrogen tanks (red), liquid oxygen tanks (blue) and the payload bay (yellow rectangle in the middle):

Skylon fuel storage and payload bay diagram
Image by Wikimedia Commons user Pline, used under the CC BY-SA 4.0 license.

Just from the drawing, you can see that the CoM of all of those things is roughly in the same place near the middle of the vehicle, where the wings are. And the engines and the hull of the vehicle also have their center of mass approximately in the same place, so overall the CoM will stay roughly the same regardless of how full the tanks are and how heavy the payload is.


*) …aaand that's pretty much the only example I can think of. There just haven't been that many "spaceplane" SSTO designs that even got far into the planning stage in real life. The closest actually flown example I can think of would be the NASA Space Shuttle, but that 1) wasn't really an SSTO, 2) used an external fuel tank, and 3) flew a vertical ascent.

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  • $\begingroup$ But where are the engines and the tubes from the tanks to the engines? $\endgroup$
    – Uwe
    Jan 25, 2022 at 18:25
  • $\begingroup$ @Uwe: I'm not sure if that's meant as a serious question, but in any case: the engines for the Skylon are (designed to be) in the wingtip pods, drawn in dark gray in the illustration above. The fuel (and oxidizer) ducts are not shown in the drawing, and I don't know where exactly they're supposed to be, but presumably they'd have to run through the wings and probably, at least to some extent, along the main fuselage. $\endgroup$ May 8, 2022 at 13:35

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