# How can Starship belly flop with a full payload?

Starship had an empty weight of under 100 tons. Of this, less than 12 tons is accounted for by the engines. Starship is supposed to be able to make atmospheric entry with a 150 ton payload in the nose. Wouldn't this shift the CG way forward and make belly flopping impossible?

How much di the main tanks weigh? How much do the header tanks and their contents weight? How much does the payload bay weigh?

Where are the center of gravity and center of pressure of Starship anyway?

• Can I ask why you have this uncertainty? I would expect the calculations and testing SpaceX have done to be the definitive work on this area of aero/astronautics. Apr 13, 2023 at 9:36
• @RoryAlsop expect ≠ know Apr 13, 2023 at 11:57
• The only folks who know, currently, are SpaceX - nobody else is at the level they are. Apr 13, 2023 at 12:54
• You are overlooking the plumbing, support structure for the engines, skirt structure for bearing loads between the booster and full propellant tanks, etc. There will generally be more structural mass lower in the vehicle to support what's higher in the vehicle. The whole reason for putting the header tanks in the nose is to bring the CoG forward, and the rear flaps still need to be larger to balance things out. Apr 13, 2023 at 15:37
• I think "150 ton payload in the nose" - is confusing. Even if it is possible down the line. May 15, 2023 at 12:45

Well, first, Starship has two winglets in the front and two in the rear, so depending on how those are deployed, they can dynamically adjust the center of pressure while in the bellyflop position. I imagine if it's running with a large payload, the forward fins would be more fully extended and the rear fins would be more folded back.

But that said, just because they can land with 150 tons aboard, doesn't mean they can land with any arbitrary object of that mass. I think there are certainly going to be limits about how the mass is allowed to be distributed to stay within the flight envelope.

The cargo adapter inside the Starship is pretty close to the center of the ship, just forward of the methane tank, so most payloads will sit well back from the nose. Only a pretty long piece of equipment would extend out towards the nose, and in that case it's unlikely to have a lot of weight up that far.

For a number of reasons, the connection point on a payload is usually going to be as near to the center of mass as is practical. You want to minimize the lever arm your cargo presents, since a long lever arm means the cargo effectively "weighs" more as far as the connector is concerned, and for all the structural elements between those points. For example, imagine tying a weight to one end of a ruler and then carrying the ruler straight out in front of you by the heavy end versus the light end. Your hand is the docking adapter, and you can easily see how much more strength is needed if the weight is way out there, if it's even possible to hold at all. And if you replace the ruler with thin cardboard, the cardboard can easily support its own weight when you hold the heavy part, but it would fold and collapse if you tried to hold it the other way.

• I don't think the last paragraph about docking adapters is really correct, even putting aside how many payloads don't actually have an actual docking adapter and are held by some other kind of payload adapter instead. There will surely be moment arm constraints on how a payload is loaded into Starship, but there are other pressing constraints for where to locate a payload's center of mass. May 15, 2023 at 16:26
• I probably said the wrong thing. I couldn't think of what to call the "bit that has a spot to clamp onto". Docking is probably the wrong term. May 16, 2023 at 12:33
• That said, I haven't been able to find any examples of payloads that have a clampy-bit and also have that bit a long way from the center of mass, except for possibly the ISS hab units, which are kind of weird hollow cylinders and don't have any points that are particularly near the CG. Even then, the CG is about halfway down the length of the unit (obviously) so the point holds that if you were stowing one inside a Starship, the cargo CG wouldn't be way up in the nosecone. May 16, 2023 at 12:36

Here's something which occurred to me.

If Starship uses a lifting entry - and it probably will - the temperatures will be higher near the nose of the ship. This will cause air molecules there to dissociate. The uncrease of gas particles will increase the pressure near the front. Further downstream, the atoms will recombine, decreasing pressure at the back. Precisely this caused the center of pressure to be 2 meters upstream of the predicted position during STS-1.