sevenperforce

I did some math on thrust balancing for a 1+7 configuration (because, again, it is super compact). I assumed a minimum throttle of 40% and a max thrust of 2200 kN. Assuming a 300-tonne-empty booster, there's actually no way to successfully hover with the 1+7 configuration. The closest you can come is lighting three engines as shown in the center. You'd think it would work (three engines at 40% gives about 0.9 gees), but the topmost engine cannot throttle below 73% or the thrust will be unbalanced. So either you hoverslam every time, or you use a different configuration. This view... That's art. It is hard to be sure but they look like they are the same size: SN5 at left; SN6 at right.

Is it though? Or is it just perspective? I think they keep landing hard on one leg and using that crush core.

Hop, skip and a jump!

I haven't heard of them saying that, no. New Glenn is supposed to skip the entry burn but all renders and discussion from SpaceX has shown both boostback and entry. I'm not sure which way altitude and velocity go at this point. I don't think we've gotten any actual numbers for staging velocity since the 12-meter carbon-fiber BFR design. Sounds about right!

I'm talking about Superheavy, not Starship. You definitely need more than one engine for the Superheavy landing burn; Elon said the absolute minimum (for hops) is two. They might be able to do a seven-engine hoverslam but that's probably pushing limits even at minimum throttle. With seven engines in a ring you can fire three or four in near-symmetry and use differential throttle to balance thrust. But the more you are throttling your engines to balance thrust, the less effective throttle range you have. You're right, the entry burn is easy -- you can fire seven or eight there with no problems. I agree. But word of Elon says the core is eight engines, not seven. Of course they could still change that. In fact they could even go to a 29-engine approach and do nine engines in the center with a 20-engine ring.

If you do one central engine with seven around it, you can certainly do a much smaller footprint. But you lose symmetry. If you want perfectly axisymmetric thrust, you'd have to fire at least 7 engines for the landing burn. Probably fine, but it's hard to know for sure; Raptor has enough throttle range to do other combinations. This gives you a skirt size of 9.2 meters with engines under the leg fairings and 9.8 meters if you want all 20 engines in a perfect circle. This assumes 10 degrees of gimbal range on all eight central engines. Obviously there are tighter packings if the central engines have a constrained gimbal range.

Yep, it switched over just in time. Flawless as usual. I would have to compare to prior starlink missions to be sure, but it felt like the time from entry burn shutdown to transonic envelope and landing was much, much shorter than usual. Did anyone else pick up on that?

I never, ever get tired of that gorgeous lift off and hearing “the vehicle is pitching downrange.”

The latter. The two ends would not move simultaneously.

This is the densest packing I can find if we give each engine 51 cm clearance: 9.4 meter skirt with one engine set under each leg.

Not to gatekeep someone else's challenge but keeping SAS on is against the rules, right? Anything that actuates the control surfaces is forbidden.

Both Raptor and Merlin use the fuel (CH4 and RP-1 respectively) for closed-loop hydraulic fluid. However that means the engine must be running in order to operate TVC for the engine. It's possible, I suppose, that they have a redundant pressurization loop for hydraulics that runs between engines, but I feel like that would produce more failure modes than it would solve.

I'm just not sure whether 0.51m would be sufficient clearance at that point. Not concerned about other engines.

Question I posed to Manley on Twitter but is worth discussing here too:

That was toasty.

Well, technically the engines need an operating hydraulic line to gimbal. But in this particular case I was thinking of two central engines lit simultaneously and attempting to provide the maximum roll authority; they would point exactly opposite each other but at an angle.

Pretty sure there's no situation where you'd want Raptors to Gimbal towards each other. They'd be fighting each other's control. They could gimbal quite close to each other for roll control, particularly if the center two engines were the only ones lit. But still probably not quite that close; you're right. And the angle would be offset anyway.

This analysis of a regenerative cooling sleeve in a methalox engine arrived at thermal minimums and maximums of 120-295 K. I'm not entirely sure what the engine bells are made of, but let's assume that they are copper (it has the most extreme thermal expansion of any metal that would likely be used). Cooper's fractional thermal expansion coefficient at 293.15 K is 1.7e-5 per degree or 0.0000017, so we're looking at something on the order of 0.298% expansion over the entire possible thermal gradient.

Sorry, not trying to be a smartass, although I think I probably was. My point is that regenerative cooling will limit the thermal range of the outer edge of the engine bell and therefore will all but eliminate significant expansion or shrinking.

Raptor is a really big engine though. The above image is a early dev model, so it has a lot of wiring and such that is not present in flight articles, but the overall footprint/cross-section is going to be very similar. They will want to put the engine itself in a frag shield, so the cross-sectional footprint of the engine is not actually going to be significantly smaller than the cross-sectional footprint of the bell itself. The Merlin 1D, which is GG, has a much smaller single turbopump and a much smaller upper footprint. While we're on the subject of dimensions...we know from the video of the Starship hop test that the entire engine is actuated via TVC rather than the nozzle alone. We know we need to have 10 degrees of gimbal in any direction, so we can calculate the gimbal footprint: You need 0.51 meters of clearance between a gimballed Raptor and any fixed engine, and 1.02 meters of clearance between any two gimballed Raptors. Will they though?

A "ring" of outer 20 doesn't necessarily mean that they would be in a perfect ring. But if they are, that's a minimum diameter of 9.6 meters. It's entirely a matter of deep throttling. Raptor is a gas-gas engine and so combustion efficiency (which is the usual problem with downthrottled engines) is a non-issue. It also has extremely high chamber pressures so unstart (where the supersonic transition moves inside the throat) isn't a problem either. Elon has said that for Raptor, the limitation on downthrottle happens in the preburners. If you design a preburner that doesn't need to downthrottle (or only needs to downthrottle to, say, 90%), it can be MUCH more robust and produce much more thrust than a preburner which needs to handle high throttle transients. Raptor also uses its turbopump to drive hydraulics for the TVC, so it's possible that going too high on thrust would overstress the TVC, but I doubt it. Welding is ridiculous! The vibrations and thermal expansion will tear them apart! Are they really doing that!?!? Elon has already said that they will weld the RVacs to Starship's skirt, so we know that's the plan. These are regeneratively-cooled engine bells so they should not experience thermal expansion. They are also much more rigid than the radiatively cooled engine bell we're used to seeing on the MVac.

Also, Starship likely will not be able to dock directly to LOP-G due to transverse loading limits on the docking port. After the ISS was constructed, the Shuttle could only dock to the end node for this reason. However, DXL and Orion would both be able to dock to Starship and ferry cargo/crew between both.

Official renders show a flared 10m skirt even though it doesn't come down to cover the bells: That last render is exactly what I think is most likely. Would fit in a 9m circle if you welded the fixed bells together.

Agreed. With only 28 engines I don't see how they need any skirt at all.

I think he's given up on doing 42 engines. And that's not enough gimbal space. If SpaceX drops the skirt altogether they might still be able to make it work with 28 engines: The example on the left has better control authority on the two central engines, which I know is desired, but less overall gimbal space. The example on the right is more symmetric and has broader gimbal on all central engines.