r/SpaceXLounge • u/SpaceXLounge • 5d ago
Monthly Questions and Discussion Thread
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u/Wise_Bass 2d ago
What's the best estimate on what Starship's payload to LEO currently is? I've read on the subreddit here that with all the updates made for the most recent version, it's rather on the heavy side right now - less than 100 metric tons to LEO.
Can they move the header tank in the nose cone, or does it need to be there for balance reasons? It seems kind of inconveniently placed for launching large payloads.
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u/SpaceInMyBrain 23h ago
Very hard to say but the close observers at Starbase think Starship's dry mass is considerably more than planned. Of course it's even more in V2 and V3 but the payload ratio will be better. But it's still estimated to be well below 100t. An Eager Space video on YT stated the extreme performance of Raptor 3 isn't just SpaceX pressing limits, it's actually needed for Starship to overcome its dry mass problems inn order to have a decent payload, one large enough to support its goals, and the tanker needs of Artemis without an unreasonable number of launches. IIRC.
Header tanks have to be in the nose for balance. They were originally going to be enclosed in each main tank but the balance issue forced them to put the LOX tank in the nose, and later they had to put the CH4 tank there also. (IIRC SN8 thru 15 only had the LOX tank in the nose.)
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u/Wise_Bass 23h ago
Do you remember what Eager Space video that was? I'll have to give that one a watch.
Header tanks have to be in the nose for balance. They were originally going to be enclosed in each main tank but the balance issue forced them to put the LOX tank in the nose, and later they had to put the CH4 tank there also. (IIRC SN8 thru 15 only had the LOX tank in the nose.)
That's a disappointment. Header tanks in the nose means you can't really take advantage of the huge potential payload fairing for large-scale LEO deployments, like if you wanted to deploy an 8+ meter wide space telescope (compared to Hubble's 2.4 meters) by opening the top of it.
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u/SpaceInMyBrain 17h ago
Some official renditions of Starship showed a huge "chomper" cargo door that extends from just behind the header tanks to the base of the cargo bay. It opens like the hood of a car. IIRC it took up almost all of the leeward area in width. So, not 8m but some damn large payloads could be released. BUT... it seems optimistic to think the ship could have a hatch that big and maintain the structural stiffness needed for reentry and the landing flip. But I'm no engineer.
Sorry, I don't recall the video. Pretty sure it was within the last 6-8 months.
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u/Spare-Language7812 4d ago
Is there a version of the starship architecture where SpaceX puts a catch tower down range so the booster can get better performance? Unsure where the natural ballistic arc would have SH come down but potentially somewhere in the Caribbean islands if they can delay staging with the reduced landing propellant needs. They could get it back to Boca with a jenky nosecone like the FH side boosters, reduced engine thrust for longevity and a smallish propellant load. Been away from Reddit for a while so sorry if this has been suggested elsewhere.
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u/Martianspirit 13h ago
I think not. There is a problem with that. The catch tower needs to be very stable. Even the best stabilized platform can not be that stable, only ones that are solid at the sea floor.
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u/mrparty1 5d ago
On flight 8 we got a nice shot from inside the Ship's skirt looking at the engines. The SL Raptors' exhaust still looked like they were creating mach diamond(s). Are the surrounding Vacuum Raptors helping to save a little (or maybe a lot) of efficiency of the center engines by limiting the expansion of their exhausts?
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u/Volens_Nolens 23h ago
No, because the flow out of the nozzles is supersonic, and no event that occurs to the gas downstream of the nozzle can propagate back to the nozzle and produce additional thrust or any other effect. You can't affect the flight of a rocket by doing something to its exhaust. So yes, it's compressing the exhaust of the engines and showing that there is residual energy left in the underexpanded flow, but it can't get that energy back.
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u/SpaceInMyBrain 1d ago
What atmosphere will be maintained inside the HLS? My best estimate is 9.0 psi. Orion can operate at 14.7 to 8.3 psi and apparently will be at 9.0 psi while docked to Gateway, with a 70/30 nitrogen/oxygen ratio. But the astronauts will have to get to a low psi pure O2 atmosphere for EVAs to avoid making the suit arms and legs too stiff to move in. Apollo suits were at 3.75 psi with the LM kept at 5 psi. No nitrogen was involved so no prebreathing was required before a Moon walk. HLS will have to match to 9.0 if docking at Gateway and I figure NASA will go with that figure even if Gateway is cancelled.
ISS astronauts currently prebreathe pure O2 for over 2 hours before an EVA so they can use their 4.3 psi suits while in an airlock. They exercise to reduce this from the previous multiple hour approach, which could even be overnight. I presume there's a transition period of a reducing N2/O2 level and reducing the pressure from 14.7 to 4.3 psi. They breathe pure O2 through masks for part (most?) of this time.
If HLS is kept at 9.0 psi at a 70/30 ratio the rebreathing time will be reduced, I assume. Save 10+ minutes? So - the astronauts would need to be in the air lock on the cargo deck for <100 minutes. Not bad, not great, time on the surface is valuable. The airlock looks sizable but will be small for 2 people exercising. Could there be a separate airlock on the crew deck above? There's room to spare. That'd also help with isolating the cabin from regolith dust. But the alternative is an entire HLS filled with a pure O2 atmosphere at <5 psi. This source notes the 9.0 psi 70/30 level was chosen for Gateway because it "maintains material flammability limits within the range currently tested and approved for spaceflight." Ergo, my conclusion is HLS will be kept at 9.0 psi while on the surface. Did I make any big mistakes?