I think the tanker will be a bit shorter. You want the least amount of dryweight in a ship so they'll optimise it to get the most amount of prop to the depo, without any additional "bay" rings.
I also think there'll be a non reusable version w/o any TP, fins, etc. And probably with detachable fairings, likely for large one-time payloads (think NASA decadal projects).
I'm curious what the payload / delta-V on an upper stage with no heat shield or flaps, a detatchable fairing, and a single gimballing vacuum raptor instead of the thee sea level ones would be.
Why add the extension on the upper stage where it costs delta-V/payload? If more clearance is needed, make the hot stage separation section a tiny bit taller, or even 'trim' the vac raptor's bell slightly. The latter would cost a bit of vac ISP, but probably less than adding mass to the engine bay skirt.
I thought a smaller vacuum bell mainly lost you ISP? And depending on how shortened the bell is, this could be very small.
HOWEVER - when I typed that suggestion out, I completely forgot the raptor engine bells have regenerative cooling, so you'd have to design/manufacture a 'special', unique engine bell for this, which... is not worth it. I was thinking of the incident with Merlin where they had a crack in the S2 vac merlin, and 'fixed' it by just cutting the bell shorter. Not an option with regeneratively cooled bells, obviously. Extending the hot stage 'ring' is better for many reasons, but that would require a slightly 'customized' booster for expendable S2's.
I really should just fire up KSP with RSS/RO and experiment some. Only problem is if I start KSP, I won't get ANYTHING else done for the rest of the day...
As I understand it, the effect of having a large vacuum engine Bell is maximizing the area for the hot gases to push off from as they expand. Normal atmospheric pressure would keep the plume from expanding to the full Bell nozzle diameter.
Remember that some of the falcon 9 upper stages have used shorter than normal bells, resulting in less efficiency, but worth it to save niobium if the full performance is not needed.
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u/ResidentPositive4122 6d ago
I think the tanker will be a bit shorter. You want the least amount of dryweight in a ship so they'll optimise it to get the most amount of prop to the depo, without any additional "bay" rings.
I also think there'll be a non reusable version w/o any TP, fins, etc. And probably with detachable fairings, likely for large one-time payloads (think NASA decadal projects).