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Why are the service modules on both Crew Dragon and Starliner expended after every flight? Clearly, shedding the mass of the service module reduces the amount of energy the heat shield needs to bleed off during re-entry, but this seems like a solvable problem that would be a boon to reusability and reducing per-flight costs.

I assume SpaceX took the economical route but I have a hard time believing a more robust heat shield would, on a per-flight basis, cost more than a bunch of service modules.

masospaghetti
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    Dragon doesn't have a service module. Just the trunk. https://spaceflight101.com/spacecraft/wp-content/uploads/sites/18/2015/08/8149779_orig.jpg It has a "service section" but it's part of the capsule and protected by the heat shield. Starliner does have an expendable service module. – Organic Marble Mar 30 '24 at 01:38
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    Un-sourced speculation, but part of this may be simplifying launch aborts, by moving things like consumable storage tanks into a separate module that the launch abort rockets do not need to accelerate and lift. Also means that the contents of those tanks are not a risk to the crew or recovery team following a normal or abort splashdown. – GremlinWranger Mar 30 '24 at 03:15
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    @GremlinWranger: Dragon 2 does not separate the trunk until after the abort burn. In fact, the aft fins on the trunk are specifically for aerodynamic stability during the abort. They serve no other purpose. (Well, they also increase the available surface area for the solar panels, which is why Cargo Dragon retains two of the four fins, even though it does not have the abort system.) – Jörg W Mittag Mar 30 '24 at 09:30

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In the case of Space X, the trunk section appears to be:

  • Structural adaptor between Falcon upper stage and dragon
  • Unpressurised storage space for cargo (since falcon is larger than dragon really needs)
  • Solar panels

The first seems cheap enough to not try to bring home, the second is possible sub optimal, but comes from the general noodleness of Falcon 9 and the fact that human space craft are low mass per volume. With the solar panels it would certainly be possible to have either folding panels, or protected in such a way that they can survive re-entry but given solar panels exposed to sunlight and radition degrade and the presence of a structural adaptor trunk anyway putting cheap unprotected panels on it is probably cheaper than engineering and flying a re-usable solution.

As a more general engineering thing, putting re-usable hardware inside a space craft pressure vessel is fine until it needs repair or service, then you can end in a complex morass of 'repair of module X damaged adjacent module Y' as technicians work in the confined space (and only one team can physically fit in there to work at a time). Some hardware may be cheaper to build and expend rather than design for service inside a pressure vessel- an example would be a pump handling (possibly toxic) fluids where doing an all welded 'only has to go in once' design in an open bay may be cheaper through life than a serviceable version with mechanical seals and joints fitting inside the crew cabin.

GremlinWranger
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  • Also worth noting that doors in a heat shield are problematic -- what if they fail to close? Shuttle had at least 6 penetrations in the black part of the heat shield (3x ET mounts and umbilicals, 3x landing gear). Dragon has two penetrations and they are both on the lee side, and only one of them opens in flight. – ikrase Mar 30 '24 at 04:12
  • @ikrase The forward ET attach on shuttle didn't have a door/penetration. – Organic Marble Mar 30 '24 at 11:37
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    @OrganicMarble - the forward attachment is pretty clever from what I remember. It's a ball joint that swivels which allows a bit of movement when the orbiter is attached (or being attached). After separation the ball swivels around so that there is a more or less flush surface there. Interestingly it's a tiny bit of the underside (a few cm) that does not have tile, just metal. – Steve Pemberton Mar 30 '24 at 12:01
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    @StevePemberton and there is an RCC "arrowhead" surrounding it. – Organic Marble Mar 30 '24 at 12:25
  • @OrganicMarble I stand corrected -- also, that's a cool design. Would the shuttle be OK if it didn't swivel? – ikrase Mar 30 '24 at 12:25
  • @OrganicMarble - I didn't know that the arrowhead was RCC, thanks. I wonder why the arrowhead appears red in this photo? Although this may have been one of the museum moves so maybe it was taped for protection or something. – Steve Pemberton Mar 30 '24 at 13:52
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    @ikrase - I found my notes on it and a photo taken during RPM (backflip) at ISS. Here is the same photo cropped and zoomed as the original was pretty high res. Slightly different than I remembered, it's a spherical bearing. The bronze outer ring in the photo is the bearing raceway, the lighter colored bronze is the bearing itself. It appears flat because it has swiveled flush, but it is capable of swiveling several degrees. In the center is the top of a piston which during separation punches out the attachment bolt. – Steve Pemberton Mar 30 '24 at 14:11
  • @StevePemberton here is a crop of the arrowhead from the STS-127 RPM. No idea why it's red in your picture. https://i.imgur.com/Q27onaO.jpeg – Organic Marble Mar 30 '24 at 14:15
  • @StevePemberton lol, I see you posted similar pix. Great minds. – Organic Marble Mar 30 '24 at 14:19
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    @StevePemberton this NASA fact sheet mentions that the arrowhead is RCC. https://www3.nasa.gov/centers/kennedy/pdf/167435main_RCCpanels08.pdf (although it doesn't use the term "arrowhead") – Organic Marble Mar 30 '24 at 14:22