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The Space Shuttle needed a high-performance heat shield that caused much grief over it's operational life. Having a large surface area on a reentering vehicle reduces temperatures by causing the vehicle to slow at higher altitudes. The External Tank had much larger surface area than the Shuttle, while weighing only about a third.

What if the LOX/LH2 tanks were located on the Orbiter instead of as a separate module? What would be the penalties for such a design? How much would this reduce ballistic coefficient, and thus temperatures? What material choices would this have opened up, and how much better would they have been?

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    $\begingroup$ That's essentially what Starship is. As Chris Bergin, founder and Managing Editor of NASASpaceflight.com likes to say: Starship tries to be what Shuttle wanted to be. $\endgroup$
    – Jörg W Mittag
    Commented Aug 22, 2021 at 9:58
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    $\begingroup$ What would be the penalties for such a design? Of the top ten penalties for such a design, eleven of them would be weight. $\endgroup$
    – Digger
    Commented Aug 22, 2021 at 14:57
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    $\begingroup$ this was also the design philosophy with Skylon, especially as it used LH2/O2, making it even more effective than starship. $\endgroup$
    – R. Hall
    Commented Sep 7, 2021 at 4:09

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It would make reentry heating much less stressful, but would make landing the 7-times-larger-volume vehicle a royal pain, and would greatly increase the surface area that needs to be insulated. And to retain the needed crossrange ability, the wings would need to be even bigger, thus upping total mass even more.

The early concept design process considered many different variations, with earlier release of ET, flyback ET, fuel in shuttle, etc.. They decided on the Shuttle Program as we knew it as the apparently optimal compromise of requirements and abilities.

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    $\begingroup$ Hmm, so keep the tank detachable and drop it before landing? Maybe put some parachutes on it so that it can be recovered. I'm sure there would be plenty of practical issues with such a design that I haven't considered, but it seems like an interesting option… $\endgroup$
    – Ilmari Karonen
    Commented Aug 22, 2021 at 9:57
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Partial answer to

What if the LOX/LH2 tanks were located on the Orbiter instead of as a separate module?

The original concept of the fully reusable shuttle system included an Orbiter with internal tankage such as you suggest.

However, there was also a separate, much larger, reusable Booster (also with internal tankage) which carried the majority of the propellants.

This drawing from Jenkins, Space Shuttle, 1992 edition, shows cutaways of two such Phase B Orbiter designs including the internal tankage on the right, and the Orbiter/Booster combo on the left.

enter image description here

If you're suggesting some kind of merged Orbiter / ET configuration which still utilized solid rocket boosters, I can not think of any work done on that. Once the propellant tanks were moved out of the Orbiter design around 1972, they never really came back. All of the Phase C designs featured variants on the Orbiter / External Tank / Booster configuration.

Designing a merged Orbiter / ET configuration which did not incorporate boosters would have been a single-stage-to-orbit design, which the Space Shuttle Program did not sign up for.

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The external tank of the Space Shuttle had a dry weight of 26,556 kg. The maximum payload of the Shuttle to a low orbit was 24,500 kg, less than the weight of the tank.

Reaching an orbit without any payload but with the external tank was impossible with this additional load of over 2000 kg above the maximum payload.

An external tank suitable for a reentry would have been much heavier than the standard tank with the cryogenic tank insulation only. The foam insulation weight was 2,188 kg.

A payload with negative weight made of unobtanium would have been needed.

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  • $\begingroup$ It would be a 1.5 stage, the same way it was. $\endgroup$
    – Abdullah is not an Amalekite
    Commented Aug 22, 2021 at 3:06
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    $\begingroup$ The space shuttle did carry the external tank effectively all the way to orbit - it was jettisoned just short of orbital velocity with a perigee still in the atmosphere. The OMS did a final orbital insertion and circularization manoeuvre, but the total delta-v requirement there is quite small. $\endgroup$
    – J...
    Commented Aug 22, 2021 at 14:54
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    $\begingroup$ You're correct that mass would be transferred to the orbiter, but I don't think it would be as severe as noted in the answer. The dry weight of the ET included more than just propellant tanks. A considerable part of the dry weight was structure to transfer the SRB thrust to the orbiter. You can delete much of that structure if you move the tanks into the orbiter. In any case, you're right that it lowers the payload capacity. $\endgroup$
    – DrSheldon
    Commented Aug 23, 2021 at 11:49
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    $\begingroup$ @J... yes, that's a very important point that may even completeky invalidate the answer. $\endgroup$
    – Abdullah is not an Amalekite
    Commented Sep 7, 2021 at 13:18

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