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I'm curious about whether or not the Space Shuttle could have been used as a lunar orbit vehicle - basically, would have it been technically capable of getting to the Moon, entering orbit, then leaving orbit?

My reasoning is that since the Shuttle was designed to carry large payloads, you could easily stuff enough fuel in there to power the already-existing thrusters to do whatever you needed to. Also, the area not taken up by fuel could be used to carry food and supplies for the extended mission.

Would it be possible?

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    $\begingroup$ Consider the never-flown Shuttle-centuar. If we replace payload mass with OMS tanks we can get a lot of delta-v out of it. I'm not absolutely sure the 1km/s would be available without cryogenic fuels but ... $\endgroup$
    – Joshua
    Nov 9, 2015 at 21:35
  • $\begingroup$ An OMS payload bay kit was planned but never flown. It was to provide 500 fps delta v. $\endgroup$
    – Organic Marble
    May 14, 2017 at 16:27
  • $\begingroup$ 500fps is 152.4m/s... meager as heck. Not even close. But it occupied only about 20% of the payload bay. I wonder about SSME payload bay kit though... $\endgroup$
    – SF.
    May 15, 2017 at 11:31
  • $\begingroup$ @SF. The worst thing about SSMEs is that they're practically impossible to light off in flight (even worse, in vacuum). So you'd have to redesign them from scratch, or do the whole ascent plus TLI in one go (have fun with the ET jett with the SSMEs running). $\endgroup$
    – TooTea
    Sep 23, 2020 at 7:06
  • $\begingroup$ @TooTea I'd imagine no relighting them, just seamlessly reduce intake from main tank, increase intake from bay tank, once switchover is complete, cut off the main tank and detach the umbilical. $\endgroup$
    – SF.
    Sep 23, 2020 at 11:45

3 Answers 3

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This exact problem was presented in The Artemis Project's "Why We Won't Fly a Space Shuttle to the Moon" article, so I'll just quote a few short excerpts from the points it is making, and the rest is then in the article:

  • Vehicle Mass

    Dry weight of Shuttle Orbiter is about 250,000 lbs, compared to the space-only LTV's [Lunar Transfer Vehicle] weight of perhaps 7500 lbs. We'd be accelerating a lot of mass that we don't really need in lunar orbit.

  • Refueling the Orbiter

    Since the Shuttle Orbiter gets to LEO [Low Earth Orbit] with near-zero extra fuel, we'd have to use other launches to get the fuel up there. You can work out the math. Use Isp=460 sec, delta-V=Isp*g*ln(Mo/Mf). The delta-V requirements are in the Mission Timeline. It's a lot of launches... continues in the article

  • Heat Shield

    The Orbiter's heat shield is designed for entry from low Earth orbit, about 25,000 ft/sec. Coming home from the moon, we'd hit the atmosphere at about 36,000 ft/sec. Whether the Shuttle could take that increase in kinetic energy is an unknown.

  • Atmospheric Entry Loads

    The Orbiter is designed for 3 g's max operational load. Assuming we didn't use rockets to decelerate while coming home from the moon, we'd hit up to 6 g's; even up to 12 if the piloting is just a bit off... continues in the article

  • Radiation, Vehicle Reliability, Operational Flexibility

    Shuttle wins all those issues. It has a proven track record. If we got to LEO and the Orbiter was fully operational (fuel cells and auxiliary power units all in the green), it would probably be as reliable for the rest of the mission as anything else we could design... continues in the article

Mentioned article then continues with another point that is limiting Space Shuttle STS (Space Transportation System) vehicle's actual landing on the Moon, so that's not applicable to our problem.

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    $\begingroup$ Re. the heat shield. The K.E. at 36K is more than twice it is at 25K. Given engineering solutions on Earth surface (where 'every ounce of solution' does not make that much difference in cost) rarely stretch more than 10% beyond 'designed/expected limits', I'd say the shuttle coming back from the moon would be doomed.. +1 for the rest though. $\endgroup$
    – Andrew Thompson
    Sep 7, 2013 at 2:43
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    $\begingroup$ Before dismissing a shuttle entry from lunar transfer, one would have to look at multiple aeropasses. The shuttle material is not an ablative and reusable, so in principle it should be able to take an arbitrarily large heat load if given time to cool down between passes. As it did for multiple missions. Then the question is whether the entry could be broken into enough passes to not exceed the heat rate capability of the system. It would take some fuel to raise periapsis and then lower it again after each pass to give time to cool down. $\endgroup$
    – Mark Adler
    Sep 7, 2013 at 2:58
  • $\begingroup$ @MarkAdler: I doubt you'd need to raise Periapsis at all. I suspect after the first pass you'd still take several days to hit periapsis again. By that time, you'd be cooled off suitably. You might have to repeat a few times, but I doubt it'd take more than a few hours to cool off the shuttles enough to do that, and by the time you were in such a low orbit, you could just reenter. $\endgroup$
    – PearsonArtPhoto ♦
    Sep 7, 2013 at 12:51
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    $\begingroup$ Even if you don't need extra orbits, you will need to raise periapsis for each entry in order to have a flight path angle at entry that is both survivable and that skips you back out. When you get to the lower orbits, I don't know if two hours is enough to cool down. That's where I imagined you might want a few more orbits, requiring you to raise periapsis out of the sensible atmosphere. $\endgroup$
    – Mark Adler
    Sep 7, 2013 at 13:35
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    $\begingroup$ is the linked Artemis Society International.... linked to NASA Artemis project?!? It is not even mentioned in wikipedia $\endgroup$
    – jumpjack
    Sep 26, 2020 at 9:20
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I found different specifications for the orbiter than those quoted from Artemis Project: max takeoff weight of 240,000 lbs (109 metric tons), and useful payload of 55,025 lbs (25 metric tons).

If we fill the payload bay with fuel for the Shuttle's orbital maneuvering thrusters, and apply the rocket equation: \begin{equation}\Delta v = v_e \ln\left(\frac{m_0}{m_f}\right)\end{equation}

Using $v_e$ of 3096 m/s (derived from the 316 sec Isp of the OMS rockets), we get about 800 m/s additional ∆v provided from the fuel in the payload bay. It takes about 4100 m/s to enter lunar orbit, and another ~900 m/s to return (assuming we can use aerobraking to slow down on arrival) so we're far short of what's needed.

The main engines on the orbiter are somewhat more efficient by mass, so 1150 m/s might be possible there, but hydrogen fuel for the SSMEs is much less dense than the fuel needed by the OMS, so unlikely to physically fit in the payload bay -- and that's still far short of the required ∆v.

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    $\begingroup$ There was an "OMS Payload Bay Kit" planned that would give a considerable amount of extra delta v. It was a pallet with fuel, oxidizer, and helium tanks. Never built or flown. wired.com/2012/03/… As late as the 90s Columbia still had circuit breakers for it in the middeck. $\endgroup$
    – Organic Marble
    May 14, 2017 at 14:57
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    $\begingroup$ Shuttle-centaur was getting close to an actual flight mission. From there, it wouldn't be too hard to route the fuel lines from a two-engine centaur into two of the three SSMEs provided we took the engines off the centaur. But you'd probably do better deploying the Centaur and leaving the shuttle in low orbit even accounting for the extra burn to get back to low earth orbit. But Centuar isn't storable. Oh wait bad idea. $\endgroup$
    – Joshua
    Jun 17, 2019 at 1:04
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    $\begingroup$ @Joshua Centaur G could get maybe 16 tons of payload to a lunar flyby, but not into orbit or back, because of propellant boiloff. $\endgroup$
    – Russell Borogove
    Jun 17, 2019 at 1:08
  • $\begingroup$ It's frustrating that all the high-performance liquid propellants dislike so much to be liquids. It's enough to make you crave a nuclear thermal rocket with a refrigerator. $\endgroup$
    – ikrase
    Jan 30, 2020 at 11:21
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Late to the party but I just watched a video by Scott Manley from August 2020 discussing the exact question, in the context of the TV Series "For All Mankind" which features Shuttles outside LEO.

Manley's tenor is that it's not principally impossible to reach the moon in a Shuttle, just impractical; the shuttle simply is not the right vehicle for the job, with wings and wheels and stuff.

In order to make that possible one would want to use the main engines, which should in principle be able to restart in zero g, after an ullage burn (thanks to OrganicMarble for explaining a video to me that he didn't even watch!) by the orbital maneuvering system (OMS) if the tanks are pressurized and electricity is available.

As others have pointed out, there is a lot of delta-v involved. For that, you'd need more hydrogen and oxygen than fits the cargo bay. Manley suggests to bring the external tank to LEO instead of ditching it at 60 km and refuel it there with hundreds of tons of fuel from some handwave source, and then fly it to the moon (into an orbit, that is). The nice thing is that you have enough spare room to bring reserves for evaporation etc.

The largest obstacle also in Manley's opinion is the high energy at re-entry into Earth's atmosphere. Re-entry would have to be done without the tank, so the main engines cannot be used for an entry burn. Manley does not discuss using the OMS thrusters for that; maybe they don't provide enough thrust to sufficiently decelerate the vehicle.

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    $\begingroup$ This makes no sense at all "In order to make that possible one would want to use the main engines, which should in principle be able to restart in zero g if the tanks are pressurized and electricity is generated by the OMS thrusters." Electricity generated by the OMS??? $\endgroup$
    – Organic Marble
    May 19, 2021 at 20:27
  • $\begingroup$ @OrganicMarble I probably misunderstood that passage from the video (which starts here); sorry for the confusion. Manley mentions pressure, electricity and OMS in close proximity, and I admit that I didn't understand a word he used ("knowledge burn from the OMS thruster"?) It's principally not inconceivable that a thruster could drive a generator turbine on the side; apparently it doesn't. Was electricity generated at all on board the Shuttle? $\endgroup$
    – Peter - Reinstate Monica
    May 19, 2021 at 21:19
  • $\begingroup$ Yes, it was generated by fuel cells. I greatly dislike Manley and his videos so I won't be watching that. The OMS were "simple" hypergolic rocket engines. No electricity generation. $\endgroup$
    – Organic Marble
    May 19, 2021 at 21:19
  • $\begingroup$ @OrganicMarble OK, sorry to hear. I am curious because obviously you are one of the most knowledgeable contributors here, often with first-hand information. Was there a specific incident that triggered your aversion? $\endgroup$
    – Peter - Reinstate Monica
    May 19, 2021 at 21:22
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    $\begingroup$ @OrganicMarble That makes sense in this context, thanks. $\endgroup$
    – Peter - Reinstate Monica
    May 20, 2021 at 7:17

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