shuttle to the moon?

[mikejohnson]

So why couldnt one of the shuttles be rigged up to orbit the moon? it certainly had room in the cargo bay for a lander.

[RAF]

Not feasable for many reasons... including not being able to slow down enough to return to Earth.

[Chew]

Fuel. If the entire payload were fuel tanks and piped to the OMS it could only raise apogee to 4200 miles.

[cjameshuff]

So why couldnt one of the shuttles be rigged up to orbit the moon? it certainly had room in the cargo bay for a lander.

The Shuttles didn't carry anywhere near sufficient orbital maneuvering propellant to reach the moon, particularly with a full cargo...made worse by the main engines, wings, landing gear, enclosed cargo bay, and a bunch of other structure that would be dead weight for such a journey. It would also be the last trip the Shuttle made, as its tiles and wings couldn't withstand a high-speed reentry from a lunar trajectory. After the Shuttle Orbiter reached low orbit, you'd need a bunch more launches to add propellant tanks and other equipment, and at that point there's little benefit to having the Orbiter involved...it's about 70 tonnes that you could fill with a much more useful vehicle.

[Chew]

So why couldnt one of the shuttles be rigged up to orbit the moon? it certainly had room in the cargo bay for a lander.

The Shuttles didn't carry anywhere near sufficient orbital maneuvering propellant to reach the moon, particularly with a full cargo...made worse by the main engines, wings, landing gear, enclosed cargo bay, and a bunch of other structure that would be dead weight for such a journey. It would also be the last trip the Shuttle made, as its tiles and wings couldn't withstand a high-speed reentry from a lunar trajectory. After the Shuttle Orbiter reached low orbit, you'd need a bunch more launches to add propellant tanks and other equipment, and at that point there's little benefit to having the Orbiter involved...it's about 70 tonnes that you could fill with a much more useful vehicle.

I remember reading it would take 19 shuttle launches to lift to low Earth orbit the necessary fuel and external tank to get one shuttle to the Moon, slow down to orbit it, return to Earth, and get back into Earth orbit.

[Jason Thompson]

So why couldnt one of the shuttles be rigged up to orbit the moon? it certainly had room in the cargo bay for a lander.

Take a look at the shuttle on the pad. It needs all the fuel in the external tank plus both solid rocket boosters plus a little kick from the onboard manoeuvring system to reach the 17,500 mph it needs to achieve to go into Earth orbit. To reach the Moon it would have to achieve a speed of 25,000 mph, and it has nowhere near enough fuel left to get that extra 7,500 mph push.

But that's only the first issue. It not only needs enough fuel to do that, it needs more fuel to brake into lunar orbit and then break out of lunar orbit once the mission is over. There are two big problems with this need for extra fuel. The first is that there's nowhere left to put it. The external tank is empty and gone. The SRBs are spent and discarded. The main engines (SSMEs) are the only engines with enough power to affect these changes in velocity, and they are fueled from the external tank. You'd have to hook up the shuttle to another fuel supply. The shuttle orbiter is designed to be hooked up to the external tank by technicians in an assembly building, not by a suited astronaut on a spacewalk. Even if you could hook up another tank of fuel, the SSMEs are not designed for restart capability. They fire once, then they have to be stripped down and re-worked before they can be fired again. You could use solid rockets instead, but there are no attach points on the shuttle for such things. Even if there were some added, solid rocket boosters are not the best solution for a man-rated vehicle in deep space due to their 'turn them on and they burn till they're done' nature. Any mistake would be disastrous.

And then even supposing the shuttle did go to and from the Moon, it would then have the problem of encountering Earth's atmosphere at about 25,000 mph after the translunar coast rather than the 17,500 mph from Earth orbit. Not only is the shuttle's thermal protection system not rated for the level of heating this would cause, the shuttle's structural assembly is not built to withstand the aerodynamic forces involved at those re-entry speeds.

So, by the time you have re-engineered the shuttle to give it restartable engines, the ability to be connected to a new fuel tank in orbit, and the structural strength and thermal protection it would need to re-enter Earth's atmosphere at translunar speeds, and constructed the systems needed to put the extra fuel tank up there for it to connect to, you might just as well have spent the cash on a purpose-built lunar spacecraft system. Re-working the shuttle, a vehicle designed solely for transport to and from low Earth orbit, to go to the Moon is like re-working an articulated lorry to make it amphibious so it could do overseas delivery of freight. Time consuming, expensive and not the best use of resources.

[mikejohnson]

That makes sense , not enough fuel , wrong engines. but wouldnt they just establish earth orbit and then rentry?  thanks

[cjameshuff]

That makes sense , not enough fuel , wrong engines. but wouldnt they just establish earth orbit and then rentry?  thanks

The only way to do that without aerobraking (which the wings and tiles can't withstand from a lunar trajectory) is to use yet more propellant, which has to be launched and carried to the moon and back.

[Jason Thompson]

That makes sense , not enough fuel , wrong engines. but wouldnt they just establish earth orbit and then rentry?  thanks

Again, slowing down to Earth orbit requires about the same amount of propellant as you needed to get out of it in the first place. Since you now need even more fuel you have more mass and again you need more fuel to affect the changes in velocity needed.

[Bob B.]

In round numbers, we require a Δv of about 3 km/s the to get to the Moon, about 1 km/s to enter lunar orbit, another 1 km/s to break out of lunar orbit, and another 3 km/s to reenter Earth orbit.  That's a total of 8 km/s.  If we use the Space Shuttle's LOX-hydrogen main engines, then mass ratio needed to attain that 8 km/s is about 6.  Mass ratio is the fully fueled mass of the space vehicle divided by its empty mass.  A mass ratio of 6 means that for every 1 kg of dry mass, 5 kg of propellant is needed.

Each shuttle orbiter had a mass of about 80 metric tons, plus each could hold about 25 tons in payload.  Let's add another 30 tons for the additional tankage (the mass of the external tank) and we're up to a dry mass of 135 metric tons.  Therefore, we'd have to launch about 675 metric tons of propellant to carry out the mission.

Edit:  corrected math mistake

[smartcooky]

And then even supposing the shuttle did go to and from the Moon, it would then have the problem of encountering Earth's atmosphere at about 25,000 mph after the translunar coast rather than the 17,500 mph from Earth orbit. Not only is the shuttle's thermal protection system not rated for the level of heating this would cause, the shuttle's structural assembly is not built to withstand the aerodynamic forces involved at those re-entry speeds.

Well, of course it wouldn't necessarily have to re-enter at 25,000 mph. They could carry out an EOI burn to put them back into the usual 17,500 mph Orbiter orbit, but that would require shitloads of extra fuel for the EOI burn, and that extra fuel would require more extra fuel to be carried from the start to carry the extra weight.

[Jason Thompson]

Well, of course it wouldn't necessarily have to re-enter at 25,000 mph. They could carry out an EOI burn to put them back into the usual 17,500 mph Orbiter orbit, but that would require shitloads of extra fuel for the EOI burn, and that extra fuel would require more extra fuel to be carried from the start to carry the extra weight.

Um, see reply #8 by me just above...

[Echnaton]

All of this illustrates that missions to the moon have to be designed under the constraint of returning to earth a space craft of the smallest possible mass.  That is not the Shuttle but could be something like, maybe, the Apollo missions which could shed no longer needed dry mass from ~160 seconds after launch to just before reentry.

[smartcooky]

Well, of course it wouldn't necessarily have to re-enter at 25,000 mph. They could carry out an EOI burn to put them back into the usual 17,500 mph Orbiter orbit, but that would require shitloads of extra fuel for the EOI burn, and that extra fuel would require more extra fuel to be carried from the start to carry the extra weight.

Um, see reply #8 by me just above...

Missed that. I just wanted to kick off a reply before heading to work. My bad!

But just one other question since you seem to understand this stuff better than most:

To achieve TEI requires enough velocity to escape lunar orbit, right? Well surely, that isn't going to be 25,000 mph. Lunar escape velocity is 2.4 km/s which translates to about 5,400 mph. Is the other 20,000 mph picked up during the coast phase due to the gravitational pull of the earth, or are we back to the scenario discussed earlier in another thread where they had to get the "safest but quickest" trajectory back to Earth to avoid exposure to the VARB, and to conserve life-support consumables on board?

[ka9q]

Yes, that very high velocity of the returning Apollo spacecraft came from falling into the earth's gravity well. Once Apollo was away from the moon, which has a mass only 1.2% that of the earth, it's as if the moon weren't there at all. Now they were back in the same sort of highly elliptical earth orbit they used to get to the moon. On their way out their velocity dropped enormously as they climbed out of the earth's gravitational well, and on their way back they got all that velocity back. So they hit the earth with roughly the same velocity they had at the end of TLI.