shuttle to the moon?

[Chew]

I recall running the numbers years ago and that between 1.5 hours before re-entry and re-entry the speed of the spacecraft doubled. It's vice versa for TLI. Bob's Apollo 11 TLI sim has the exact numbers; they can roughly be used in reverse.

"What goes up, must come down" can be appended to read "at the same speed when at the same distance regardless whether going up or coming down."

[ka9q]

I was disappointed that Constellation didn't propose any improvements over Apollo that threw away so much hardware during each flight.

I think the emphasis should be on reusing spacecraft instead of launchers (e.g., the shuttle). Yeah, it's expensive to throw away a launcher after a single flight into orbit, but it's even more expensive to keep sending spacecraft all the way to the moon and throwing them away after a single use just because the tanks are empty.

It's not the cost of the spacecraft themselves so much as the cost of getting them up there that I'm trying to save through reuse.

What we need is one spacecraft that can repeatedly go (I won't say "shuttle") between lunar orbit and the surface, and another that can repeatedly go between lunar orbit and earth orbit. If propellant can be made from lunar materials, it could be much cheaper than sending propellant up from the bottom of earth's deep gravity well.

Since we want to return the earth-moon "shuttle" to low earth orbit to avoid having to launch it again, it can't use an Apollo-style direct return. Aerobraking seems like the only feasible answer; it would target the atmosphere at an altitude that would slow it into an elliptical orbit rather than capturing it completely. Additional maneuvers and braking passes could then produce a circular low earth orbit with relatively little fuel expenditure, and the crew would then transfer into a conventional earth orbital spacecraft for the return to earth. The number of aerobraking passes required would depend on the tolerance of the heatshield and that of the crew to additional radiation exposure and time in space.

The lunar landing vehicle could be launched from earth without a crew and take a leisurely low-energy path via the earth-sun L1 point into lunar orbit, where it would await the first crew to arrive on the orbit-to-orbit vehicle.

In fact, that method could be used even with an Apollo-style (one-use) LM to reduce the total launch costs of a lunar mission; call it the "double LOR" method. The CSM would enter lunar orbit alone and rendezvous and dock with the waiting LM before landing, then rendezvous and dock again with the ascent stage after the surface crew returns.

[smartcooky]

In fact, that method could be used even with an Apollo-style (one-use) LM to reduce the total launch costs of a lunar mission; call it the "double LOR" method. The CSM would enter lunar orbit alone and rendezvous and dock with the waiting LM before landing, then rendezvous and dock again with the ascent stage after the surface crew returns.

Would it be possible, given the low gravity of the Moon to build a bigger, stronger Lunar Descent/Ascent Vehicle (LDAV) that carries down with it enough fuel (in the form of an exchangeable fuel module) to launch the entire LDAV back into orbit?

The missions would look something like this
1. The crewless LDAV is sent on the slow trajectory via Earth-Sun L1 as you suggest.

2. The first crew arrive in the Earth Return Vehicle (ERV) dock with and transfer to the LDAV. They carry out their surface mission, then lift off and dock with the ERV, transferring to make the trip home. After ejecting the fuel module, the LDAV is left in lunar orbit.

3. Prior to each subsequent mission, a fresh fuel module for the LDAV is launched on the slow trajectory via Earth-Sun L1 and inserted into a similar lunar orbit to the LDAV.
 
4. On arrival of each subsequent mission, the crew capture the exchange fuel module and install it in the LDAV before proceeding with their surface mission.

The ERV and LDAV are reused - only the fuel modules and the earth launchers for them and the ERV are disposed of, and the launchers can be much smaller if they are only launching fuel tanks and the ERV.

Perhaps a single type of launcher could be designed to accomplish both jobs?

Does this sound feasible? or practical? Or, am I missing some really obvious, insurmountable flaw?

[Donnie B.]

One problem would be making engines that were reusable (many times) without servicing. 

Another issue is that the fuel itself makes up most of the launch mass of an Apollo-style mission.  I'm not sure you'd get all that much savings with your profile, since all the fuel still needs to be lifted out of Earth's gravity well.

[cjameshuff]

Another issue is that the fuel itself makes up most of the launch mass of an Apollo-style mission.  I'm not sure you'd get all that much savings with your profile, since all the fuel still needs to be lifted out of Earth's gravity well.

The propellant does, at least initially, but the rest of the vehicle can stay in lunar orbit. Consider sending a tanker and a multiuse lander, and conducting multiple expeditions to the surface during a single mission, refueling after each. Then park the lander into some stable orbit, and send another tanker when you do another mission. And when you make use of propellant from off-planet sources (LOX/CH4 from polar ices, for example), you drastically change the situation.

[raven]

Mass concentrations make long term stable orbits around the moon difficult, I think. Still, the GRAIL satellites gravitational mapping will hopefully help solve this.

[Jason Thompson]

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

No problem.

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

I wouldn't say that. I've picked up a lot from reading here and elsewhere but there are people here who understand it far better than I do. I'm not an actual rocket scientist.

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,

Others have already answered, but basically yes.

In both TLI and TEI the idea is not so much to escape the gravity of Earth or the moon but to raise the orbital height to a point where the gravity of the other becomes dominant. You then start falling toward the other body. Freefalling from that distance will inevitably result in a speed of about 25,000 mph when you reach Earth.

[ka9q]

Would it be possible, given the low gravity of the Moon to build a bigger, stronger Lunar Descent/Ascent Vehicle (LDAV) that carries down with it enough fuel (in the form of an exchangeable fuel module) to launch the entire LDAV back into orbit?

You've hit on the crucial element in the design of a reusable LDAV, as you call it: the ability to make a round trip. However I was thinking of fueling it on the surface from locally produced propellants and carrying enough into orbit to do a subsequent powered descent.

The buzzword is "in-situ resource utilization" and I think it's the only way we'll ever go anywhere beyond earth orbit. Apollo did absolutely none. It didn't even use solar power, the one in-space resource already in near universal use in the 1960s.

The most plentiful resource on the moon, aside from bulk materials for construction and radiation shielding, is oxygen. Like the earth, the moon's crust is something like half oxygen; all you need is energy to extract it. While you can get plenty of solar power during the lunar day, you still have the problem of staying warm during the long night. I think the only practical near-term solution is nuclear fission because of its extremely good energy-to-weight ratio.

So if you're going to have nuclear reactors for power, why not use them for propulsion too? The raw materials for chemical fuel are rare on the moon, but a nuclear thermal rocket doesn't need fuels and oxidizers; it only needs reaction mass. Hydrogen is ideal but I wouldn't want to squander the moon's limited polar reserves if there alternatives. I wonder if pure oxygen could be used as a propellant without eating up the engine.

[ka9q]

The ERV and LDAV are reused - only the fuel modules and the earth launchers for them and the ERV are disposed of, and the launchers can be much smaller if they are only launching fuel tanks and the ERV.

It's easy to forget since it wasn't visible, but 2/3 of the mass of a loaded LM was propellant. This is why I think lunar propellant propellant will be the only sustainable way; it'll just be too expensive importing it from earth.

[Abaddon]

Another issue is that the fuel itself makes up most of the launch mass of an Apollo-style mission.  I'm not sure you'd get all that much savings with your profile, since all the fuel still needs to be lifted out of Earth's gravity well.

The propellant does, at least initially, but the rest of the vehicle can stay in lunar orbit. Consider sending a tanker and a multiuse lander, and conducting multiple expeditions to the surface during a single mission, refueling after each. Then park the lander into some stable orbit, and send another tanker when you do another mission. And when you make use of propellant from off-planet sources (LOX/CH4 from polar ices, for example), you drastically change the situation.

Well, if we are going to speculate. I would consider a stepping stone approach. Say, establish a significant station at L1. If large enough, maybe this would allow for servicing of the posited lunar lander without requiring a return to Earth for same, and resupply without going all the way to the moon. Return trajectories to Earth from L1 would naturally be lower velocity with commensurate weight savings. Service trips for the reusable lunder (lunar lander) would be less fuel expensive as it only need reach L1. Missions of any type would become many smaller missions, not a single big one. Pure speculation, I have done no math on it, nor intend to. It's just an idea.

ETA: Yes, I can spell, this wee keyboard is my nemesis.

[Chew]

Useless trivia: the difference in velocity, from LEO, needed to get to the Moon (i.e., raise your apogee to the altitude of the Moon) and escape velocity is only about 95 m/s.

[Bob B.]

I wonder if pure oxygen could be used as a propellant without eating up the engine.

Just speaking from a performance perspective, oxygen is a poor propellant due to its high molecular weight.  From what I've been able to research, the nuclear rockets tested back in the 1960s operated at chamber temperatures around 2500 K.  Maybe with modern materials we can operate hotter today, but for the sake or argument, let's assume not.  At 2500 K there's not going to be much dissociation with any of the commonly available propellants, so at a molecular weight of 32, oxygen is a poor performer.

I just ran some sample calculations for a theoretical engine, assuming chamber temperature = 2500 K, chamber pressure = 40 atm, and nozzle exit pressure = 0.1 atm.  The calculations were for three propellants: oxygen, water, and hydrogen.  Calculating specific impulse, here's what I got:

Oxygen:  214 s
Water:  307 s
Hydrogen:  834 s

Oxygen pretty much sucks, water is about equal to hypergolic propellants, and hydrogen is clearly the best.

As a side note, I did a bunch of studies on this a couple years ago to see if I could find an alternative fuel that could yield a good performance without having the huge density penalty associated with liquid hydrogen.  I found that at 2500 K, methane and propane looked to be better than hydrogen.  The specific impulse is only about 75% of hydrogen, but the higher densities mean that the tanks can be dramatically smaller.  The higher mass ratios attainable with methane or propane mean that it's possible to get more Δv out of every kilogram launched.  Of course this is based on typical tank construction.  If super-lightweight tanks can be made from modern composite materials, the the advantage might swing back to hydrogen.  I also found that at higher operating temperatures, where significant dissociation will occur, nothing beats hydrogen.  It's possible to obtain a specific impulse exceeding 1500 s with hydrogen at high temperatures.  Even with the density penalty, no other propellant is going to beat that.

[cjameshuff]

It's easy to forget since it wasn't visible, but 2/3 of the mass of a loaded LM was propellant. This is why I think lunar propellant propellant will be the only sustainable way; it'll just be too expensive importing it from earth.

That still leaves 1/3 being the LM itself. Send two LMs and you've sent enough mass in LMs to refuel one.

The actual ratio's not quite that favorable, because a reusable system can't use the two-stage approach, but there's still a benefit to a reusable vehicle, and big tanks are more efficient for moving propellant around.

So if you're going to have nuclear reactors for power, why not use them for propulsion too? The raw materials for chemical fuel are rare on the moon, but a nuclear thermal rocket doesn't need fuels and oxidizers; it only needs reaction mass. Hydrogen is ideal but I wouldn't want to squander the moon's limited polar reserves if there alternatives. I wonder if pure oxygen could be used as a propellant without eating up the engine.

You can get oxygen anywhere on the surface, but you need some substantial equipment to extract it, and have little in the way of abort options. I favor propellants derived from the polar ices. Yes, there's limited reserves, but if we're doing enough on the moon that we are actually making a dent in those, we can easily use them to send expeditions to more plentiful sources of volatiles elsewhere. Use some of that propellant to snag a few iceballs and park them at L1, and you've more than compensated for what you've extracted from the poles.

And I favor LOX/CH4 for its density, storability, and the fact that you could build simple vehicles from local materials without a massive operation to extract and enrich fissile materials (particularly given the probable lack of concentrated uranium ores).

[raven]

I always thought, if we could get them to work without clogging the engines, aluminium/oxygen could be something. Hydrogen is the best, but while water may be reasonably plentiful on the moon at the poles, aluminium is literally everywhere.

[Bob B.]

And I favor LOX/CH4 ...

Are the materials and processes available to synthesize methane on the Moon?  I've already heard about manufacturing methane on Mars, but my understanding in that the process uses carbon dioxide from the Martian atmosphere.  If you know of a process that will work on the Moon, I'm interested in hearing more about it.