Pondering LEO to Moon

Date: Oct 4, 2004

Pondering LEO to Moon

Date: Oct 5, 2004

Good idea!

However Wouldnt it be simpler just to use the shuttle launches to put a combned lander/space ship in leo along with extra fuel and send it off to the moon on its own?

I suspect one would need a lot less fuel that way. (though I can see that the multiple dockings necesarry would be interesting)

Ill have to think more about a mission profile

Dusty

Date: Oct 5, 2004

However Wouldnt it be simpler just to use the shuttle launches to put a combned lander/space ship in leo along with extra fuel and send it off to the moon on its own?

In fact it would. But it would have two disadantages:

1. It would take time to develop a orbital module with capabilities similar to the Shuttle.

2. Development adds expense. Using the shuttle as a lunar orbiter would probably be cheaper for something like one mission a year.

Obviously, if we wanted to make missions to the moon common, then we really need to build a moon taxi. If we have a moon taxi, then it makes sense to have a space station to dock it with for staging. If we wait for a space station, then we get the ISS and go nowhere.

With the plan I'm considering, development consists of nothing more than a few fuel tanks, stripped down engines (possibily), and interconnects. The lunar module could come out of existing designs. Given that the thrust doesn't even need to be very high, the tank/rockets only have to hold the lightest of loads. As long as there's a rigid beam to press against the rear of shuttle, tethers could be more than sufficient for holding the mated craft together. Magnetic interlocks are another option.

It's an idea, anyway.

Date: Oct 5, 2004

It could be done. Infact, the shuttle, early on in the program, did have the capability to use extra OMS (Orbital Menuvering System) Propellant carried as a pallet or kit that was carried in the payload bay. Supposedly, this would have allowed the orbiter access to 600 mi high orbits. Using much larger external kits would of course allow it access to much higher orbits.

The problem isn't so much getting there, it's getting back. How do you propose to slow the orbiter down after its lunar mission? Do you use it's wings and thermal protection system and do a direct reentry? Nobody has tried a reentry using a lifting body at that speed. When the shuttle reenters after a LEO mission, it is only going about 4.9 to 5.2 miles per second. A lunar mission will create reentry conditions more severe: entry velocities very close to escape velocity of nearly 7 mi per second. Doesn't sound like much, does it? Only a 50% increase in speed, right? True, but that 50% increase in speed results in (7/5)^2 more kinetic energy! This is double the kinetic energy of a standard reentry! This higher kinetic energy will result in higher reentry temperatures: returning Apollo spacecraft had to survive heating on the order of five to six thousand degrees F!

So either you modifiy the shuttle and TPS to survive the reetnry heating, or you take the delta-v penalty, bite the bullet, and carry enough propellant to slow it down by retro thrust. Now you've got to carry a whole lot of propellant to do that--so there goes any payload you may have wanted to take.

Sure you can orbit an ET with an orbiter still attached. Given enough time, and a solar powered cryogen refrigeration plant to prevent boiloff, you could even completely refill an ET in orbit. It would take a lot of ground launches--dozens atleast, probably more like 30, to fill an orbitting ET with cryogen. You can easily fly an orbiter/ET to the moon--and you might even have enough to get back. Such a mission would cost--just from the logistic support launch cost point of view--15-20 billion dollars.

Or you can take about half that money, develop a new Heavy Lift booster, and shoot the whole thing to the moon directly. There are reasons why the Apollo-Saturn 5 took the configuration that it has. From an energetics point of view, Low Earth Orbit Reundevue is expesnsive--and Lunar Orbit Reundevue is the least expensive in terms of mass. As long as you are limited to chemical propellants, you will have this probelm. When you go nuclear thermal--new avenues are opened, but conventional propellants are pretty limited in what can be done from an energetics point of view.

Date: Oct 5, 2004

I must say, I was pondering the issue of delta V requirements. Apollo (as we all know) came in hot and used aerobraking (what a benign sounding name for the "fairground ride from hell"!)

whether we consider my sceme or the originall, both are likly to have large delta V and consiquent heavy fuel requirements

I was just pondering the relative virtues of taking an additional 100 tons of wings, landing gear, aerodynamic controll surfaces etc for a half million mile trip to the moon and back over that of, maybe, developing a shuttle launched moon taxi perhaps.

BTW how much Delta V would we need?

eg from LEO parking orbit to Lunar parking orbit and back again into LEO plus of course the trip from lunar orbit (LLO) to the surface and back again!

Coukd this all be done with one capsule?

I am thinking in terms of a single capsule acting as both lander and transit accomodation, it would be combined with a lunar lander stage which would be left on the surface like with apollo the two of which would in turn be attached to a drive stage (one or two stage ) that would take the ship from LEO to LLO and back again. the drive stage would be unmanned and left in lunar orbit during the surface mission. the crew would have to re-aquire the drive stage before returning home

Dusty

Date: Oct 5, 2004

I was kind of hoping for retro-propellant, as opposed to aerobraking. As you said, aerobraking would put an unknown amount of stress on the Shuttle. Now the Apollo mission used a Delta-V that allowed it to reach the moon in 4 days. But the (very cushy) Endevour orbiter is outfitted with EDO (Extended Duration Orbiter) equipment. This allows it to fly for ~16 days, possibly longer. If we were to consider a mission that took 7 days to LLO, 2 days on the moon, and 7 days to LEO and reentry, what kind of Delta V would be required?

The only info I can find shows a delta V of 4.4 km/s from LEO to LLO. Yet (as I understand it) Delta-V is a function of how fast you want to get somewhere. More Delta V == a faster trip to the moon. Less Delta V == a slower trip. But since we have already achieved escape velocity, we'll still get there even if we're throwing things overboard as propulsion. (Actually, the air resistance would overcome this minor boost, but who's counting?

)

(Here's the calculator I found. Either I don't know how to use it, or it's simply wrong. It seems to give me a very low ∆v for some crazily high ratios. e.g. 100,000,000 kg full mass, 104,300 dry mass, and an ISP of 450 only gives a ∆v of 3,089 m/s. Considering that the shuttle has a launch mass of 2 kilotons and acheives a ∆v of 7-9 km/s, that just doesn't seem right.)

Assuming that it would take too much fuel to boost the retro propellent up with the shuttle, what about staging the retro propellant in LLO? Just like the tanks that are launched into LEO, we could launch tanks to LLO a year or so ahead of time. With a relatively low ∆v, the tanks could reach LLO shortly before the mission launches.

Date: Oct 5, 2004

my understanding is that "Delta V" represents the space crafts ability to change velocity. it is not time dependant as such, a low thrust for a long time may produce the same delta V as a high thrust for a short time.

This velocity change ability is determined by the rocket equation though my math isnt good enough to know how to modify the rocket eqation to allow for launches (when some of the thrust is being used to support the dead weight of the rocket rather than for increacing its velocity-maybe I should work on it! I used to be quite good at maths!)

The velocity change is also vector dependent. Changing from an equtorial orbit to a polar one would involve zero "speed" change but would require an enormous delta V (I think rather more than you would need for a ground launch in the first place!)

For a delta V of 4.4Km/s for LEO to LLO would sugest that we would need a similar Delta V capability to return to LEO if we are using engine breaking ie arround 9Km/s overall.! (at this point note we havnt included the delta V requirements for the lander stage!)

To put this in perspective this is roughly the same as the mission delta V that a shuttle, At launch, weighing over 2000 tons has!! Obviously (for the reason mentioned earlier re launch modification for rocket equation) a launch ready 2000 ton shuttle already in orbit would achieve better performance but...

(At this point my head is beginning to spin, am I talking BS or am I correct! the math is frightening! perhaps LEO to LLO isnt as much as 4.4 KM/S, Perhaps that is the "round trip" delta V requirement which would make things easier. But it would still be hard!!)

I think we are beginning to see that a return trip to the Moon is very VERY hard! However you do it!

It was one hell of an achievement!!

I think, we can also see that sending 100 tons of shuttle to the moon and back is going to take as much fuel as launching it in the first place! (Unless im talking crap!)

Moon taxi it is then!

Dusty

Date: Oct 5, 2004

my understanding is that "Delta V" represents the space crafts ability to change velocity. it is not time dependant as such, a low thrust for a long time may produce the same delta V as a high thrust for a short time.

Your understanding is the same as mine. The only difference is that I understood that when you're talking about destinations, you're talking about how fast you want to get there. i.e. If I want to get to the moon in two days, I need a higher delta-V (thus velocity) than if I wanted to get there in a week.

I did some checking and perhaps what I'm thinking about is that there are a variety of ways to produce an orbital transfer. One is to spiral outward toward your target. This takes the most fuel. Another is a Hohmann transfer which involves lengthing your orbit into a parabola and allow your apogee to intersect with that of your target.

Still another method is the Interplanetary Highway, which has extremely low Delta-V requirements. I don't know if it's applicable to a LEO/LLO transfer, however.

It may be possible that I'm confused and that the minimal energy to reach the moon is indeed 4.4 km/s. If that's the case, then we need a lot of booster tanks to move the shuttle into that sort of orbit and back again.

Two other comments:

1. This page is of interest to this discussion. However, I'm not looking for efficiency; I'm looking for cost. With politics what it is, sometime the less efficient solution is the cheaper one.

2. Does anyone have a spreadsheet or a program that I can download to properly calculate delta-Vs and/or orbital transfers? I'm still learning all these numbers, so I'm hesitant to develop my own.

Thanks!

Date: Oct 6, 2004

Yes, to a certain extent it is true that the duration of the voyage is dependent upon the speed. However, when we are talking about chemical rockets and the delta-V needed to go from LEO to LLO, we are usually talking about the minimum amount of kinetic energy needed to place the space craft on a Trans-Lunar-Trajectory. The kind of trajectory is called a Hohman transfer orbit: this is an elliptical orbit whos apogee extends to the moon's 'sphere of influence,' i.e., where the moon's gravity becomes dominant. A TLI, Trans-Lunar-Insertion burn is needed to reshape the spacecraft trajectory so that it becomes circularized around the moon. This represents the minimum energy requirements for achieving the mission. More on this later--I have to leave and take my daughter to school! Bye.

Date: Oct 6, 2004

Based on our discussion here, I've done up a spreadsheet with some figures. It was actually easier than I thought after I realized that I didn't need the ground launch ∆v calculations. (Where burn time is an important consideration.)

The sheet is divided up into two tabs: one for a one way space shuttle mission and one for a one way CSM/LEM mission. The calculations start with the required ∆v, and calculate from there. Exhaust velocity is derived from an Isp of 450. The weight of the external tanks/boosters is nothing more than a wild ass guess. I looked at the shuttle's external tank and found the weight ratio to be about 0.04. Since the Mass Ratio for this propulsion method is 1.0, I took the total missions mass, multiplied it by .04, then took a number four times as large. Thus I expect that the tank/engines mass is actually too large.

You can download the calculator at http://iambatman.homeip.net/Moon Shot Costs.xls.

Feel free to play with the numbers.

A few things became quite obvious, though:

1. LHOx is the ONLY way to go for a moon shot. Kerosine engines would simply be too costly.
2. The shuttle moon shot is doable, but the costs would be at least 3 billion dollars.
3. If we reconstructed *existing* lunar module designs, we could probably launch a mission for ~1-2 billion in hardware.

The spreadsheet doesn't take into account all the costs, only the final costs of using Delta II rockets for launching the matable tanks/engines. I assume that one shuttle launch would be necessary in both cases. i.e. Either the shuttle goes up and makes the trip itself, or the shuttle takes the CSM, LEM, and passengers up. Either way, we can assume that cost is fixed at about $500 million.

Cheaper missions may be possible if the tanks/engines were launched on a cheaper $/lb solution. I have no figures for the Delta-III or IV launch vehicles, so those may make sense. However, once the Falcon Vs are ready, the cost of a mission may drop substantially.

Thoughts?

Date: Oct 6, 2004

Looks like you've got the basic idea. I once looked at the kinds of engines that an already established lunar colony might use in the future. There was a Ben Bova novel (the name escapes me) about a time in the future in which the Russian dominated space travel, and Russians had established a lunar colony in which atleast some of it was used as a lunar gulag (which is totally ridiculous considering the transportation costs.) Anyways, one of the things mentioned was a lunar orbit shuttle/ballistic lander that burned aluminum dust with liquid oxygen. Both are materials readily derived from the lunar crust, given sufficient energy that is. The advantage of such a system would be mainly that a rocket engine would not need to use the precious hydrogen that was (then, before the discovery of lunar ice) not easily come by.

Doing a little figuring I came to the conclusion that such an engine would be unlike anything ever done before. You would have to burn the aluminum with excess oxygen, otherwise only molten aluminum oxide would form and there would be no gasses to expand out of a nozzle. So hot oxygen gas must be working the fluid.

Well, as it turns out, oxygen gas with hot aluminum oxide particles is pretty similar in many respects to the exhaust products of a solid propellant rocket engine using aluminum dust and ammonium perchlorate. Some of the parameters (the value of k: the ration of specific heat at constant pressure to specific heat at constant volume--a critical paramater in the design of rocket engines...) are nearly identical to a conventional solid rocket motor. Performance calculations suggested that at a moderate 300psi chamber pressure a modest specific impulse of 150 seconds was achievable.

So then the hard part came: how to you fuel an engine that burns metal with oxygen, and has a flame as hot and oxidizing as a cutting torch?

I came up with the idea of storing the fuel as a wire: mechanically injecting wire--something analogous to a wire feed arc welder--fed from large spools. The wire is fed to the combustion chamber and burned with streams of hot oxygen gas. The hot oxygen gas comes from cooling the combustion and thrust chamber with liquid oxygen. The material for the combustion chamber was a difficult pick: I chose a titanium thrust chamber with a thick ceramic-fiber liner--the liner could even be a thorium oxide/aluminum oxide fiber matrix. I suppose it could work--I never actually built a prototype, but I came to the conclusion that, although clunky and convoluted, it might be just possible to fly such a contraption.

Date: Oct 6, 2004

I came up with the idea of storing the fuel as a wire: mechanically injecting wire--something analogous to a wire feed arc welder--fed from large spools. The wire is fed to the combustion chamber and burned with streams of hot oxygen gas. The hot oxygen gas comes from cooling the combustion and thrust chamber with liquid oxygen. The material for the combustion chamber was a difficult pick: I chose a titanium thrust chamber with a thick ceramic-fiber liner--the liner could even be a thorium oxide/aluminum oxide fiber matrix.

Sounds complicated. And isn't most of the Aluminum on the moon already oxidized?

It's just a thought, but wouldn't ships with banks of Guass guns work better? Potentially, small aluminum grains or spheres could be accelerated out of the ship at rates far exceeding the velocities obtained by burning the wire. Using banks of rapid-fire guass cannons might even make the thrust comparable to the alumnium wire concept. At the very least, the guass cannons could thrust for a bit longer and acheieve the same (or better) ∆v.

The powerplant might be a problem, however. Options would include large banks of solar panels (heavy) or a small (externally mounted?) nuclear generator. Maybe we can swipe one off the Prometheus.

Date: Oct 6, 2004

Interesting that you came up with the same idea for aluminum (wire) combustion, as others have for boron fiber combustion -- as a better (certainly safer) alternative to hydrogen/oxygen fuel cells for automotive power -- see http://www.eagle.ca/~gcowan/boron_blast.html

As for lunar surface shuttles (particularly cargo-only types), I don't think anything beats Zuppero's steam NTR concept. See http://www.neofuel.com/

Date: Oct 6, 2004

Actually, all of the aluminum on the moon is oxidized. I pondered the idea after assuming the moon was already colonized with a healthy mining infrastructure. It was an idea for creating rocket propulsion without necessarily the need for hydrogen, water, or other organic fuels. Only the aluminum--which has a heat of combustion of nearly 20000 Btu per pound--which puts it at slightly higher heat of combustion (atleast on a weight basis) than gasoline and rp-1, but less than half as much for hydrogen which has a lower heat of combustion of nearly 55000 Btu per pound.

Yes, the engine concept was complicated--what, for example, do you do if one of your wire injectors jams? It's not an easy idea to create. I even looked at using aluminum beads and looked at using a powder and premixxing this with liquid oxygen to create a slurry. However the liquid slurry is just asking for a flash through which would end your rocket trip real quick!

Interestingly, when I was in highschool back in about 1986, the aluminum/oxygen rocket motor idea gave me an idea for a very exotic thermochemical fuel cell which uses aluminum dissovled in mercury as the fuel and uses oxygen for the oxidizer. Aluminum is very soluble in mercury, and the advantage here is that the aluminum oxide that forms floats to the top of the mercury bath where it can be scraped off and dumped on the ground. A fuel cell powered in this way could achieve nearly two volts per cell. This was an idea I had for powering a long range rover on the moon (also assuming of course that lunar mining infrastructure was already in place.) Again, it is complicated and would most likely fail soon after starting because of the accumulation of oxides on one or both electrodes. But it was exciting to be thinking about something no one had tried before....

As for using mass drivers for propulsion---I think it could be done. I had looked at what the requirements would be to shove an asteroid around (using asteroidal regolith as a 'propellant.') It takes a while, but even with a relatively modest energy supplies (I assumed a small nuclear fission reactor, no bigger than about 10MWe--certainly big by space standards!) Given time even realtively large delta-v's can be achieved, although if you want to keep most of your rock you must achieve an exhaust velocity of greater than 10km/s. 10km/s is roughly equal to about 1100 seconds of specific impulse, or about what a good NTR using hydrogen for propellant can achieve. Any lower than that, and you start to use ALL of your asteroid as propellant.

Incidently, what happens to all of those 'exhaust' pellets that are ejected anyway? The bad idea about using mass drivers is that unless you can achieve solar escape velocity (say close to 50km/s) then you're most likely going to create some serious navigation hazards by dumping millions of tons of artificial meteoroids in wildly chaotic orbits about the sun! You can see that even moving a few dozen asteroids will dump almost half of their collective masses into creating belts of chaotically shifting debris. Given enough traffic, we'll end up creating a mine field in space!

You can argue that space is big--true, the chances of getting whacked is pretty small. However, what happens when after a century or so we've dumped billions of these things all around. What then? Are the chances of getting whacked 10% now? Given enough trips back and forth, a collision will be inevitable. Something to ponder!

I myself have liked the idea of VASIMIR. A nuclear/electric VASIMIR engine could be tuned to use different propellants. It could use oxygen (derived from lunar regolith sent into orbit about the moon or L4 or L5) to fly out to an asteroid. So what if it takes 10000 tons of oxygen to do it? It arrives at the asteroid, sets up its mining operations. Maybe it extracts water, hydrogen, ammonia and other volatiles. Maybe some precious metals like gold and PGM's. It uses asteroid hydrogen for a higher energy return burn and brings back to our L4 or L5 base several kilotons of volitiles.
It loads up on more oxygen propellant (derived from metal extraction from lunar regolith) and does it again. No belts of ballistic projectiles whirling around the sun, only the natural objects (and some human-made space junk) to hit.

Anyways, just an idea or two....