It can't travel slower without burning huge amounts of fuel.

About 3/4 of the way to the moon (176,000 mi or 283,000 km from earth) is the earth/moon gravitational equipoise. Any closer to the earth, and the earth's gravity pulls a stationary object back. Any closer to the moon, and the moon's gravity will pull a stationary object to it.

Figuratively it's like a hill you must coast over. Leaving the earth at 25,000 mph, the spacecraft is constantly slowing. By the time it crosses the earth/moon "gravipoise", (top of the hill) it's only going a few thousand mph. Then the moon attracts it more strongly, and it starts accelerating "downhill" toward the moon.

On the way back to earth the same thing. Even if you cross the gravipoise point at 1 mph, afterward sliding downhill to earth for two days you'll be at about 25000 mph.

The question is what's the cheapest way to slow down? Haul massive amounts of propellant to the moon and back? Or use the earth's atmosphere. Atmospheric braking is by far the lowest cost way (at least for returning to earth).

For a lunar shuttle it's a tough choice. It must shed about 7000 mph (11,200 km/hr) to enter earth orbit. That requires a lot of fuel. However beefing up the lightweight structure to handle aerobraking and adding the aerobraking hardware would also be costly.

Technically speaking aerobraking for orbital entry just sheds some speed, the remainder is shed by rockets. A more advanced technique called aerocapture in theory can achieve orbit with little or no propellant usage:

http://www.inspacepropulsion.com/tec...e_physics.html

Simple, just attach a huge magnet to the front of the shuttle and use big electromagnets on the space stations. They could pull it towards them with an opposite polarity until the shuttle needs to slow down, then they just reverse the polarity so they are the same.

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Enter the World of Athran

[quote="joema"]that's more what i was looking for. makes sense.

trying to imagine how it would work on the mine...regolith braking perhaps? i can't see it, at least not with anything alive on board.

wolf's magnets...lol. that's funny to imagine even if it is impractical. I can remember playing with magnet sets and trying to make them float, seems one was always running off in who knows what direction with no way to control it. The only thing that worked was to have several ring magnets and put a pencil through the middle to hold them.

maybe you should build a big space pencil wolf

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None to speak of

A magnet floating on simple, fixed magnets is inherently unstable, but if you have some diamagnetic material, you can do this:



(From Diamagnetic Levitation with Carbon Graphite. See also Diamagnetic Levitation Using Bismuth.)

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0 1 1 0 1 0 0 1 1 0 0 1 0 1 1 0 1 0 0 1 0 1 1 0 0 1 1 0 1 0 0 1 1 0 0 1 0 1 1 0 0 1 1 0 1 0 0 1 0 1 1 0 1 0 0 1 1 0 0 1 0 1 1 0 ...

i removed the picture for the sake of lowbandwidthees, see above post to see it.

that picture may work but it'd be hard to get a ship into that position if it is traveling that stinkin fast! just move a couple of these puppies a bit closer to earth though...

On a more serious note, sudden acceleration is harder on the body than slow acceleration. As long as you have to be carrying fuel, why not burn it slowly once you're "over the hill" and at least avoid a fast, hard and tricky acceleration at the last second? Imagine it as a celestial letting off the gas when you see a yellow light a block ahead, instead of a head jerking slam on the brakes at the last second. Would that work?

*keep in mind acceleration is any change in velocity, not just the gaining of speed.

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I don't think there are significant gains to burning fuel slower. If you burn it at 1/4 the rate, but produce 1/4th the thrust you must burn 4 times as long for the same delta V. If there were major advantages to burning fuel slower, all space vehicles from the Apollo Service Module to the Space Shuttle OMS engines would have smaller engines and longer burn times.

Regarding the original question of the shuttle reaching the moon, even if the entire payload bay was full of the highest specific impulse propellants known (liquid flourine and liquid lithium, Isp about 500), it still couldn't make it. Those would provide about 1954 meters/sec delta V, but you need 3129 meters/sec to reach escape velocity. Still not enough.

i appreciate rocket scientists.

thanks!

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Have a bit of time on my hands so bring back this thread.

When the Shuttle gets back in opperation, I understand the plan was to continue building the ISS in orbit.

Would it make sense, should they still be intent on returning to the moon, to have the Translunar vehicle built in orbit, and crew transfering to it from a shuttle.

But then we still have the braking issues to contend with as we go from one gravity well to another.

Any further thoughts on that one. If GW wants to go to Mars by way of going back to the moon, (Is that what he said he wanted to do? :-? ) This would need to be sorted out for a permenant moon base.

Apollo style rockets, if they could recover the details for that would be soooo 1960's

Anyone got any ideas?

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Perhaps you are psyhic. I was gunna post about this yesterday and totally forgot.

I don't recall if it was discussed or not, but I was thinking that the best way would be a three stage stage system.

By building a base on the moon (we can send the materials in packets and the crew in a similar way to Apollo to start with) and building up the IIS including Hangers for Earth/Moon Shuttles, we could run a shuttle from the IIS to a lunar orbit. A Lander would then lift off from the moon dock with the shuttle and once it has on board whatever is going down it could undock and land while the shuttle returns to the IIS. Crews on the IIS and Moon would be responsible for keeping the shuttles and landers going, with their construction and maintenance a good step towards building a ship for travel and exploration of Mars. By building Hangers it would allow a craft to be taken inside the IIS and refitted after each run while others where making their runs. The shuttle or its successor would be the means of trnsport to the IIS. The Moon/ISS shuttle could be purpose built so it would not need to suffer from the troubles of trying to get the Space Shuttle to the moon, and so would the Lander at the Moon Base, though it could probably be modelled off the ideas behind the LM or Delta Clipper for the most part.

What do you think? Jay, any major problems I didn't notice?

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It must be fun to lead a life completely unburdened by reality. --- JayUtah

You can't reason an irrational person out of an irrational belief. --- Noclevername

Apollo: The History and the Hoax
Enter the World of Athran

I dont know why instead of building a dull useless spacestation, they didnt build a spaceship to explore the solar system already.

Because (as Robert Heinlein and Larry Niven have both pointed out) in terms of delta v, Earth orbit is half-way to anywhere in the Solar System!

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For those interested, this DVD is in the works and should be very interesting:

http://www.rocket.aero/me163.html

thanks

Dwight
ps no, I'm not affiliated with them

I am definitely NOT a rocket expert but I did work on payload on the Orbital Science's MicroStar satellite (ORBCOMM Block II and MUBL programs). Those satellites used an electromagnetic propulsion system for orbit keeping (a coil embedded in the hull that acted with or against the earth's magnetic field as needed).

Is there any way this could be used to reduce or eliminate the fuel load?

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Is there any way this could be used to reduce or eliminate the fuel load?

Well, possibly. The strength of a magnetic field decreases with distance, so interacting with Earth's magnetic field gives you less thrust magnitude as your distance increases. And one problem with geomagnetic propulsion is that the thrust vector direction is limited.

Orbital trim manuevers can typically be done as low-magnitude manuevers. In addition to the geomagnetic system you describe, Boeing also offers ion propulsion for orbital trim in the 701 satellite chassis. But low-magnitude manuevers are not suitable for all manuevers and mission types. We've shown that low-energy transfers can be done with ion propulsion, but at a tremendous cost in mission duration.

The chief difficulty in operating a shuttle from Earth orbit to lunar orbit and back using an Apollo-type trajectory (i.e., a 3-4 day transfer) is that you require a delta-v of about 13,000 fps on the outbound leg and on the inbound leg: 10,000 fps near Earth and 3,000 fps near the moon. In the pure rocketry solution you must reach Earth parking orbit with a spacecraft capable of 26,000 fps delta-v.

You can shave 10,000 fps off that figure by devising a way to brake into Earth orbit from a return trajectory without using an engine -- perhaps some form of aerobraking. That seems great until you realize that to engineer a spacecraft capable of aerobraking removes many of the advantages of a pure-vacuum spacecraft. What you gain in terms of lower delta-v requirements you lose in structural inefficiency. Historically those problems have been sixes. If field-effect and geomagnetic propulsion could be extended and refined to help effect orbital captures, that would perhaps reduce the fuel loadouts. But the current thrust magnitudes offered by those systems are orders of magnitude too small to have an engineering advantage in trajectories for manned translunar missions.

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Clavius Moon Base

How about magnetic propulsion for deorbiting? No need for so much OMS fuel. Of course, the systems for magnetic propulsion will probably negate the savings.

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I was thinking about it less for manned missions than for transfer of material (things that don't have a consumable issue or time limit). You could tranfer the bulk of the material in this matter and the crew in a smaller ship.

Just a thought.

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It's a good thought. Where possible we should undertake more fuel-efficient methods of transfering materiel, and this means thinking outside the box in terms of propulsion.

For the sake of safety and consumable budgets, manned missions to the moon will likely follow Apollo-type trajectories, and may be constrained to use Apollo-type system designs. The gist of what I said above is that while it seems intuitively valid to make a "pure" spacecraft for transfer between parking orbits, that solution may not offer much advantage because of the fuel requirements.

Apollo spacecraft used the S-IVB for the initial 10,000 fps TLI manuever. Then that stage was discarded. Staging is a time-honored and well-understood method for improving spacecraft system performance. It would be wise to consider a TLI stage for future spacecraft. Apollo SPS performed the 3,000 fps LOI and TEI manuevers. The final 10,000 fps EOI manuever was simply omitted, as the spacecraft never entered Earth orbit upon return. That delta-v was simply folded into the re-entry delta-v. De-orbit from Earth orbit is a 25,000 fps maneuver usually accomplished by aerobraking following a ~500 fps de-orbit burn. Re-entry from a translunar trajectory is a 35,000 fps maneuver accomplished in Apollo by aerobraking.

The attractiveness of the shuttle solution is that you have spacecraft optimized for each phase of the mission. You have a cruiser optimized for orbital and translunar flight. You have a moon lander optimized for landing and takeoff at the lunar surface. And you have an Earth orbiter suitable for liftoff and landing on Earth. The latter -- and only the latter -- would have to be aerodynamic, thus incurring the efficiency penalty for aerodynamically robust structures. But if the transfer shuttle must also be aerodynamically engineered in order to performe aerobraking, then you have to ask what is fundamentally the difference between the cruise phase and the Earth launch/landing phases in terms of equipment design? Is it really an advantage to have two separate spacecraft under those circumstances?

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Clavius Moon Base

Just a thought here, but maybe we're looking at this the wrong way. If our intention is to set up some infrastucture on the moon for jaunts to mars etc, would it not be logical to (eventually) contstruct a relatively large spacecraft that is moon based?
ie: you have a craft that is re-fuelled on the moon using local resources, the craft would have enough fuel for takeoff / landing on the moon and the tei, eoi tli and loi manuevers. I've not done any proper calculations but it would seem feasable in the low gravity / airless environment of the moon you could have a craft that could take off and still be packing enough spare fuel to get to earth orbit and back with a reasonable payload.

Then all you need is an aerocynamic craft similar to the existing space shuttle for the trip from leo and back.

Anybody with a better knowledge of rocket science than me got any thoughts on this?

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