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Sailing to Orbit on Moondust‏

Posted by: crowd1 - Fri May 14, 2010 12:37 pm
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Sailing to Orbit on Moondust‏ 
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Post Sailing to Orbit on Moondust‏   Posted on: Fri May 14, 2010 12:37 pm
I am new here and I have an idea for a space transportation system.

This system accelerates a space vehicle on a suborbital trajectory to orbital velocity by using a stream of high speed particles. The space vehicle captures the energy of these particles by using a large sail. The sail needs to be made of an extremely strong, light-weight and heat-resistant material (e.g., Kevlar). Also, the sail needs to be as large as possible to distribute the heat and to provide a large target area for the source of the particles. The sail is only as large as weight and storage area on the space vehicle allow though.

The source of the particles is an Earth orbiting satellite. It uses solar-powered ion engines to accelerate itself to just under Earth escape velocity. Once it achieves this velocity and is in the correct orbital position, it streams out small dust-like particles ahead of itself. Since the satellite is already traveling at 7-8 miles per second, it only needs to accelerate the particles to a few hundred miles per hour. Just fast enough to allow them to cover the required distance (i.e., the distance it will take the space vehicle to accelerate to orbital velocity) with sufficient accuracy. Depending on the size of the sail, an accuracy of 0.001 degrees may be needed.

It is clear that the total mass of the particles in the particle stream needs to be several times the mass of the space vehicle in order to get the space vehicle to orbital velocity. If the particle streamer is launched into orbit by a traditional rocket with a full load, the overall cost of the system is several times the cost of a traditional rocket system. It is in the means of obtaining the particles that the true savings of this system are realized. This proposal includes several methods for this (listed in order of complexity). While each subsequent method presents greater technical challenges, it also offers greater cost savings. The methods are:

1. Particles Launched from Earth to Orbit - As mentioned already, this method results in no savings, but provides a means of testing other aspects of the system.

2. Particles Launched from Earth to Suborbit - This method makes use of existing low-cost technology to launch the particles into a suborbital trajectory. Once outside the atmosphere, the particles are projected from their suborbital host vehicle into the path of an orbiting satellite. The orbiting satellite (which may also be the particle streamer) has a scoop that collects the particles and stores them. The orbiting satellite also uses ion engines to make up for the loss in velocity. The fuel for these engines is included in the collected particles (as small gas capsules mixed in with the solid particles). To reduce the force of the impact on the collecting satellite, only 1-2% of its total mass is collected at a time. This means that 100+ suborbital launches need to be made to get a full load. These suborbital launches are small, cheap and frequent though. An overall reduction in system cost should be achieved.

3. Particles Launched from the Moon - This method is similar to the previous method. Particles collected from the Moon's surface (i.e., regolith) are projected into the path of a satellite in lunar orbit. The satellite has a scoop to collect and then store the particles. Fuel for the ion engines is launched from Earth and then transferred. Without an atmosphere and with less gravity, little energy is required to get the particles off the surface of the Moon. Since the velocities involved for lunar orbit are a fraction of those for Earth orbit, the satellite also collects more particles at a time and/or collects them more frequently. Additionally, accelerating to just under Earth escape velocity takes less delta-v since the particle streamer only needs to achieve lunar escape velocity and then Earth's gravity does the rest. Of all the methods, this may be the most efficient and ideal means of obtaining particles for the system.

4. Particles Collected from an Asteroid/Comet - This method requires a long range spacecraft to travel to a near-Earth object and collect particles for the launch system. Though this method does not require a high velocity impact for particle collection, it requires some sort of drilling/digging as well as significant station-keeping. This might, in the end, be less efficient than the other methods.


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Post Re: Sailing to Orbit on Moondust‏   Posted on: Sat May 15, 2010 2:00 am
Interesting concept. A method of having your reaction mass delivered to the vehicle "just in time" to use it for propulsion without the cost of having to lug it around with you.

Only negatives I can see is that it will require pretty precise positioning of the target, and you burn a lot of energy getting all those dust particles individually to the vehicle, much more than if they were a conventional tank of fuel delivered, especially thru the Earth's atmosphere (which is probably impractical).


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Post Re: Sailing to Orbit on Moondust‏   Posted on: Mon May 17, 2010 11:49 am
Thank you so much for the response James. I realized after I posted this that I didn't even ask for feedback or anything. My bad.

Anyway, I definitely agree with the issue of precise targeting. It would take accuracy at least as good as (or better than) current anti-satellite missiles. A huge challenge for sure.

I'm not sure I understand your second statement about getting the particles individually to the vehicle thru the atmosphere though. Perhaps I didn't explain it right, but I was picturing several sources for these particles and for most of them (the more cost-effective ones) the transfer occurs outside the Earth's atmosphere. They are either transferred from a suborbital trajectory, the Moon or a near-Earth object and accelerated using ion engines. It's the greater ISP of the ion engines that result in overall system cost reduction.

In the case of collecting particles from a suborbital trajectory and from the Moon, an orbiting satellite would scoop up the particles in batches. It would scoop up 1-2% of it's own mass at a time which would slow it down some, but not enough to knock it out of orbit. The satellite would use it's ion engines to make up for the lost velocity.

Let me know if that makes sense.

Thanks.


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Post Re: Sailing to Orbit on Moondust‏   Posted on: Mon May 17, 2010 2:39 pm
Posting on a public forum implies you are soliciting commentary from the peanut gallery.

It was a reference to point #2. I don't think the other users of LEO will appreciate dust being scattered around, even in a low, sub-orbital trajectory. One accidental pass thru your cloud in a crossing orbit will effectively sand-blast a satellite.

The way I am picturing this approach is that instead of carrying on-board fuel, you time the particle "uploads" to arrive from trailing (behind) so the collection process provides ISP and then the craft shoots it back for an additional boost. But there is no or little onboard storage to minimize its mass and to maximize the DV (unless a return trip is in the mission plan). The particle projector tracks the vehicle along its flight path for as long as its under acceleration, very similar to a radio/laser datalink.

That is the only way I see this having an advantage over conventional systems and overcomes its own inherent inefficiency.


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Post Re: Sailing to Orbit on Moondust‏   Posted on: Tue May 18, 2010 8:25 pm
I'm missing something. Your pusher satellite will lose as much momentum as the suborbital vehicle gains. It will have to transfer all of that momentum to the streamed particles, so that they can in turn transfer it to the launched vehicle. How do you do that, and where do you get the energy for it?

Then there is the question of the orbit. The pusher satellite will need lots of momentum. Fine, so you fire your ion thruster, which will push it outwards slowly. But that will only make it gain potential energy, not any momentum. So what you need to do is to get it into a highly elliptic orbit, and then when it falls back in from the farthest point, you launch the suborbital craft and give it a shove as you woosh by, circularising the orbit of the pusher again in the process. That could work, but it could be a problem if the orbit is in the equatorial plane, and extends beyond GEO.

Finally, you need a lot of acceleration. The launched vehicle is on a suborbital trajectory, which means it'll fall back to Earth. How quickly depends on the height it's launched to, but it'll be less than an hour. So, you need to get it up to orbital speed in very little time. That means that it needs a lot of momentum quickly. Which means lots of dust, or dust at very high speeds, but the latter is problematic because the energy required to accelerate it (and the heating of your sail) rises quadratically with the speed. So lots of dust at relatively low relative speeds it is, as you said. It's an orbital shotgun, not a machine gun.

As James said, you risk hitting some innocent bystander with it. Have you looked at the concept of the orbital skyhook?

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Last edited by Lourens on Sat May 29, 2010 2:28 pm, edited 1 time in total.



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Post Re: Sailing to Orbit on Moondust‏   Posted on: Wed May 19, 2010 11:40 am
Thanks for the response James. You bring up some points that I had not fully considered. Of course, that's why I posted the idea here.

I don't think the risk of running into the dust cloud in a suborbital trajectory (for delivery method #2) is very high. I was not imagining a very large cloud. More like a puff. It would only be there for a few minutes before it fell back to Earth and wouldn't be any more of a hazard then any of the numerous suborbital vehicles currently planned for space tourism. Like these other suborbital vehicles, the trajectory would be mapped out to avoid collisions with objects already in LEO.

I do agree with you that there is a potential hazard with the particle stream on its way to the sail though. I think the only way to deal with this is to make sure the stream is traveling faster than escape velocity (to avoid any particles entering orbit) and to guarantee that the stream is very accurate. Again, accuracy would be the key. Any particles hitting the sail would be slowed down sufficiently to be knocked out of orbit. At least until the vehicle and the sail were close to orbital velocity. This may put an upper limit on the speed the vehicle could achieve with the particle stream and sail alone (the final push coming from a rocket). If any particles in the stream missed the sail, their velocity would keep them from entering Earth orbit. I was already thinking that the particle streamer would be traveling just under escape velocity, so it would not take a whole lot to push the stream past escape velocity.

Naturally, even with amazing accuracy and speeds greater than escape velocity, the particle stream is still a hazard. The path of this stream would definitely need to be mapped out to avoid collisions with objects in Earth orbit. Still, is it any more of a hazard than megawatt lasers firing into space (lightcraft) or cables stretching out to GEO (space elevator)?

I'm not sure I understand your take on the system in the latter half of your response. Not sure I see much of a difference from what I was already proposing. The system makes use of 2 vehicles. One with a sail and one with a load of small dust-like particles. The vehicle with the particles accelerates to near escape velocity and projects a narrow stream of these particles ahead of itself. The other vehicle pops up out of Earth's atmosphere on a suborbital trajectory, deploys it's sail and enters the particle stream. It does not carry any fuel.


Last edited by crowd1 on Wed May 19, 2010 12:32 pm, edited 1 time in total.



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Post Re: Sailing to Orbit on Moondust‏   Posted on: Wed May 19, 2010 12:24 pm
Thanks for the response Lourens.

You are correct that some momentum would be lost by the particle streamer (or particle pusher, whatever we want to call it). This vehicle would be traveling at speeds approaching escape velocity when it ejected the particles, so the particles would only need to be accelerated to a few 100 miles per hour (or less) to create a long narrow stream. Depending on the mass of the particle streamer, the loss of momentum would be a fraction of the particle's muzzle velocity (to use a gun analogy). As the stream approached the other vehicle with the sail, it would be moving at a very high speed, but it would only be slightly faster than the particle streamer.

As far as the energy to generate the stream, I was thinking solar. Like I already mentioned, it's just a few 100 mph, so a fan/wheel/pump could probably do it(???).

I was also already imagining a highly elliptical orbit to get the particle streamer to the appropriate velocities. And you are correct about the acceleration of the vehicle with the sail needing to happen fast. I was definitely thinking within minutes instead of hours. The sail would need to be very strong and heat resistant. I was hoping that a reasonable balance could be reached between strength of the sail, heat-resistance, duration of acceleration, G-forces and weight.

As far as the skyhook scheme goes, that is a perfect analogy. Let's talk about the hazards of a swinging cable in LEO or the accuracy required to maneuver a suborbital vehicle to within a few feet of a dangling hook outside the atmosphere. And how about the unobtainium material you need to construct this cable? My idea may be a little crazy, but if you can talk about a skyhook with any sort of "could-work" attitude, then I don't think I am too far behind. :)


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Post Re: Sailing to Orbit on Moondust‏   Posted on: Wed May 19, 2010 1:15 pm
crowd1 wrote:
I'm not sure I understand your take on the system in the latter half of your response.


Sorry, I think I was stuck on the idea that you somehow wanted to capture and store the particles onboard the target vehicle and that it were a "transmission" from surface to orbit.

The more I think about this the more I like it. Its very elegant the way it takes advantage of the relative velocities of differential orbits. You really don't even have to "launch" the particles. You could just have your streamer approaching perigee of an eccentric orbit cast its "bucket of sand" rearward, decelerating it to ensure that it would pass low enough to both catch the sub-orbital target at its apex and the particles that miss would get dragged into the atmosphere. This would solve the problem of your streamer losing momentum as it donated ISP to the target object.

The only real practical problem is like you say, building a sail tough enough to survive absorbing the energy of the particles while still being lighter (and cheaper) than conventional alternatives.


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Post Re: Sailing to Orbit on Moondust‏   Posted on: Thu May 20, 2010 11:04 am
Thanks James. I feel a little less insane now. I've read some of the other posts on this forum about anti-gravity, motion induction engines and other miscellaneous forms of hocus-pocus/bad-physics; and I was fearing my idea was in the same genre.

And I like your notion of projecting the particles behind the particle streamer and directing them into the upper atmosphere. That could be a lot easier than trying to accelerate them to escape velocity. And, like you said, you would not lose any momentum with the particle streamer.


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Post Re: Sailing to Orbit on Moondust‏   Posted on: Thu May 20, 2010 11:59 am
Yes it almost doesn't pass the "free lunch" test, but really all you are doing is transferring energy from one to another with the leverage of orbital momentum (courtesy of the main body gravity).

This doesn't make it practical by an stretch of the imagination. There are lots of technical issues with this technique and it may never prove to be efficient enough for real use. But it is a good topic for a paper, a thesis, or even nice research grant. lol.


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