Using orbital debris as reaction mass- profiting from orbital mass-drivers

[Northstar1989]

As I'm sure you're all aware, debris in orbit around Earth is a HUGE problem, and a major hazard to satellites and new launches, due to the literally hundreds of pieces of debris in orbit... Any proposal to actively manage that debris has fallen on deaf ears though, due to the EXTREME cost of most such plans... This is something I've been thinking about a lot lately.

But what if we could actually PROFIT off that debris? What if we could use it as *reaction mass*? Thus, an idea was born...

Lately, I've been running a number of mods in KSP, one of which is a selection of parts from the Stanford Torus mod including a Mass Accelerator. Not satisfied with the performance of the part, I created my own version with realistic force (up to 11,760 kN/s- the same as the proposed electromagnets for Star Tram gen-1) and power consumption levels, as well as reducing the mass (from 25 tons to 10- closer to real world levels for an aluminum electromagnet coil of that size), cost (one ring was more expensive than the largest rockets- 185,000 Funds. I reduced that to 12,500), and improving the structural integrity-related stats so a coil of these wouldn't explode from their own force when properly braced and fired at close to full power...

I intend to re-release the re-balanced parts as a standalone "mini-mod" in the near future, as the license (CDDL-1) is open source and explicitly allows modification and re-distribution.

Anyways, the mass accelerators made it into a list of infrastructure projects I was/am considering for my collaborative community game:

http://forum.kerbalspaceprogram.com/threads/97427-Announcing-Northstar-s-Collaborative-Kerbal-Career-Campaign-where-to-next?p=1509692&viewfull=1#post1509692

To cut to the chase, one of the ideas I considered was setting up orbital "Momentum Exchangers"- basically space stations around Kerbin and the Mun that would have batteries and a stack of networked Mass Drivers. There stations would be used to fire payloads in opposite directions, taking advantage of Newton's Lwas to maintain a stable orbit (basically, they would fire one payload retrograde, then turn around and fire another prograde, or vice-versa). At first, I was just thinking of using them for crew capsules and landers I wanted to de-orbit, and payloads I wanted to boost into a higher orbit.

But then today, the thought occurred to me- why not use these mass drivers on debris as well? Not only would that DRASTICALLY increase the amount of reaction mass eligible for my mass drivers to work on, it would also allow me to shoot debris retrograde at g-levels that would kill a Kerbal (I'm playing with Deadly Re-Entry enabled, and probably TAC Life Support in the near future- Kerbals can die if g-forces exceed certain levels), but also not worry about whether the re-entry trajectories were survivable (in fact, from a reoleplaying perspective, it would be PREFERABLE if the debris burned up before hitting the ground...)

Further, why not use this in real-life? The benefits would be much more substantial. Not only does it take a lot more Delta-V to get a kg of anything to orbit, the orbital velocities are MUCH higher. Which means, a mass driver that can shoot payloads retrograde at, say, 190% of orbital velocity to de-orbit them (on a retrograde re-entry trajectory) can give debris or crew capsules a LOT more velocity without sending them into stable retrograde orbits. And, an even MOAR POWERFUL mass driver, which actually shoots debris retrograde so fast as to send them on escape-trajectories from the Earth-Moon system (I should point out now, there is no theoretical maximum speed on mass drivers other than the speed of light- although the length of electromagnets required grows exponentially longer with each additional m/s of velocity you want to give the ejected mass...), well the potential benefits there are almost unimaginable.

The plan would look something like this- NASA or some other space agency would build a LONG and POWERFUL coil of electromagnets in a low orbit above Earth (due to the Oberth Effect, more energy is gained from reaction mass when you are moving faster- and LEO is also the cheapest orbit to reach). Since all this would be is basically a really long coil of aluminum wire, it would be easy enough to assemble on-orbit without super-complicated or expensive machinery (the mass driver wouldn't be extremely precise- a 1 or 2 degree variance in trajectories isn't going to matter a lot when de-orbiting debris). The longer the coil of mass drivers, the better, since this would increase the amount of energy that could be gained from each kg of debris/reaction mass, and reduce the recoil acceleration on the mass drivers (so as to not damage any control systems attached to the electromagnets, or payload waiting to be fired in the opposite direction).

A small ion-powered tug (to reduce fuel-mass you have to launch for it) would rendezvous with debris over the course of weeks/months, and tug these debris back to the mass driver over weeks/months/years (depending on the mass of the debris- although more massive debris could potentially provide more impulse to the mass driver, making it more valuable...)

When the debris arrived in the mass driver, it would be loaded up at the prograde end of the electromagnet coil, and fired RETROGRADE at as high a velocity as possible, so as to either de-orbit the debris or send it on an escape trajectory (or impact-trajectory with the Moon). Ideally, this would be done at periapsis of the mass-driver's orbit, so as to provide the most energy possible from the reaction mass/ debris.

The mass driver would then load up a payload (which would usually have attached to the mass driver station BEFORE the firing of the debris) at the retrograde end of the electromagnet coil, and fire it PROGRADE (by reversing the direction of the electric current from the first firing- unlike in KSP, electromagnet mass accelerators don't have innate polarity) with enough velocity to return the mass driver to its original orbit. For payloads requiring particularly large boosts (such as an interplanetary mission), the mass driver could build up energy by firing several pieces of debris in succession (at each periapsis of its orbit), before firing the payload with enough velocity to consume ALL of the accumulated energy.

The bread-and-butter of the mass driver payloads would probably be small satellites bound for geosynchronous orbit- which would actually be a good thing, as the most economical/easiest payloads to accelerate this way would be those with a small diameter (as this would allow the same mass of electromagnet-wire to be used to build a longer coil). Although any payload would work- as long as it were thin enough to fit through the mass driver coil (obviously the larger the diameter of the mass driver, the wider a variety of payloads it could be used for), larger ones would actually reap more economic benefit- as the first 100 m/s of Delta-V in a mission always requires more fuel than the second or third... Even a tiny nudge of, say, 50 m/s to a Mars Mission would represent a lot more fuel savings (and thus launch-mass savings) than a push of 150 m/s to a large number of much smaller satellites (assuming the ISP of the fuel were the same). So not only is this scalable- the benefits are actually GREATER with heavier payloads...

The only major obstacle to this I could foresee, aside from initial investment costs (the mass driver coil would have to be sufficiently large/powerful to save GEO satellites more launch mass than the fuel required to tug debris to the mass-driver's orbit), would be the high electrical power requirements. This could definitely be solved by having a high electrical storage capacity and low operational frequency (with something like solar panels providing the power to slowly charge the batteries or capacitors), but something like Microwave Beamed Power from a ground-based installation would probably be a much more cost-effective solution (not only would you have to launch much less solar panel and battery/capacitor mass to orbit- the same Microwave Beamed Power installation could also find additional use in launching small Microwave Thermal Rockets...)

It sounds like a great idea, at least on paper, and gets "free" momentum without violating any of the laws of physics- basically it steals momentum in the prograde direction from debris that don't need it (said debris then either re-enter, or shoot into much higher retrograde orbits, depending on the size of the push), and gives it to payloads that *DO* need it- like interplanetary-probes or satellites bound for geosynchronous orbit. The energy comes from electricity- which ultimately either comes from solar panels (or nuclear reactors) on the mass driver, or a rectenna and Microwave Beamed Power at a ground installation.

Let me know what you guys think of this idea.

Regards,

Northstar

P.S. This idea is similar to, but distinct from, Momentum Exchange Tethers. Another great idea that nobody's seriously tried to develop...

P.P.S. This idea also works in KSP- so I intend to see if I can't post some pictures of my own attempts to re-create it sooner or later. Feel free to do the same yourselves, once I post the up-rated version of the Stanford Torus mod Mass Accelerators (the versions from the base mod are far too weak/heavy compared to real life for this to be practical...)

Edited October 31, 2014 by Northstar1989

[shynung]

One problem I see with using this kind of propulsion: impact hazards from the exhaust.

Exhaust gases emitted by typical rockets, both chemical and electrical, disperse after a relatively short distance (a few kilometers) enough to render them essentially harmless to other spacecrafts unlucky enough to be in their orbital path. Solid objects spewed by mass drivers stay more-or-less in chunks even after thousands of kilometers away from the engine itself.

Given that an impact from a single paint fleck produced this crater on a Space Shuttle's front window:

It can be said that this mass driver engine is as good as a propulsion system as it is as a weapon.

That, alone, brings political problems to the owners of the spacecraft. But I digress.

[Northstar1989]

One problem I see with using this kind of propulsion: impact hazards from the exhaust.

Exhaust gases emitted by typical rockets, both chemical and electrical, disperse after a relatively short distance (a few kilometers) enough to render them essentially harmless to other spacecrafts unlucky enough to be in their orbital path. Solid objects spewed by mass drivers stay more-or-less in chunks even after thousands of kilometers away from the engine itself.

Given that an impact from a single paint fleck produced this crater on a Space Shuttle's front window:

http://upload.wikimedia.org/wikipedia/commons/1/12/Space_debris_impact_on_Space_Shuttle_window.jpg

It can be said that this mass driver engine is as good as a propulsion system as it is as a weapon.

That, alone, brings political problems to the owners of the spacecraft. But I digress.

That's a perfectly valid point- but we're easily capable of plotting the trajectories of the exhaust. And in the simplest/cheapest version of this plan, the debris would be given retrograde momentum in a range that would de-orbit it: which would REDUCE impact hazards in the long run, and is part of the whole point of this plan...

Though, yes, the fact that the mass-driver could double as an anti-satellite weapon might actually serve as a *selling point* to certain politicians... [evil cackle]

Regards,

Northstar

Edited October 30, 2014 by Northstar1989

[shynung]

That's a perfectly valid point- but we're easily capable of plotting the trajectories of the exhaust. And in the simplest/cheapest version of this plan, the debris would be given retrograde momentum in a range that would de-orbit it: which would REDUCE impact hazards in the long run, and is part of the whole point of this plan...

So it's like VASIMR, where the engine can alter its exhaust velocity on-the-go. Though, that also puts a difficult twist on the engine's operations: the exhaust velocity must (A)not be larger than twice the orbital velocity (so the exhaust doesn't go retrograde, smacking into stuff), or (B)be larger than earth's escape velocity (so the exhaust gets flung out into solar orbit). Option B is fine for satellites, since they only need thrust for stationkeeping, but heavy payloads like interplanetary missions would use option A for the need of high thrust. Given that the specific impulse of the engine is limited by that plan, that means a large propellant bin full of scrap, which would go head-on against the rocket equation (all that mass needs to be moved around, too).

Alternatively, one can install some sort of grinding device into the engine, so that any orbital scrap used as propellant is ground to particles small enough to cause little damage to other spacecrafts, but it would raise the needed power requirements, and ultimately effective I_sp, as that powerplant mass would still need to be hauled around wherever the engine goes, not to mention the grinder itself.

[Streetwind]

As I'm sure you're all aware, debris in orbit around Earth is a HUGE problem, and a major hazard to satellites and new launches, due to the literally hundreds of pieces of debris in orbit... Any proposal to actively manage that debris has fallen on deaf ears though, due to the EXTREME cost of most such plans...

And your proposal pretty much has the same issue - it's extremely expensive. Using the mass driver to boost payloads is not going to make a dent in the construction and operation costs. At best it may fire once or twice a year.

It is true that there is a lot of space debris, but that fact needs to be taken in the same way that you take the fact that there are a lot of asteroids in the asteroid belt. In practical application, despite the huge number of asteroids, you could fly blind at full engine power through the densest spot and the probability of hitting something would be borderline negligible. In fact, the probability of any two given asteroids being within viewing distance of one another is borderline negligible.

Space debris is the same. It is considered dangerous not because there's a huge inpenetrable cloud of it, but rather because a significant collision - even if it is as unlikely as winning the lottery - will instantly scrap your billion dollar spacecraft, creating more debris in the process that might scrap other billion dollar spaceraft (kessler syndrome). And that would really suck a lot, to put it mildly.

Additionally, much like asteroids, the various pieces of debris have extremely divergent trajectories, and the proximity to Earth greatly increases the dV requirements for matching planes. So yes, you might have an ion powered tug that gets really good gas mileage... it might be able to match orbit with a dozen or so objects before requiring refueling. Do note though that xenon gas is excessively expensive. It costs a thousand times more per kg than regular RP1/LOX fuel, while its Isp is only ten to thirty times higher. So while you're saving on the number of refueling operations performed, you're not saving on fuel costs... in fact, the ion drive might be more expensive to run over time, depending on launch costs for fuel.

And now the biggest issue... a lot of space debris is really small. Like, a few grams and smaller. Flecks of paint, metal splinters, single screws, that sort of thing. So even if your space tug managed to catch ten of them, it would be completely negligible as reaction mass for the mass driver, and it would be completely negligible in terms of making a dent in the proliferation of space debris. You would have to focus on the really large pieces, of which there are a constantly decreasing number: their orbits decay, they collide with something and fracture into smaller parts, and there are nearly no new ones produced nowadays that debris awareness is fairly high. Derelict satellites are pushed into graveyard orbits with the last of their propellant upon decomissioning them. You could pluck these wrecks out of their graveyard orbit and use them as reaction mass, but that will not be cleaning up any actually dangerous debris (the stuff not in graveyard orbits).

To be a real solution for the danger that space debris poses to spacecraft, a cleanup proposal needs to prove how it can be cost effective at dealing primarily with this giant cloud of really small objects that are near-impossible to detect and track, have completely unpredictable orbits, and are of absolutely zero value to retrieve. If the problem was as easily solved as "send a spacecraft to rendezvous with the debris", we'd likely be doing that already.

Edited October 30, 2014 by Streetwind

[Dodgey]

One solution I did hear of for space debris did include a mass driver system but not in the way NorthStar described it. Instead of one larger sat and multiple smaller ones this system was just a fleet of smaller ones. Basically they would attach to a piece of debris and hold onto it, then at the correct time it would "fling" that piece of debris behind it, deorbiting one piece and changing its orbit to meet up with another piece of debris. It uses the debris as reactionmass to change its orbit to catch another.

[Northstar1989]

@shynung

So it's like VASIMR, where the engine can alter its exhaust velocity on-the-go. Though, that also puts a difficult twist on the engine's operations: the exhaust velocity must (A)not be larger than twice the orbital velocity (so the exhaust doesn't go retrograde, smacking into stuff), or (B)be larger than earth's escape velocity (so the exhaust gets flung out into solar orbit). Option B is fine for satellites, since they only need thrust for stationkeeping, but heavy payloads like interplanetary missions would use option A for the need of high thrust. Given that the specific impulse of the engine is limited by that plan, that means a large propellant bin full of scrap, which would go head-on against the rocket equation (all that mass needs to be moved around, too).

You *COMPLETELY* mis-understand the proposal. The whole idea is that you DON'T permanently move the mass driver station from its low orbit. If you fire a piece of debris retrograde, you fire a payload prograde. Energy (and momentum) is 100% conserved no matter how heavy or light the mass driver station- a heavier station will gain less velocity from firing debris (and crew capsules) retrograde, but will also lose less velocity from firing payloads prograde by the same measure. Thus, the mass of the mass driver station is entirely *IRRELEVANT-* in fact, as I pointed out in the OP, *heavier* stations are preferable, as they will experience less of a recoil shock when firing debris.

Also, you are firing satellites on GTO orbits from LEO with this method, using orbital debris as reaction-mass. This saves on the fuel costs to put them in GEO in the first place- it has nothing to do with stationkeeping fuel (other than that the fuel you would have had to used to get to GTO can be used for stationkeeping instead...)

Alternatively, one can install some sort of grinding device into the engine, so that any orbital scrap used as propellant is ground to particles small enough to cause little damage to other spacecrafts, but it would raise the needed power requirements, and ultimately effective I_sp, as that powerplant mass would still need to be hauled around wherever the engine goes, not to mention the grinder itself.

Because the mass driver station is not being accelerated in the long run, it doesn't matter how massive it is. Read the OP before you comment. A grinding station would actually be highly profitable, as it would allow you to harvest more velocity from each piece of reaction mass- but energy is always conserved whether you are accelerating a small station by a lot or a massive station by a little.

@Dodgey

One solution I did hear of for space debris did include a mass driver system but not in the way NorthStar described it. Instead of one larger sat and multiple smaller ones this system was just a fleet of smaller ones. Basically they would attach to a piece of debris and hold onto it, then at the correct time it would "fling" that piece of debris behind it, deorbiting one piece and changing its orbit to meet up with another piece of debris. It uses the debris as reactionmass to change its orbit to catch another.

This makes more sense, as you don't have to haul the mass driver around, and get USEFUL momentum-change out of that reaction mass instead.

Regards,

Northstar

Edited October 31, 2014 by Northstar1989

[Northstar1989]

@Streetwind

And your proposal pretty much has the same issue - it's extremely expensive. Using the mass driver to boost payloads is not going to make a dent in the construction and operation costs. At best it may fire once or twice a year.

Unfortunately, you're quite wrong.

Most of the mass you launch to LEO for a GEO satellite is actually fuel to get it to GEO in the first place. Larger rockets are a LOT more expensive- this is why they say it costs $10,000/kg to get anything to orbit, not such and such price per launch. If you have to launch 30 or 40% the initial mass to orbit, you save a LOT on satellite placement costs. And due to the Rocket Equation, if 90% of launch mass is fuel to get to GEO, then maybe 60% or 70% of that fuel is to get to GTO, and only 30 or 40% of that is fuel needed to circularize...

You can also launch twice as many satellites at a time on a given launch vehicle if each satellite only weighs 30 or 40% as much...

It is true that there is a lot of space debris, but that fact needs to be taken in the same way that you take the fact that there are a lot of asteroids in the asteroid belt. In practical application, despite the huge number of asteroids, you could fly blind at full engine power through the densest spot and the probability of hitting something would be borderline negligible. In fact, the probability of any two given asteroids being within viewing distance of one another is borderline negligible.

Physical distance doesn't matter. We're not talking about a chemical rocket that flies blind through the debris-belt, and tries to scoop up as many pieces as possible. We're talking about a TINY ion-probe that engages in rendezvous with each individual piece of debris, and SLOWLY brings it back to the mass driver utilizing the most fuel-efficient route possible. The only thing that matters is how far apart objects are in terms of Delta-V: which when they are in the same orbit is almost negligible (you can enter into a phasing orbit only 1 or 2% higher/lower, and wait MONTHS for positioning for the next rendezvous if necessary...)

Space debris is the same. It is considered dangerous not because there's a huge inpenetrable cloud of it, but rather because a significant collision - even if it is as unlikely as winning the lottery - will instantly scrap your billion dollar spacecraft, creating more debris in the process that might scrap other billion dollar spaceraft (kessler syndrome). And that would really suck a lot, to put it mildly.

And de-orbiting each piece of debris reduces that risk. Nothing about this idea requires a huge, impenetrable cloud- in fact that would only make implementation more difficult.

Additionally, much like asteroids, the various pieces of debris have extremely divergent trajectories, and the proximity to Earth greatly increases the dV requirements for matching planes. So yes, you might have an ion powered tug that gets really good gas mileage... it might be able to match orbit with a dozen or so objects before requiring refueling. Do note though that xenon gas is excessively expensive. It costs a thousand times more per kg than regular RP1/LOX fuel, while its Isp is only ten to thirty times higher. So while you're saving on the number of refueling operations performed, you're not saving on fuel costs... in fact, the ion drive might be more expensive to run over time, depending on launch costs for fuel.

The launch costs DWARF the fuel costs. Propellant-grade Xenon can typically be obtained for about $850/kg, although prices range as high as $1200 per kg, according to Wikipedia (which only cites one source for the price, which appears to be on the high end. Even if other fuels were FREE, but you needed to launch 10x as much, they would be MUCH more expensive.

Do the math: $100,000 in launch-costs for 10kg of a free propellant with around 400s ISP (basically only LH2/LOX has ISP that high- and is *HIGHLY* cryogenic, meaning it can't be stored for long periods of time in orbit without expensive cooling systems...) vs. $11,200-$10,850 in launch+propellant costs for a single kg of Xenon. And because of the Rocket Equation, you need MORE than 10x as much mass of fuel for a fuel with 1/10th the ISP...

And now the biggest issue... a lot of space debris is really small. Like, a few grams and smaller. Flecks of paint, metal splinters, single screws, that sort of thing. So even if your space tug managed to catch ten of them, it would be completely negligible as reaction mass for the mass driver, and it would be completely negligible in terms of making a dent in the proliferation of space debris.

It wouldn't be negligible as reaction mass for the driver. If you could catch even 100 kg of tiny pieces of debris like that, and shoot them out the mass driver at Earth escape-velocity (11.2 km/s), then that's the same impulse as 100kg of fuel with an Isp of 1141.7 seconds. And because you don't impart any net-energy to the mass driver (any speed it gains is later transferred to payloads), you completely circumvent the limitations of the Rocket Equation (yes, if the mass driver is loaded down with reaction mass it will gain less speed by firing it retrograde at a given speed- but it will also lose less speed when firing payload prograde at a given speed- energy and momentum is conserved).

You would have to focus on the really large pieces, of which there are a constantly decreasing number: their orbits decay, they collide with something and fracture into smaller parts, and there are nearly no new ones produced nowadays that debris awareness is fairly high. Derelict satellites are pushed into graveyard orbits with the last of their propellant upon decomissioning them. You could pluck these wrecks out of their graveyard orbit and use them as reaction mass, but that will not be cleaning up any actually dangerous debris (the stuff not in graveyard orbits).

Just the opposite- the larger pieces would actually be LESS profitable to use as reaction mass, as they would require larger ion tugs just to pull them back to the mass driver. It would actually make more sense to send a tiny tug that cuts little pieces off larger debris and hauls it back to the mass driver in a large number of trips than it would to send a larger tug that can do it all in one go. Not only are ion engines difficult to scale, but you have to launch more mass to orbit in the first place if you send up a heavier tug...

The BEST ion tug would actually be the SMALLEST ion tug, with the ability to cut larger debris into smaller pieces. This way, you can use the same tug for both large derelects and tiny screws...

If there were a *USE* for orbital debris, people might also be a lot less hesitant to create it. If NASA had known we would be re-using the External Tank on the Space Shuttle, for instance, they could have increased the total launch mass by 10-20%, and put three times the "payload" in orbit with each launch (if you count the External Fuel Tank as a payload), rather than dumping the EFT on a sub-orbital trajectory just shy of achieving circularization...

To be a real solution for the danger that space debris poses to spacecraft, a cleanup proposal needs to prove how it can be cost effective at dealing primarily with this giant cloud of really small objects that are near-impossible to detect and track, have completely unpredictable orbits, and are of absolutely zero value to retrieve. If the problem was as easily solved as "send a spacecraft to rendezvous with the debris", we'd likely be doing that already.

This would give value to retrieved debris AND work well on tiny pieces of debris (in fact, the smaller the better- as you can obtain more energy from each piece of reaction-mass without having to grind anything up into smaller pieces that way). The hard part would be finding+detecting the smallest pieces of debris in the first place, but we could always make a handy profit off the larger pieces in the meantime. There are dozens of decommissioned satellites, and even a handful of intact rocket stages in orbit- probably enough reaction mass to launch at least a couple-dozen satellites on GEO-Transfer Orbits (GTO) if you cut/ground them up and launched each tiny piece retrograde with sufficient velocity...

Regards,

Northstar

Edited October 31, 2014 by Northstar1989

[Northstar1989]

Another *REALLY COOL* thing is that you could even set up another mass-driver in a retrograde orbit, and bounce reaction mass between the two instead of de-orbiting or ejecting it from the Earth-Moon system (to be precise, you would want to shoot debris into designated elliptical graveyard-obits, and send ion tugs to retrieve them: fuel for which would be the only reaction mass consumed in this plan...) Each mass-driver gains speed, but there is no change in net momentum (kind of like if two people stand on skateboards and push against each other- both start moving in opposite directions, but momentum is conserved). You still need electrical energy to run this system though.

This would allow you to re-use the same reaction mass an effectively unlimited number of times, and essentially circumvent the Rocket Equation by coupling each impulse in the prograde direction with an equal impulse in the retrograde direction. Of course, I'm not sure why you would WANT to put payloads in retrograde orbits, unless their ultimate destinations were outside the solar system (in which case, you are ultimately stealing momentum from the Earth-Moon system...)

Regards,

Northstar

Edited October 31, 2014 by Northstar1989

[NERVAfan]

Additionally, much like asteroids, the various pieces of debris have extremely divergent trajectories, and the proximity to Earth greatly increases the dV requirements for matching planes. So yes, you might have an ion powered tug that gets really good gas mileage... it might be able to match orbit with a dozen or so objects before requiring refueling. Do note though that xenon gas is excessively expensive. It costs a thousand times more per kg than regular RP1/LOX fuel, while its Isp is only ten to thirty times higher. So while you're saving on the number of refueling operations performed, you're not saving on fuel costs... in fact, the ion drive might be more expensive to run over time, depending on launch costs for fuel.

Could you use argon instead? It is dramatically cheaper.

And now the biggest issue... a lot of space debris is really small. Like, a few grams and smaller. Flecks of paint, metal splinters, single screws, that sort of thing.

This is something I've wondered about for a while. Cubesats deorbit quickly since they are small and have a high surface area/mass ratio. So why don't the paint chips and stuff deorbit really fast?

[Northstar1989]

Could you use argon instead? It is dramatically cheaper.

Yes. Some types of ion propulsion rely on Argon instead. It actually has a better ISP than Xenon for the same per-atom energy, due to its lower molecular mass (but it also has lower thrust). It also suffers from lower fuel-density than Xenon, and requires lower storage temperatures.

Another option is Nitrogen, which can be skimmed off the upper atmosphere:

http://en.wikipedia.org/wiki/Propulsive_fluid_accumulator

This is something I've wondered about for a while. Cubesats deorbit quickly since they are small and have a high surface area/mass ratio. So why don't the paint chips and stuff deorbit really fast?

Some pieces of debris ARE short-lived. Others are in highly elliptical orbits, and so spend most of their time high above the Earth, and only a short time in lower orbits. But the debris that would be most useful for mass drivers are simply objects with very high density (and thus very high ballistic coefficients)- like screws and tools dropped off space stations...

Regards,

Northstar

[shynung]

You *COMPLETELY* mis-understand the proposal. The whole idea is that you DON'T permanently move the mass driver station from its low orbit. If you fire a piece of debris retrograde, you fire a payload prograde. Energy (and momentum) is 100% conserved no matter how heavy or light the mass driver station- a heavier station will gain less velocity from firing debris (and crew capsules) retrograde, but will also lose less velocity from firing payloads prograde by the same measure. Thus, the mass of the mass driver station is entirely *IRRELEVANT-* in fact, as I pointed out in the OP, *heavier* stations are preferable, as they will experience less of a recoil shock when firing debris.

Also, you are firing satellites on GTO orbits from LEO with this method, using orbital debris as reaction-mass. This saves on the fuel costs to put them in GEO in the first place- it has nothing to do with stationkeeping fuel (other than that the fuel you would have had to used to get to GTO can be used for stationkeeping instead...)

Because the mass driver station is not being accelerated in the long run, it doesn't matter how massive it is. Read the OP before you comment. A grinding station would actually be highly profitable, as it would allow you to harvest more velocity from each piece of reaction mass- but energy is always conserved whether you are accelerating a small station by a lot or a massive station by a little.

I was under the assumption that the mass driver would act as an engine, i.e. docks to a client spacecraft and pushes them around, using orbital debris as propellant. Yes, I did not completely read your post, but with that wall of text, something's bound to be missed.

However, the exhaust velocity problem is still there. It is in the mass driver owner's best interest to not fire a piece of debris into a retrograde orbit, lest his spacecraft would risk get hit by its own exhaust. However, collecting the propellant for the mass driver is a complicated task, to put it lightly. If the spacecraft owners were concerned about mass-efficiency of the system (effective I_sp), they'd set the system to fire into a retrograde escape orbit, which brings its own energy requirement problems on the table.

Ideally, the exhaust velocity should match the mass driver's current exhaust velocity, so that it would fall straight down into the atmosphere, to present the least risk to neighboring spacecraft. This means adjusting to different payload mass by adjusting the exhaust mass, which can limit the effective I_sp, in addition to aggravating the propellant collection difficulties.

Here, the mass driver owner is presented with a dilemma: (A)fire the propellant on a retrograde escape trajectory to conserve propellant mass (exhaust velocity > 19.5 km/s, effective I_sp about 1950 seconds), requiring massive and powerful (=expensive) powerplant in the process, or (B)fire the propellant at the spacecraft's current orbital velocity (about 8 km/s at LEO), to skimp on the power generation system, at the cost of propellant mass efficiency (effective I_sp about 800 seconds), putting considerable loads on the propellant collection system in the process.

Which plan do you think is better? Or do you have a better alternative?

Edited October 31, 2014 by shynung

[KSK]

I'm likely missing something here, but if you're ferrying all the debris to a single place (the mass driver), why not just glom it all together at that point (or put the small stuff in a tin can), and use the last of the propellant in your ion tug to de-orbit the whole lot in one go? Other than coolness - and it is definitely cool - I'm not seeing a great advantage to using the mass driver.

[Dodgey]

Because you use the debris as reaction mass to accelerate other satellites into higher orbit, essentially stealing the momentum off of the debris and giving it to something else.

[KSK]

Because you use the debris as reaction mass to accelerate other satellites into higher orbit, essentially stealing the momentum off of the debris and giving it to something else.

Ahh, I see. That still seems like a false economy though. I don't have any numbers to back this up but I imagine you'd use quite a bit more propellant to harvest your debris (to use as propellant) than you would need to simply boost the satellite directly. Not to mention that you'll need to expend additional propellant to rendezvous the satellite with the mass driver.

It's a cool idea but I'm still not seeing any advantages over a combination of conventional satellite launchers and a separate debris capture-and-deorbit system of some kind.

[Dodgey]

Well you aren't taking into account the fact that you can theoretically launch the debris at a much higher velocity then they used to be, all you are doing is bringing the mass accelerator reaction mass, which it then imparts a great deal of energy into. So basically you could accelerate a 10g piece of debris at 100m/s retrograde and accelerate a 1kg satellite at 1m/s prograde, or just turn the power up on your mass accelerator to increase the speeds.

[Northstar1989]

Well you aren't taking into account the fact that you can theoretically launch the debris at a much higher velocity then they used to be, all you are doing is bringing the mass accelerator reaction mass, which it then imparts a great deal of energy into. So basically you could accelerate a 10g piece of debris at 100m/s retrograde and accelerate a 1kg satellite at 1m/s prograde, or just turn the power up on your mass accelerator to increase the speeds.

10 m/s? 100 m/s?

*chuckle*

You could launch debris are up to 30,000 m/s (the current max speed of our largest mass accelerators- on very small masses on Earth), which equates to an ISP of 3058 seconds.

Ahh, I see. That still seems like a false economy though. I don't have any numbers to back this up but I imagine you'd use quite a bit more propellant to harvest your debris (to use as propellant) than you would need to simply boost the satellite directly. Not to mention that you'll need to expend additional propellant to rendezvous the satellite with the mass driver.

It's a cool idea but I'm still not seeing any advantages over a combination of conventional satellite launchers and a separate debris capture-and-deorbit system of some kind.

It's a matter of mass-leveraging. Using 10 kg of Xenon propellant, you could easily drag at least 2 or 3 metric tons of debris back to a LEO mass driver (the Delta-V gap between the debris belts and typical station low orbits is VERY small). Then, you could grind the debris up into tiny pieces (Which would be fired separately), and gain the equivalent of 20 or 30 more times as much reaction mass at an ISP of up to 3058s (the best ion thruster currently in development, the Dual Stage 4-grid ion thruster, has an ISP of 19,300 seconds).

Not only that, but you aren't limited by the Rocket Equation, since your mass driver doesn't get carried with the payload- unlike the fuel for a comparable impulse using conventional propulsion. The mass driver also allows you to accelerate the payload as fast as the payload can withstand being accelerated (with very small payloads, you *could* reach over 100 g's of acceleration), which means you take much greater advantage of the Oberth Effect than with an ion thruster- which are the only other means of propulsion with this kind of specific impulse.

Regards,

Northstar

Edited October 31, 2014 by Northstar1989

[Kulebron]

What about energy vs impulse problem? Energy expended grows quadratically with exhaust speed, or impulse. Basically, the more Isp you want to extract, the more energy you need to spend. This makes good Isp of this propulsion quite impractical, because you need bigger energy engine and this eats all the gain.

[merendel]

What about energy vs impulse problem? Energy expended grows quadratically with exhaust speed, or impulse. Basically, the more Isp you want to extract, the more energy you need to spend. This makes good Isp of this propulsion quite impractical, because you need bigger energy engine and this eats all the gain.

Asumeing the thing is powered by fairly large capacitors for the actual shots the higher energy requirements are more of a rate of fire issue. It does not mater if you have a generator producing 1 Megawatt of power or a solar array that only produces 1 kilowatt of power if your feeding all the output into a capacitor to release in 1 go. Its just going to take significantly longer to charge up for a shot.

As to the concept of the whole system I'm not sure of the longterm viability of the collection aspect. Even assuming its cost effective it's still a finite resource to tap. I doubt space programs will switch to leaving orbital debris agian just because theres a plan to capture it as its still a hazard until the street sweeper shows up. On the other hand I could see them useing spent second stages as reaction mass. Assumeing a first stage that gets the payload partway to orbit (and falls back for recovery spaceX style) and a second stage that finishes orbital insertion the second stage could be used by the station to launch the payload up to a higher orbit. Even fairings might be worthwhile to not jetison while on suborbital for use as mass driver ammo.

Edited October 31, 2014 by merendel

[Northstar1989]

Asumeing the thing is powered by fairly large capacitors for the actual shots the higher energy requirements are more of a rate of fire issue. It does not mater if you have a generator producing 1 Megawatt of power or a solar array that only produces 1 kilowatt of power if your feeding all the output into a capacitor to release in 1 go. Its just going to take significantly longer to charge up for a shot.

The absolute amount of energy required per shot can indeed still be a problem- Kulebron is right about that. You have to either accept lower exhaust velocities for larger pieces of debris (remember, there are "safe" bands of velocity at *both* below 2x orbital velocity and above escape-velocity), or grind larger pieces of debris up unto smaller chunks. It's not easy to build a capacitor bank that can hold the energy to expel a 2-ton spent upper stage at 30,000 m/s- it's much easier (and lighter) to grind that debris up into 1 or 2-kg chunks, and expel each of *those* at 30 km/s...

Of course, one way to DRASTICALLY lessen the generation and storage requirements on-orbit is to make use of Microwave Beamed Power. If you *beam* the power to the mass driver, you can leave all the heavy solar panels or nuclear reactors on the ground, and won't need nearly as much storage-capacity (if you can beam up power at half the rate it's consumed, for instance, then you only need enough storage for half the per-shot usage... And there are few limits to the rate at which you can beam up power, other than thermal-management issues...)

As to the concept of the whole system I'm not sure of the longterm viability of the collection aspect. Even assuming its cost effective it's still a finite resource to tap. I doubt space programs will switch to leaving orbital debris agian just because theres a plan to capture it as its still a hazard until the street sweeper shows up. On the other hand I could see them useing spent second stages as reaction mass. Assumeing a first stage that gets the payload partway to orbit (and falls back for recovery spaceX style) and a second stage that finishes orbital insertion the second stage could be used by the station to launch the payload up to a higher orbit. Even fairings might be worthwhile to not jetison while on suborbital for use as mass driver ammo.

I never said it would be a *disorganized* dumping of debris in orbit. More likely, space programs would start generating extra debris in a new designated graveyard-orbit *MUCH* lower than current graveyard orbits (as the debris wouldn't have to remain stable nearly as long) not too far above the position of the mass driver. A lot of extra reaction mass could be obtained just by not de-orbiting spent upper stages, and using them in the mass drivers to recycle their momentum instead...

It might also make sense to establish two mass drivers and such orbits- one for polar satellites and debris (and for utilizing any debris with a greater than 45-degree inclination), and another for equatorial satellites and debris (and for utilizing debris with a less than 45-degree inclination).

And, like I said before, if you also set up a mass driver in a retrograde orbit, you could just shoot debris between the prograde and retrograde mass drivers (into special elliptical graveyard orbits for each) to ensure you never ran out of reaction mass for the mass drivers...

Regards,

Northstar

[Northstar1989]

Ahh, I see. That still seems like a false economy though. I don't have any numbers to back this up but I imagine you'd use quite a bit more propellant to harvest your debris (to use as propellant) than you would need to simply boost the satellite directly. Not to mention that you'll need to expend additional propellant to rendezvous the satellite with the mass driver.

Aside from the figures I already pointed out before (about how you can greatly leverage your initial reaction mass using this system), I also wanted to point out that mass-drivers would have GREAT synergy with a system of Propulsive Fluid Accumulators...

Essentially, if you made the debris-retrieval tugs Nitrogen-fueled instead of Xenon-fueled, you would have an essentially *unlimited* amount of reaction mass to work with by skimming additional Nitrogen off the edge of Earth's upper atmosphere. Nitrogen-electric propulsion doesn't work well for manned or deep space missions because of its very low TWR (the lighter the atoms in your exhaust stream, the better your ISP but the worse your thrust. Nitrogen is much lighter than Xenon...), but it would work GREAT for a debris-retrieval system like this where the distances and Delta-V gaps are relatively small, and the most important thing is that you don't expend more money retrieving the debris than you gain from using the mass driver to accelerate payloads...

You could also stick a couple nitrogen-electric plasma thrusters on the mass driver, and accelerate it entirely with Nitrogen skimmed off Earth's upper-atmosphere if you wanted... This would make transfer orbits to anywhere beyond Low Earth Orbit EXTREMELY low-cost (after the large initial investment cost of establishing the system), as the Propulsive Fluid Accumulator + Mass Driver system would be self-sufficient, and could impart an effectively unlimited amount of Delta-V on any departing spacecraft, limited only by the size of the mass-driver and its access to electrical energy...

Propulsive Fluid Accumulators exasperate the energy problem though- you need a way to feed them large amounts of electricity inside the edge of the upper atmosphere, where solar panels create too much drag to be feasible for this below about 150 km orbits... The BEST solution for this is probably to simply beam them power from the surface of the Earth or higher (and more stable) orbits, the same as with powering the mass driver...

Regards,

Northstar

P.S. Atmosphere-Skimming Propulsive Fluid Accumulators were another idea I proposed on my list of infrastructure projects to try and tackle in KSP. However, without that ability to utilize Nitrogen in plasma thrusters like in real life, or skim atmosphere from ABOVE the Karman Line, the system would be DRASTICALLY less effective in-game than in reality...

Edited November 1, 2014 by Northstar1989

[Bill Phil]

Well, someone saw Gravity.

Debris is not as bad as you think. Sure, it's not great, but for the last 6.5 decades, no major collisions have happened. Kessler syndrome is about debris increase, but eventually the debris becomes molecule sized, and so the problem solves itself, eventually.

[Northstar1989]

Well, someone saw Gravity.

Debris is not as bad as you think. Sure, it's not great, but for the last 6.5 decades, no major collisions have happened. Kessler syndrome is about debris increase, but eventually the debris becomes molecule sized, and so the problem solves itself, eventually.

Debris ARE a major issue, and there have been not one but TWO major collisions in the past two decades. Here's one of them:

http://en.wikipedia.org/wiki/2009_satellite_collision

And the whole point of this idea is to make debris USEFUL, as reaction mass, while simultaneously disposing of it.

Regards,

Northstar

[No one]

How are you going to find the debris?

If most of it is paint flecks and screws and stuff, won't that be really hard to see?

[Northstar1989]

How are you going to find the debris?

If most of it is paint flecks and screws and stuff, won't that be really hard to see?

Those aren't the really dangerous bits. The most dangerous debris are those "massing 1 kg or more" according to NASA. Some debris are as large as entire rocket upper stages or decommissioned satellites. Those pieces should be possible to find.

Like I've repeatedly pointed out, if there was a *use* for debris (as reaction mass), you might also see more generation of them...

Regards,

Northstar