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Instead of dumping massive garbage packs once in a while, using a robotic arm, could some sort of garbage canon eject the same mass in small pieces, at higher velocity, so that it contributes to maintain orbit of the station? (saving reboost fuel and allowing more accurate garbage's deorbiting trajectory) Has there been some studies about this?

For instance a single Nickel-Hydrogen battery cell looks pretty much like something that could fit into a barrel and propelled by some pushing mechanism*.

So could other type of garbage be split and ejected in small mass fractions. How complicated inefficient or impractical is this?

*since astronauts have to do a lot of exercise, some springs could be wound up in the process, for this purpose.

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    $\begingroup$ This is a very interesting idea-- maybe not with gas however. it seems to me that the revolution in this is the fact that no consumables are being used, or at least valuable consumables, and so the terrible ISP of such a technique could be overlooked. if pressurised gas was being used to propel the reaction mass(garbage) then it would negate this nuance. perhaps if it was just a pusher design, using a platform within the barrel to propel it, then it might work $\endgroup$ – Reuben Farley-Hall Mar 18 at 9:38
  • $\begingroup$ @ReubenFarley-Hall, you're right, i'll edit in this way. $\endgroup$ – qq jkztd Mar 18 at 10:20
  • $\begingroup$ One problem is that any kind of mechanism to do that looks a lot like a satellite-killer, unless it grinds the stuff up into very, very small particles. $\endgroup$ – tfb Mar 18 at 14:19
  • $\begingroup$ @tfb Would firing micro-pellets increase its potential use as a satellite-killer? $\endgroup$ – Michael Richardson Mar 18 at 20:24
  • $\begingroup$ Such a reboost system would need to be designed as part of a station from the beginning. I suspect, though, that the cost from the increased mass of the garbage launcher and extra structure would FAR outweigh the advantages compared to what they do now: use visiting vehicle thrusters to reboost. $\endgroup$ – CourageousPotato Mar 19 at 8:07
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this answer shows a period of time when the ISS lost only 10 meters of altitude per day, and this answer shows a period of time when it lost about 100 meters a day. Based on this kind of analysis that requires a delta-v of between 6 and 60 mm/sec. The ISS is about 400,000 kg.

update: this answer shows more extreme periods with altitude losses of between 7 and 400 meter per day! But let's use the factor of 10 range for our working space and assume that in an emergency rocket propulsion is still available.

So for the first range described the daily impulse would have to be between 2400 and 24,000 kg m/s (Newton-seconds) or about 120 to 1200 Estes "D engines" or about one "M" engine per day.

If you had for example 1 kg of waste to shoot per day, you'd have to shoot it at 2400 to 24,000 meters per second. That ranges from suborbital (it would hit the atmosphere hard in half an orbit) to escape from Earth's gravity even though it's retrograde (24,000 - 7800 - 11,200 = 5000)!

For comparison, Wikipedia's Muzzle velocity says:

Firearm muzzle velocities range from approximately 120 m/s (390 ft/s) to 370 m/s (1,200 ft/s) in black powder muskets, to more than 1,200 m/s (3,900 ft/s)[4] in modern rifles with high-velocity cartridges such as the .220 Swift and .204 Ruger, all the way to 1,700 m/s (5,600 ft/s) for tank guns firing kinetic energy penetrator ammunition. To simulate orbital debris impacts on spacecraft, NASA launches projectiles through light-gas guns at speeds up to 8,500 m/s (28,000 ft/s).

So you will need a pretty substantial cannon to do this, and videos of "tank guns firing kinetic energy penetrator ammunition" will probably show some "kick" or recoil.

The ISS' structure is minimal; it is no heavier than is absolutely necessary to hold itself together, keep the air pressure inside and astronauts alive, and hold up when the Space Shuttle grabs ahold of it gently or a capsule docks.

It's not built to withstand such kicks.

So you'd have to shoot 10 or 100 smaller masses per day.

Good luck with advertising the ISS as a viable research platform for microgravity experiments!

The alternative would be to shoot 10 or more kg of waste per day at lower velocity, but you may run out of waste unless you break that pallet of old nickel batteries up into pieces and shoot a little bits of it at a time.

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The earliest example of a rocket engine is a steam rocket, which you could scale up and heat by burning garbage. The combustion exhaust could also be used to get a few more micronewtons of thrust.

You might even be able to use waste compounds as your reaction mass (assuming a reasonable boiling point) instead of water. You probably will need a significant amount of oxidizing agent on hand, to be able to burn arbitrary junk.

Even more challenging, NASA’s Logistics Reduction and Repurposing project proposed converting the garbage to methane, and then using that in a conventional liquid-fueled rocket engine. Still need to find oxidizer somewhere.

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  • $\begingroup$ So you will have to supply water for reaction mass and oxygen to burn the garbage. This is starting to sound like the carp dinner where you cook the carp on a board, then throw away the carp and eat the board. $\endgroup$ – Organic Marble Mar 19 at 3:33
  • $\begingroup$ The steam rocket Walter HWK 109-500 was introduced in 1942, but the first flight of Aggregat 2 was in 1934. You are wrong by about 6 or more years. Aggregat 2 used alcohol and liquid oxygen like the later Aggregat 4 also known as V-2. See Wikipedia $\endgroup$ – Uwe Mar 20 at 22:00

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