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Collecting gas from a planet by skimming the atmosphere


SomeGuy123
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Ok, so I was going to ask if it's even possible to do, but I think I have an idea of how to do it. 

The trick is, you have to burn less gas in propellant than you gain by skimming the atmosphere of a planet, or there's no point.  

So what you do is, you start out in a circular orbit.  You do a burn that lowers the periapsis until it just barely skims the top of the atmosphere.  Your spacecraft is basically a cylinder, with a conical maw at one end.  You try to collect all the gas that hits the maw you can.  Some fins are used to keep the maw facing prograde.

So you skim the planet, gaining gas.  Basically it's like a rocket engine in reverse - the momentum change is the velocity of the gas * (the mass of gas collected + mass of gas that bounces off your craft and fails to be scooped).

So what you need to do now is use some of that gas to regain the velocity you lost.  You must have a rocket engine with an exhaust velocity much higher than your orbital velocity when you skimmed the atmosphere. 

So you have a space station that you flyby at apoapsis.  It has a big nuclear reactor or solar array, and it beams power or laser light to your spacecraft as it flies by at apoapsis.  In elliptical orbits like this, this part of the orbit lasts a long time, so there's plenty of time, and you want to fly by as close as possible to the power transmitter.

You use the beamed power to run some kind of high efficiency plasma rocket to raise your periapsis to what it was before you did this maneuver, and then you repeat it.

Once your spacecraft is done collecting gas in repeated orbits, you use the same beamed power station flyby to circularize your orbit again, then you do more flybys to do the burns to transfer to the destination for the gas.  

If the gas collection missions is to collect specific gases - like helium-3 or oxygen or whatever - you'd want to process the gas onboard and consume the unwanted gases as propellant.

Doable?  Could you set up the orbits so you always make a very close flyby the power beaming station at every apoapsis?

Also, I figured that Jupiter would be much harder, but looking at this, I don't know if it matters that much.  Jupiter does have a higher orbital velocity, but as long as you just barely skim the edge of Jupiter's atmosphere, you'll be fine.  You would need a faster plasma rocket engine to regain the velocity you lost with each orbit but it's probably quite doable.  It might take more orbits to collect the same amount of gas, however - the gas you encounter skimming jupiter's atmosphere at orbital velocity would be much hotter than skimming Neptune or Earth or Venus.

Edited by SomeGuy123
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Jupiter has a low orbit velocity of 40km/s (higher if you want an elliptical orbit). To benefit at all from scooping up the gas, you'd need an exhaust velocity of at least 40km/s, or an Isp of 4,000. That's into ion engine territory, well above the Isp you'd get from a thermal rocket.

You could probably just about break even on earth though.

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The slowdown at the scooping phase near periapsis would tend to drop your apoapsis more than your periapsis. That alone rules out a single orbit scoop-raise-scoop cycle. Multiple orbit cycles, like scoop-raise pe-raise ap-scoop cycles might work, but you will have to burn at both apses (is that the correct plural?) to stop you from eventually (re-)entering. If you can burn to keep your apoapsis while scooping and still gain propellant you could do single loops, but I suspect a low circular orbit might work better then.

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19 hours ago, Emperor of the Titan Squid said:

a bussard ramjet, with magnetic scoops

Bussard ramjets have been found to create more drag than thrust in their original role of relativistic interstellar flight. I doubt they'd do better in... thicker waters.

And the more advanced versions (such as the hydrogen-boron fusion scramjet) need relativistic velocities to work.

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7 minutes ago, DDE said:

Bussard ramjets have been found to create more drag than thrust in their original role of relativistic interstellar flight. I doubt they'd do better in... thicker waters.

And the more advanced versions (such as the hydrogen-boron fusion scramjet) need relativistic velocities to work.

Ironically, this may in fact be a feature, not a bug.  There's various schemes that would let you launch a spacecraft from the solar system starting at moderately relativistic velocities.  A gigantic mass driver or cyclotron around the sun or an iron pellet beam you'd ride to get up to speed.  It's slowing down that's the hard part, not getting up to interstellar cruise speed.   (because if your spacecraft starts out at 0.1 C, leaving sol, getting to that speed without consuming any onboard fuel, it's a lot easier to manage the rocket equation)

Edited by SomeGuy123
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46 minutes ago, SomeGuy123 said:

Ironically, this may in fact be a feature, not a bug.  There's various schemes that would let you launch a spacecraft from the solar system starting at moderately relativistic velocities.  A gigantic mass driver or cyclotron around the sun or an iron pellet beam you'd ride to get up to speed.  It's slowing down that's the hard part, not getting up to interstellar cruise speed.   (because if your spacecraft starts out at 0.1 C, leaving sol, getting to that speed without consuming any onboard fuel, it's a lot easier to manage the rocket equation)

But that's a magnetic sail and not a ramjet... although in theory it could be used as a retromotor.

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4 hours ago, DDE said:

But that's a magnetic sail and not a ramjet... although in theory it could be used as a retromotor.

Better to use the hydrogen / deuterium you capture in an fusion engine to generate power to boost the sail, the rest you use as an retromotor but this is mostly to get rid of it, as an bonus it might ionize hydrogen in front of you 

On 22.8.2016 at 7:08 PM, peadar1987 said:

Jupiter has a low orbit velocity of 40km/s (higher if you want an elliptical orbit). To benefit at all from scooping up the gas, you'd need an exhaust velocity of at least 40km/s, or an Isp of 4,000. That's into ion engine territory, well above the Isp you'd get from a thermal rocket.

You could probably just about break even on earth though.

Only a point if you are after helium / helium3, for hydrogen its easier to go after ice moons who all gas giants have. 
 

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Just put an inductor on a low near-Sun orbit, and a scoop inside.
The Sun magnetic field induces the power in the inductor, the power powers a magnetic scoop, the magnetic scoop attracts and sucks the charged solar wind into a bag.
The hydrogen dissipating from the bag makes a mix with the solar wind and gets resucked back into the bag.

Delta-V budget would be nearly the same, but you don't need to dive.

Edited by kerbiloid
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  • 2 weeks later...
On 8/23/2016 at 11:30 AM, DDE said:

But that's a magnetic sail and not a ramjet... although in theory it could be used as a retromotor.

Yes, sorry.  I was implicitly assuming - and if you think about it, any practical starship has to have this technology - that you leave Sol using a "magnetic sail".  (it's more like a gauss gun shaped engine)  Then, in flight, robots tear down the magnetic sail engine, toting the parts into plasma furnaces, and the ship manufactures using mainly the elements from that engine the ramscoop braking engine.  

This is also how you respond to things becoming damaged from particle impacts or wearing out over an interstellar voyage that might take centuries.  You have to have the ability to manufacture any part on the entire ship, so that nothing is irreplaceable, and there must be no single modules that will cause the ship to fail if they shut down.  Obviously, a major impact or an engine explosion would still send enough pieces of your ship into the void to probably doom the mission but anything short of that would be recoverable.  And of course the beings riding the ship need to be just as modular, their minds distributed across multiple blocks of circuitry where no block is critical or contains the only copy of important information.

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2 hours ago, SomeGuy123 said:

This is also how you respond to things becoming damaged from particle impacts or wearing out over an interstellar voyage that might take centuries.  You have to have the ability to manufacture any part on the entire ship, so that nothing is irreplaceable, and there must be no single modules that will cause the ship to fail if they shut down.

I'm afraid that the erosion even from tiny impacts will irreversibly deplete your supply of matter. If you have matter to spare, you should put it into the shield in the first place.

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On 9/1/2016 at 0:13 PM, DDE said:

I'm afraid that the erosion even from tiny impacts will irreversibly deplete your supply of matter. If you have matter to spare, you should put it into the shield in the first place.

Wait, what?  No.  There's studies on this for the Daedalus project.  Impacts are rare, the problem is they damage the shield.  And every now and then, black swan impacts can go through the shield and hit other places.  That's why your ship has to be able to fix itself and also not have any key parts.  Your logic doesn't even follow - if erosion is that fast, a thicker shield won't help.  The reason for self repair is because if all the impacts hit a particular part of the shield by sheer chance, you can move mass from spares or the rest of the shield to shore up the damaged area.

For THAT matter, there's other approaches.  Damping layers can cause the fragments of an impact to stick inside the shield so you don't lose matter.  You would need to restore these damping layers after each hit.  Pristine shield components would be far stronger at tolerating impacts that damage components, losing less matter per impact, so being able to rebuild the shield is also critical.

You could also use electric charge and other more complex mechanisms to aid things.  And collect interstellar gas, fusion it to heavier elements, and use it to restore your supply of matter.

Edited by SomeGuy123
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