Max,

Firstly, the power expended by an ion drive is primarily dissipated by accelerating the ions - not forming them.

Secondly, the ion densities in either region are many orders of magnitude lower than are found in commercial drives. H+ densities of the order of 10^10 per m^3 are common, with ion drives running at about 10^20 ions per m^3.

Thirdly, the vanishingly low densities require a large 'collector' - which in turn incurs mass penalty.

In short, you're far better off carrying a small tank of Xenon.

James,

lets get down to the math. In the orbit of the ISS the atmospheric density is 10-11kgm-3 . Lets have a spacecraft with collecting dimensions of 20m by 20m that's an area of 400m2 , if we are moving at 8kms-1 we shall be covering a volume of 3,200,000m3 per second, in a day we shall have covered a volume of 2.8*1011m3 . so to get how much mass we shall have collected in a day we shall get the density 10-11kgm-3 multiplied by the volume 2.8*1011m3 .

So in just a day we shall have collected 2.8kg of propellant.

When collection is over the collectors can be retracted to reduce on drag.

Clearly that's feasible and should be implemented.

Max,

Nothing wrong with your figures - but may I ask how you propose to 'collect' a neutral object travelling at 8km/s?

I used to work at an institute with a 2-stage light gas gun - we worked on the Stardust mission - and I scratch my head as to how one might coerce a nitrogen molecule to do anything other than embed itself into a chunk of aluminium, or sputter off an atomic cluster. Glancing collisions don't really exist at that speed.

The situation is *not* hydrodynamical.

James,

for ions we don't have to catch them we can let them pass through our electromagnetic field and give them a boost in their direction of travel this is similar to ramjets.

For collecting ions and neutral atoms we shall create a vacuum to suck them in, this could be placed facing opposite to the orbital direction. This will enable propellant collection.

May be shock absorbers like aerogel or an air tight flexible material like a balloon could also be used but am not so certain.

This system should be tested.

Max,

>For collecting ions and neutral atoms we shall create a vacuum to suck them in,:

:D

You are proposing a pump array 20m x 20m in size. Let's say we use off-the-shelf items to keep the cost down.

A turbomolecular pump (I've used quite a few in my time) with a mouth about 10cm in diameter will weigh about 3 or 4 kg, including its controller. So, that's 140 tonnes of pumping gear.

I think that I cannot agree with your statement that, "Clearly that's feasible and should be implemented."

James,

I still don't understand how you got the mass of 140 tonnes.

The number of particles collected will depend slightly on the area of the mouth of the pump but mainly on how high a vacuum we can create.

The surroundings are already a high vacuum. You cannot "suck", creating a vacuum causes the environment to "push" the material towards you, but there's no useful "push" in space.

However, you're missing a key point, what is the change of momentum for your craft if you collect 2.8kg of mass travelling at 8km/s? You've got an effective brake there, useful for de-orbiting but not much else.

To use that material for thrust, you then need to accelerate it to greater than 8km/s, how much power does that take? Compare with a much smaller craft carrying its own reaction mass.

George,

from what I know we can create vacuums of density 10-18 kgm-3 and we are in a region of density 10-11 kgm-3, this pressure difference will suck in the atoms and ions into the spacecraft.

Ion thruster engines have exhaust velocities of 30kms-1 which is far more than 8kms-1.

I think there are further issues, that would make it troublesome.

1) What kind of ions would you "collect"? Propellantwise, heavy ions would be beneficial for the thrust/ impulse. In any case I doubt (but am not sure) that an engine could be designed to deal with a random "mixture" of ions.

2) Also regarding impulse. you neglect the impulse of the ions you are collecting - they are moving along the field lines of the magnetic fields, which will most likely not be parallel to your intended velocity vector, so you actually have to change their direction. Also they will likely impact your spacecraft, which might be damaging, in any case mean impulse exchange in a non-desired direction.

3) Regarding the sucking in - you are relating two different things. You are relating the two static pressures. But first off all these atmospheric regions cannot be handled like a continuous fluid and even if you do that, you still have to regard the dynamic pressure as well (caused by the velocity of the ion) to receive the total pressure.

Especially the fact that any interaction between the ion and the spacecraft means an exchange of impulse and the fact that for turning the velocity vector of the ion into the desired direction you require not neglactable energy, I doubt that this is "obviously" feasible.

Besides your idea has been played with for interstellar propulsion, they are called Bussard Collectors, resp. Bussard Ramjet.

Max,

>from what I know we can create vacuums of density 10-18 kgm-3

Not really.

A good turbo, with a scroll backing pump will get you 10^-9 mbar.

1 bar is ~1kg/m^3, so that's a density of 10^-12 kg/m^3.

To cover an area of 20m x 20m, you need 100 turbos per m^2 (each the size of a large mug) which leads to the loony-tunes mass I quoted.

One cannot have a large funnel and a single collecting head to compress the gas to a usable density. This is a free-molecular flow regime - the molecules are ballistic and have mean free paths larger than the scoop you propose (as Volker correctly pointed out).

They will not 'flow' when a pressure differential is applied - as they cannot 'sense' that differential as they don't collide with each other in transit. No amount of vacuum will 'suck' them.

Do you not recall the Wake Shield free flyer? Or the reason why LDEF had a particular attitude?

May I suggest that the easiest thing to do now is admit that this is a poor idea?

I'd respect that decision.

ME: Ion thruster engines have exhaust velocities of 30kms-1 which is far more than 8kms-1.

Indeed but ion thrusters use xenon which has a mass of 131 AMU, you would be collecting mostly hydrogen. For the same thrust, that means you need 131 times as much electrical power.

Thanks for all the answers, I have one last question.

Is it possible to ionize the incoming atoms using electromagnetic waves so that on approach we apply an electromagnetic field to trap them?

Max,

Bussard thought of this a long time ago, using a UV laser to ionize inbound neutral hydrogen so that the eponymous ramjet could gather fuel.

Alas, even fusion turned out to be insufficiently energetic to make this anything other than an elaborate brake.