Author Topic: How can developed space colonies help earthlings reach orbit?  (Read 6813 times)

Offline turbopumpfeedback2

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So let us assume the following:

there are widespread space colonies in the solar system. However, they only have  the propulsion technology that has been used on real space missions up to now.

Therefore only standard chemical propulsion and ion propulsion (current).

How could they help people from earth reach low earth orbit?
« Last Edit: 11/06/2015 07:54 pm by turbopumpfeedback2 »

Offline turbopumpfeedback2

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The question essentially is: can they help that earth launch system become reusable?

Offline KristianAndresen

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If one of these widespread colonies is in LEO, then there are a number of ways it could assist ships from Earth. While rotating to provide artificial gravity, it could sport a rotovator or skyhook. Or, if a ship from Earth was able to reach the same altitude as the colony but not quite orbital velocity, the colony could launch extra fuel along a linear accelerator just as it passes by, in such a way that the fuel pod matches velocity with the ship, enabling a fast docking.

Offline Paul451

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can they help that earth launch system become reusable?
only standard chemical propulsion and ion propulsion (current).


By "standard/current", do you mean... ?

"Provide enough fuel in LEO, at low enough cost, to allow a full propulsive deorbit burn that zeros velocity before re-entry, allowing simply subsonic re-entry"

or

"Build a rotovator (rotating tether) with a tip velocity low enough (relative to the ground) to allow a reusable (suborbital) SSTO to transfer cargo to orbit"

Offline QuantumG

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Suppose there was cheap refuelling available in LEO, does your second stage rocket now need heat shielding for descent? Assuming ~5 km/s of delta-v on your second stage, retropropulsion could make reentry as effortless as a suborbital flight. If you suppose some more LEO infrastructure, you could actually fly your second stage on a suborbital trajectory and provide the final circularization burn with on-orbit propellant. This allows your first stage to be bigger - and easier to reuse. A lot of the problems of a fully reusable two-stage rocket go away if there's cheap propellant and relevant infrastructure in LEO. Not to mention that a LEO-to-elsewhere infrastructure makes it possible to focus the reusable launch system on the ground-to-LEO leg.
Human spaceflight is basically just LARPing now.

Offline KelvinZero

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can they help that earth launch system become reusable?
only standard chemical propulsion and ion propulsion (current).


By "standard/current", do you mean... ?

"Provide enough fuel in LEO, at low enough cost, to allow a full propulsive deorbit burn that zeros velocity before re-entry, allowing simply subsonic re-entry"

or

"Build a rotovator (rotating tether) with a tip velocity low enough (relative to the ground) to allow a reusable (suborbital) SSTO to transfer cargo to orbit"
I would interpret rotovator as too far ahead for the OP in which case propellant for deorbiting seems a nice trick to make our reusable vehicles suffer less stress and last longer.
Also:
(*) Having more destinations/ infrastructure in orbit might allow more launches from more locations pretty much any time, and missing windows might only add minutes of waiting. Perhaps we can also reduce the time between launching and arriving at a LEO station, allowing the launch vehicle to be as minimalist as possible. Pretty much as soon as the rockets stop burning you are disembarking again.

(*) This might be a bit too obvious, but of course the main way by far that a colonised solar system would help people reach low orbit is by providing destinations that justify airline-like operations with both reusable vehicles AND production lines that do not get shut down after only a handful are built.

um.. I want to add a new one but then I will need to rationalise it fits into the OP..
I have always liked the idea of a magnetic rail in orbit that goes around the entire earth. Sort of an orbital loop but without the tower. It could be used for accelerating suborbital vehicles up to orbital velocity and also doing the reverse, negating the need for a deorbit burn while also negating the need for station keeping propellant.

The reason I think this might fit in the OP whereas the rotovator might not is that we already do have magnetic rails that move things at pretty high velocity and we use them daily. It is just that they are sitting on the earth. I argue it is only the absurd scale that would push this even further into scifi-land than a rotovator in most people's minds.
 
A rigid ring would not be stable in orbit so this would require some new approach that is harder to fit in the OP. We could start much smaller though, perhaps a rail of a few hundred km, and maybe we just use it for internal commuting, dropping garbage (and station-keeping), helping just a bit with deorbiting vehicles at first. There could be a natural evolution between exploiting magnetic rail just a bit and the whole monstrous orbital ring thingy.
« Last Edit: 11/07/2015 12:29 am by KelvinZero »

Offline original_mds

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1. Bulk transfer fuel/volatiles to Earth Orbit from celestial sources (e.g. bulk tankers of hydrocarbons from Titan, oxidizers from refining or other sources)

2. Beamed power satellites for improving mass fractions of shuttles (e.g. via electric turbine stages that leverages lessons learned with the US DOD experiments with nuclear-powered jet engines)

3. Provide transfer stations for LEO

4. Provide other finished goods required for living in space to LEO to reduce per-person required up-mass
Good things are coming.

Offline turbopumpfeedback2

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can they help that earth launch system become reusable?
only standard chemical propulsion and ion propulsion (current).


By "standard/current", do you mean... ?

"Provide enough fuel in LEO, at low enough cost, to allow a full propulsive deorbit burn that zeros velocity before re-entry, allowing simply subsonic re-entry"

or

"Build a rotovator (rotating tether) with a tip velocity low enough (relative to the ground) to allow a reusable (suborbital) SSTO to transfer cargo to orbit"

Rotating tether is ok as long as the material used was used in real applications with lengths of at least few meters.

Offline Paul451

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Rotating tether is ok as long as the material used was used in real applications with lengths of at least few meters.

A rotovator doesn't have as extreme requirements as a full "space elevator". Much shorter, much lower mass, much more reasonable strength. Usually within reach of kevlar, spectra, or similar fibres already manufactured in bulk.

A non-rotating "skyhook" is even easier. Much lower strength requirements, and tidal forces keep it stable. Plus you can use the Earth's magnetic field as the engine to raise the tether's orbit - no fuel required! However, the velocity gained by the incoming payloads is much lower for any given mass of tether. And as with any orbital infrastructure, you are limited to a single orbital plane (or, more likely, equatorial orbit). It may simply never be worth the money.

I have always liked the idea of a magnetic rail in orbit that goes around the entire earth. Sort of an orbital loop but without the tower. It could be used for accelerating suborbital vehicles up to orbital velocity and also doing the reverse, negating the need for a deorbit burn while also negating the need for station keeping propellant.

Remember that the axis of rotation during the magnetic acceleration of payloads, is on the opposite side of the ring from the point of acceleration, not at the centre. Therefore the ring will be constantly pushed out of orbit (even more than the inherent instability issues with a rigid ring.)

A short orbiting mag-rail (several hundred kilometres "short") would serve as a momentum-exchange engine. Using a high-efficiency electric drive to gain (or lose) orbital altitude, then transfer it to the payload in a burst. The more massive the better, and hence the need for non-terrestrial resources.

It could de-orbit launch vehicles (as I proposed with simple orbital refuelling), gaining velocity and altitude. Or kick payloads into higher orbits, losing velocity and altitude.

It could even, in theory, "catch" suborbital launches that reach orbital height (but not velocity), and "decelerate" them up to orbital velocity. But... damn...

If you suppose some more LEO infrastructure, you could actually fly your second stage on a suborbital trajectory and provide the final circularization burn with on-orbit propellant.

Is the "more infrastructure" an orbital ship which drops into an eccentric orbit to intercept the incoming payload and accelerate it back to orbit before it hits the atmosphere?

If not, how does orbital fuel help a launch vehicle?
« Last Edit: 11/07/2015 02:37 am by Paul451 »

Offline turbopumpfeedback2

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Thanks for great replies!

Suppose there was cheap refuelling available in LEO, does your second stage rocket now need heat shielding for descent? Assuming ~5 km/s of delta-v on your second stage, retropropulsion could make reentry as effortless as a suborbital flight. If you suppose some more LEO infrastructure, you could actually fly your second stage on a suborbital trajectory and provide the final circularization burn with on-orbit propellant. This allows your first stage to be bigger - and easier to reuse. A lot of the problems of a fully reusable two-stage rocket go away if there's cheap propellant and relevant infrastructure in LEO. Not to mention that a LEO-to-elsewhere infrastructure makes it possible to focus the reusable launch system on the ground-to-LEO leg.

This refueling of the second stage is really simple but very effective.

This could help the Space Shuttle. Just refuel the external tank in orbit,  make a backward burn for deorit. Then those heat resistant tiles can be strip off ... And land ET with a parachute.

Offline KelvinZero

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Remember that the axis of rotation during the magnetic acceleration of payloads, is on the opposite side of the ring from the point of acceleration, not at the centre. Therefore the ring will be constantly pushed out of orbit (even more than the inherent instability issues with a rigid ring.)
I was imagining free orbiting components, certainly nothing rigid. Also heavy. LEO Suburbs and industries all accessable by the same 'rail' (though not really just a rail since it is not a single rigid thing) So hopefully there would be time to average out the effects of outgoing and incoming vehicles, both to-from the earth and to-from the solar system.

You could also have massive vehicles moving around only slightly above and below the ring velocity just to redistribute momentum as required.

The final product would be complicated Im sure. We would just start smaller as you said. I see a ring as more of a final evolution if LEO just gets ever more crowded.

Offline QuantumG

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Is the "more infrastructure" an orbital ship which drops into an eccentric orbit to intercept the incoming payload and accelerate it back to orbit before it hits the atmosphere?

That's what I was thinking of, yes. There's other, more creative methods too.
Human spaceflight is basically just LARPing now.

Offline Robotbeat

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Like rotovators.
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Offline KelvinZero

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This one is another huge stretch to fit within the OP but one method we already use extensively to change the velocity of spacecraft is aerobraking.. so..

..now, admittedly we have not developed anything like the technology to place a thousand-kilometer long column of air (or any other material from lunar dust to candyfloss) in an eccentric orbit..

..but if we did, we could claim that the method of the vehicle that can exploit it is already established and in practice, alongside chemical and ion propulsion :)

Offline AlanSE

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um.. I want to add a new one but then I will need to rationalise it fits into the OP..
I have always liked the idea of a magnetic rail in orbit that goes around the entire earth. Sort of an orbital loop but without the tower. It could be used for accelerating suborbital vehicles up to orbital velocity and also doing the reverse, negating the need for a deorbit burn while also negating the need for station keeping propellant.

The reason I think this might fit in the OP whereas the rotovator might not is that we already do have magnetic rails that move things at pretty high velocity and we use them daily. It is just that they are sitting on the earth. I argue it is only the absurd scale that would push this even further into scifi-land than a rotovator in most people's minds.
 
A rigid ring would not be stable in orbit so this would require some new approach that is harder to fit in the OP. We could start much smaller though, perhaps a rail of a few hundred km, and maybe we just use it for internal commuting, dropping garbage (and station-keeping), helping just a bit with deorbiting vehicles at first. There could be a natural evolution between exploiting magnetic rail just a bit and the whole monstrous orbital ring thingy.

In what way, exactly, do you have in mind that an orbital ring is unstable? It kind of is, but it's mathematically more complex than a simple yes/no answer. If you consider movement that changes the orbital plane (left or right, so to speak), then it's neither here nor there. If a section of the ring is thrown off course, then it oscillates, but the oscillations do not grow. They just continue to oscillate.

Movement that affects altitude and orbital velocity should be called unstable, because those cause different parts of the ring to have different periods.

How bad would the instabilities be if you could put the ring under slight tension? I'm not talking about tension like in a space elevator, I have in mind a conventional nylon tether. The goal isn't to fundamentally hold its orbital position in an unnatural state. Just provide some pull to dampen out unwanted movements in localized sections.

This, alone, wouldn't be enough. You would need some inertial dampening and computer control systems. If you launched something to/from a suborbital trajectory, you may need time for the tether to transfer that impulse to the entirety of the ring. If you didn't, the quick tug could break it. So instead, you have large space stations attached to the tether, and you allow the relative position of the stations to drift slightly. You have pulleys control the forces exerted on the stations so that they move slowly and anticipate future launches and catches. None of these things I've described are easy, but space pulleys and computer stability analysis are very modest development hurtles for a mega-structure. It's the actual catch/launch mechanisms (that can work at orbital velocities) that are the real problem.

Offline Paul451

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Is the "more infrastructure" an orbital ship which drops into an eccentric orbit to intercept the incoming payload and accelerate it back to orbit before it hits the atmosphere?
That's what I was thinking of, yes. There's other, more creative methods too.

Well? Don't keep it a secret, elaborate. It seems to be what the OP started the thread for. Other than skyhooks, rotovators and sub-orbital docking, what else is there that lowers the fuel requirements of a second stage?

In what way, exactly, do you have in mind that an orbital ring is unstable?

A rotating ring isn't orbiting. Eventually tidal/gravitational perturbations will cause it to drift until one side brushes the planet, destroying the ring. A ring will therefore need constant maintenance. But the requirements to maintain the ring's position are a tiny fraction of those required to build the thing, so it's not much of a burden unless you're aiming for it to survive the fall of the civilisation that build it.

However, it will additionally experience vibration and resonance issues within the structure itself. The length is so huge that it will wobble like crazy. That may exceed our ability to build a planet-spanning ring with any realistic material, even if we had the industrial capacity. But I don't believe that was the sort of instability KZ was talking about.

Offline KelvinZero

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A rotating ring isn't orbiting. Eventually tidal/gravitational perturbations will cause it to drift until one side brushes the planet, destroying the ring. A ring will therefore need constant maintenance.
Yes my knowledge is pretty much limited to the famous anecdote concerning Larry Niven's Ringworld, that resulted in giant engines being strapped to it in a later story :)

There are all sorts of variations and I expect there is one that avoids station keeping propellant, but it probably needs serious micromanagment.

Offline Asteroza

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From an involved infrastructure standpoint, a reverse bomber catching suborbital craft is the lowest mass option.

Whether a HASTOL setup, with a suborbital vehicle and a rotorvator, beats a laser SPS firing on a beamed power heat exchanger SSTO is up for debate, though if there is already substantial infrastructure in orbit to support industry, a beamed power infrastructure may be by default available and retaskable. Though the rotovator is attractive if there is substantial downmass in traffic flow due helping surface dwellers...

Offline Hanelyp

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Looks to me like a reverse bomber would need (back of envelope) twice the delta-V it delivers to the payload.  It needs to first slow to match the payload, then to accelerate again to a stable orbit.  And at higher than high ISP engine acceleration.  Unless the reverse bomber is getting propellant cheap from somewhere other than the ground this doesn't work out economically.

The brilliance of tether assist is the tether doesn't need to drop momentum to meet up with the payload, and can then use a low thrust system to make up the momentum it transfers.

Offline turbopumpfeedback2

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Looks to me like a reverse bomber would need (back of envelope) twice the delta-V it delivers to the payload.  It needs to first slow to match the payload, then to accelerate again to a stable orbit.  And at higher than high ISP engine acceleration.  Unless the reverse bomber is getting propellant cheap from somewhere other than the ground this doesn't work out economically.

I agree that a reverse bomber (funny name :-) ) delivers half of its total dv to suborbital payload. Probably in the best case.

Actually, if the reverse bomber is in circular orbit and then deaccelerates, its distance to the planet decreases, while payloads distance to planet increases.

So it will need to make a nontrivial maneuver to match the speed vector of the payload. This would be interesting to optimize in a simulation. Maybe I do it during some of the future weekends.

The assumption is that space colonists aim to lower the cost to orbit for the people on earth. Let's say they I willing to give fuel, but do not want to give up reusablity of their vehicles. 
« Last Edit: 11/09/2015 10:12 pm by turbopumpfeedback2 »

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