Boosted Orbital Tether and Orbital Runway upgrades

[MatterBeam]

Hi!

I've come up with four ideas for cheap access to space, involving orbital tethers.

The first has already been thought up of: the orbital tether pulls a suborbital tether into orbit. However, my variant has the tether station itself being used as a platform which can change its orbit to increase or decrease rendezvous velocity.

The second involves a tether being used as a 'runway': the tether trails behind a space station. A spacecraft latches onto the tether and starts braking. This speeds up the spacecraft and slows down the space station, effectively pulling the spacecraft from a suborbital trajectory into orbit.

The third involves flywheels. The runway is attached to flywheels spinning in the opposite direction. When the spacecraft brakes, it pulls on the flywheels and slows them down, instead of slowing down the space station. The flywheels are kinetic energy reserves that allow for propellantless operation. 

The fourth involves replacing the tether with a series of pulleys. The relative velocity betweent he spacecraft and the tether can be very high in a simple runway, making braking hard. With the pulleys, the tether 'expands' like a bungee cord. The relative speed between spacecraft and tether is divided by the number of pulleys for easy braking.

These ideas and designs are described and illustrated with calculated examples here:

Boosted Orbital Tether

Orbital Runway upgrades

[p1t1o]

The idea of "braking" on a "runway" in space really confused me until I clicked the link!

I was like "But what about the landing pad for my space helicopter?"

Im not a big fan of tethers, but thats just because they bug me, not because I have anything against the science.

[MatterBeam]

Just now, p1t1o said:

The idea of "braking" on a "runway" in space really confused me until I clicked the link!

I was like "But what about the landing pad for my space helicopter?"

Im not a big fan of tethers, but thats just because they bug me, not because I have anything against the science.

I first described it as a fishing line in space...

[FleshJeb]

I just gave them a brief read-through, but I think you might be ignoring conservation of momentum with the flywheels.

There are some very interesting ideas related to tethers and mag-sailing in this famous story: http://www.davidbrin.com/tankfarm.htm

[MatterBeam]

2 hours ago, FleshJeb said:

I just gave them a brief read-through, but I think you might be ignoring conservation of momentum with the flywheels.

There are some very interesting ideas related to tethers and mag-sailing in this famous story: http://www.davidbrin.com/tankfarm.htm

A spaceship massing 10 tons and travelling at 8450m/s relative would have 84.5MN/m of momentum. My calculations give the flywheels a total angular momentum of about 1575MN.m according to http://calculator.tutorvista.com/angular-momentum-calculator.html

The flywheels have more than enough momentum. Kinetic energy is used on a 1:1 basis, minus losses to braking, and they have a decent reserve for that too.

Edited January 27, 2017 by MatterBeam

[YNM]

I'm not very sure, but you know how hard it is to rendezvous slowly in space, right ? Wouldn't it be even harder in high relative velocity ?

[Bill Phil]

3 hours ago, YNM said:

I'm not very sure, but you know how hard it is to rendezvous slowly in space, right ? Wouldn't it be even harder in high relative velocity ?

Rendezvousing isn't the hard part (except for how unintuitive it is). The hard part is docking. But the Russians have had that automated for a long time now. The real difficulty is getting up there...

But it would be way harder to do this at high relative velocity. But if it could be done, it would be a much more feasible option than a space elevator and potentially very useful...

[magnemoe]

4 hours ago, Bill Phil said:

Rendezvousing isn't the hard part (except for how unintuitive it is). The hard part is docking. But the Russians have had that automated for a long time now. The real difficulty is getting up there...

But it would be way harder to do this at high relative velocity. But if it could be done, it would be a much more feasible option than a space elevator and potentially very useful...

Rendezvousing is not hard at low speeds, note that real life is far slower than standard in KSP, in KSP we often eyeball the encounter then do a 20-40 m/s rendezvous burn, and an correction burn and get close during next orbit. 

Here the encounter speed is many km/s, just hitting something at this speed as in anti satellite or anti icbm weapons is hard. Here you are supposed to grab an wire so both timing, vector and angle of attack are critical, because the speeds and because you are on an different trajectory than the wire will give an window of a 3 seconds with an 20 km wire and you do not want the wire to hit the ship. 
Miss and you will continue suborbital. 

Edited January 27, 2017 by magnemoe

[MatterBeam]

On 27/01/2017 at 1:45 AM, YNM said:

I'm not very sure, but you know how hard it is to rendezvous slowly in space, right ? Wouldn't it be even harder in high relative velocity ?

You only need to cancel velocity along X and Y axis, so that the very high Z-axis velocity will hit you dead center.

On 27/01/2017 at 4:56 AM, Bill Phil said:

Rendezvousing isn't the hard part (except for how unintuitive it is). The hard part is docking. But the Russians have had that automated for a long time now. The real difficulty is getting up there...

But it would be way harder to do this at high relative velocity. But if it could be done, it would be a much more feasible option than a space elevator and potentially very useful...

The 'boosted' part of the name comes from the initial rocket booster than pushes it up to 1000km altitude. 4500m/s is required, but that's only a mass ratio of 3.8. Getting to orbit requires roughly 9000m/s, or a mass ratio of 13.8. So for every ton the BOT puts into orbit, you save 10 tons compared to a full two-stage rocket.

On 27/01/2017 at 9:36 AM, magnemoe said:

Rendezvousing is not hard at low speeds, note that real life is far slower than standard in KSP, in KSP we often eyeball the encounter then do a 20-40 m/s rendezvous burn, and an correction burn and get close during next orbit. 

Here the encounter speed is many km/s, just hitting something at this speed as in anti satellite or anti icbm weapons is hard. Here you are supposed to grab an wire so both timing, vector and angle of attack are critical, because the speeds and because you are on an different trajectory than the wire will give an window of a 3 seconds with an 20 km wire and you do not want the wire to hit the ship. 
Miss and you will continue suborbital. 

You can shoot out the last loop of wire to match the velocity of the incoming spacecraft and greatly extend the capture window.

[Snark]

On 1/26/2017 at 10:40 AM, MatterBeam said:

The first has already been thought up of: the orbital tether pulls a suborbital tether into orbit. However, my variant has the tether station itself being used as a platform which can change its orbit to increase or decrease rendezvous velocity.

The second involves a tether being used as a 'runway': the tether trails behind a space station. A spacecraft latches onto the tether and starts braking. This speeds up the spacecraft and slows down the space station, effectively pulling the spacecraft from a suborbital trajectory into orbit.

How does this make the access to space cheaper?  Yes, it saves dV for the spacecraft... but at the expense of borrowing momentum from the tether station.  Instead of the spaceship needing to spend fuel to rendezvous with the station, the station itself will need to spend fuel to maintain its orbit.  How does this save cost?

On 1/26/2017 at 10:40 AM, MatterBeam said:

The third involves flywheels. The runway is attached to flywheels spinning in the opposite direction. When the spacecraft brakes, it pulls on the flywheels and slows them down, instead of slowing down the space station. The flywheels are kinetic energy reserves that allow for propellantless operation.

The problem isn't kinetic energy.  It's momentum conservation.

On 1/26/2017 at 6:04 PM, FleshJeb said:

I think you might be ignoring conservation of momentum with the flywheels.

^ This.

On 1/26/2017 at 8:42 PM, MatterBeam said:

A spaceship massing 10 tons and travelling at 8450m/s relative would have 84.5MN/m of momentum. My calculations give the flywheels a total angular momentum of about 1575MN.m according to http://calculator.tutorvista.com/angular-momentum-calculator.html

The flywheels have more than enough momentum.

[Emphasis added by Snark]

(Linear) momentum and angular momentum are different things, completely.  Apples and oranges.  They have different units.

Linear momentum is conserved.  Angular momentum is also conserved.  But you can't convert one to the other; they're not interchangeable.

You're going to have to spend some propellant somewhere.

[Bill Phil]

58 minutes ago, Snark said:

How does this make the access to space cheaper?  Yes, it saves dV for the spacecraft... but at the expense of borrowing momentum from the tether station.  Instead of the spaceship needing to spend fuel to rendezvous with the station, the station itself will need to spend fuel to maintain its orbit.  How does this save cost?

The tether station can be made to have a large momentum. This can be done in different ways. Increase mass or increase the semi-major axis of the orbit. Yes, it will lose momentum. But that can be provided by the occasional boost (which is mentioned in the article). It may or may not make space travel cheaper. But the margins on launch vehicles will be lessened. Heck, if you put it into a huge orbit, you can use it a bunch of times before you need to reboost it. Only trouble is a distinct lack of launch opportunities for a long time between praises. It could work. Should we build it? That's a question for the people with the money.

Orbital momentum is angular momentum...

It's another form of momentum exchange tether. It exchanges momentum.

[Snark]

1 hour ago, Bill Phil said:

The tether station can be made to have a large momentum. This can be done in different ways. Increase mass or increase the semi-major axis of the orbit. Yes, it will lose momentum. But that can be provided by the occasional boost (which is mentioned in the article). It may or may not make space travel cheaper. But the margins on launch vehicles will be lessened. Heck, if you put it into a huge orbit, you can use it a bunch of times before you need to reboost it.

Well, sure.  It just means you have to spend all the money ahead of time (to launch the station in the first place), and then you can save money on the spaceships for a while until you have to boost the station again, which will need more money.  You still have to boost, and the amount of momentum you need is precisely equal to the amount of momentum that the ascending rockets saved.

So there's no net savings, and unless you've got something else going on to save fuel somehow, it won't save any money in the long run-- which is what the OP was suggesting.

To make a financial analogy:  Let's say you have two people, A and B.  Both are trying to earn a living.  This requires long, hard hours working at a thankless job.

• A works 12-hour days, every day.  He comes home exhausted and miserable and is wiped out all the time.
• B sees that and says, hey, that's no fun.  I need a better strategy.  So what I'll do is save up a year's worth of expenses in the bank.  Then after that, I can live the same lifestyle as A, but I only have to work 8-hour days instead of 12-hour days, because I'll just withdraw a bit of cash from the bank each time.  See?  I save all the misery!  And I can keep doing that until the bank account runs low, then I'll have to fill it again.

How does B represent any kind of win over A?  He's going to have to bust his hump trying to save up that year's salary in the first place... and then he's going to have to work extra hard to replenish the bank account when it runs low.

At the end of the day:  if B's daily expenses are the same as A's... and if his hourly wage is the same as A's... then over time, on average he's going to have to work the same number of hours per year as A.  Rearranging "I'll work harder for a while, then take it easy for a while" doesn't give him any net savings.

[YNM]

20 hours ago, MatterBeam said:

You only need to cancel velocity along X and Y axis, so that the very high Z-axis velocity will hit you dead center.

So had been every space (and earth ie. land/water/air) rendezvous. But you realize why they try to slow down long before the target right ? And you know, at least by hand, that's a pretty complicated thing ? And now, you're asking for it to be done hundreds of time faster...

 

Here's a challenge - do it in RO/RSS, say to a 200x200 orbit at best inclination. If you manage to, then I'm impressed.

Edited January 29, 2017 by YNM

[wumpus]

13 hours ago, Snark said:

Well, sure.  It just means you have to spend all the money ahead of time (to launch the station in the first place), and then you can save money on the spaceships for a while until you have to boost the station again, which will need more money.  You still have to boost, and the amount of momentum you need is precisely equal to the amount of momentum that the ascending rockets saved.

Except there are known ways to achieve high Isp as long as you are willing to spend the time waiting for it (right now mostly ion-based systems).  I'd also assume that most of the mass of the tether-bearing satellite came from a captured asteroid (again moved with ions).

What gets me about such proposals is the length of the tether, and how it would have to play out (and the resulting angular momentum).  Any sort of difference in speed has to be accounted for in the tether.  I've found this enough of an issue to not consider the momentum vs. angular momentum issues but presumably "merely" require eventual spending of propellant.

If there's one thing that KSP has taught me, is that Isp matters more the higher up the stages you go.  If you could somehow use high Isp systems (regardless of when they were spent) for orbital insertion, that is a clear win (I just can't imagine the amount of tether you would need to "reel out/reel in" a satellite from suborbital velocity to orbital velocity).

[Snark]

1 hour ago, wumpus said:

Except there are known ways to achieve high Isp as long as you are willing to spend the time waiting for it (right now mostly ion-based systems).  I'd also assume that most of the mass of the tether-bearing satellite came from a captured asteroid (again moved with ions).

Well, sure.  That can give you a net benefit, summed up succinctly as:  "The benefit of this system is that you can get more of your momentum from high-Isp space-station drive, instead of from low-Isp lifter rockets."

It's just that the OP didn't mention that, nor did anyone earlier in this thread, as far as I can tell-- it was framed as "saves fuel because you get a boost from the station", which does not compute.  The benefit here is wholly predicated on the station having a higher Isp than the lifter-- that's the crucial point, should be called out clearly up front.

[magnemoe]

1 minute ago, Snark said:

Well, sure.  That can give you a net benefit, summed up succinctly as:  "The benefit of this system is that you can get more of your momentum from high-Isp space-station drive, instead of from low-Isp lifter rockets."

It's just that the OP didn't mention that, nor did anyone earlier in this thread, as far as I can tell-- it was framed as "saves fuel because you get a boost from the station", which does not compute.  The benefit here is wholly predicated on the station having a higher Isp than the lifter-- that's the crucial point, should be called out clearly up front.

I think most here thought that the station would have high isp engines. That part is solid, more of an issue how much this recovery equipment would add to the weight. 
More an issue is how you unwind the tether fast enough, yes you could take energy from one flywheel and use to spin up the spindle but not sure how this would effect the tip, that is unless you had an rocket on the tip. 
I would not used pulleys that would complicate stuff a lot, you would have to get them up to high speed and the tether running over the wheel would generate problems, first is the braking then you turn the tether around 180 degree, remember we have insane high speed and energy here, releasing an single tether would simply be extremely hard. 

Finally is catching the tether, yes it has been done but at 1/100 of the speed and only with small stuff. The braking wires on carriers grab planes  but they are easy to catch as they are on the deck the plane has landed on so its just two dimensions, still they use multiple wires and sometimes the plane don't catch. 

 

[MatterBeam]

There are two versions of the Boosted Orbital Tether: simple and flywheel.

The simple version is just a fancy weight on a string. Braking on the string slows down the station. To boost it back up, you use high Isp engines. A chemical rocket will have an Isp of 300-400, an electric engine can reach 10000 Isp. Therefore, recovering the lost kinetic energy using a few hundred kg of propellant is much more economical than using several dozen tons of rocket fuel.

The second version builds up a kinetic energy reserve in a spinning set of flywheels. When the payload starts braking on the tether, the flywheel pulls in the opposite direction. This slows down both the flywheels and the spacecraft. When they both come to a stop, the station hasn't lost any kninetic energy or altitude: this method requires zero propellant, so it is 'free'.

If you want an hour's reading on the details of this design, go here: http://bbs.stardestroyer.net/viewtopic.php?f=5&t=166030

[FleshJeb]

1 minute ago, MatterBeam said:

The second version builds up a kinetic energy reserve in a spinning set of flywheels. When the payload starts braking on the tether, the flywheel pulls in the opposite direction. This slows down both the flywheels and the spacecraft. When they both come to a stop, the station hasn't lost any kninetic energy or altitude: this method requires zero propellant, so it is 'free'.

@Snark covered my point for me, but I'll expand on it:

To prepare to catch the next spacecraft, you have to spin up the wheel again. This will spin the platform in the opposite direction. You will still have to use your engines to cancel that spin. It's not free.

However, as you said, the high-ISP engines might make this a worthwhile strategy. I'm still concerned about the possible failure modes.

[VincentS]

As stated multiple times before, the flywheel trick won't work. If it worked, you could build a reactionless engine  : 
1) shot a pellet using a railgun at one end
2) catch the pellet using your flywheel-augmented sling.
3) rinse and repeat. (moving the pellet back to the railgun has no net effect on momentum)

You gain momentum using 1) recoil, and lose no momentum when catching it at the other end 2) ; 3) has no net effect.

[magnemoe]

12 minutes ago, VincentS said:

As stated multiple times before, the flywheel trick won't work. If it worked, you could build a reactionless engine  : 
1) shot a pellet using a railgun at one end
2) catch the pellet using your flywheel-augmented sling.
3) rinse and repeat. (moving the pellet back to the railgun has no net effect on momentum)

You gain momentum using 1) recoil, and lose no momentum when catching it at the other end 2) ; 3) has no net effect.

The flywheels do not conserve momentum, they simply make accelerating the suborbital rocket to orbital speed far easier at the cost of increased mass and complexity, 
Problem is that the suborbital rocket is 8 km/s slower than the runway, 
More everyday, you have an rope trailing an car moving at 100 km /h you sit on an bicycle and grab the rope, you can not grab to hard as the shock would tumble you, grabbing it but not so hard will burn your hands hard. if the rope was extended and then winch you in it would be manageable. 

This was used in real life to grab people on ground from an plane, you have an balloon at the end of an rope, plane grabs your rope then lets its wire wind out to reduce the g force on you before winding you in. 
Worked well and was used from WW2 up to the 1970s where long range helicopters with mid air refueling became an better option. 

My problem here is the speeds and energies involved who is 100 times larger than anything done.
 

[VincentS]

You are obviously correct magnemoe, I was reacting to the following (emphasis is mine) :

1 hour ago, MatterBeam said:

The second version builds up a kinetic energy reserve in a spinning set of flywheels. When the payload starts braking on the tether, the flywheel pulls in the opposite direction. This slows down both the flywheels and the spacecraft. When they both come to a stop, the station hasn't lost any kninetic energy or altitude: this method requires zero propellant, so it is 'free'.

I should have quoted this the first time for clarity sake.

[Snark]

3 hours ago, MatterBeam said:

The second version builds up a kinetic energy reserve in a spinning set of flywheels. When the payload starts braking on the tether, the flywheel pulls in the opposite direction. This slows down both the flywheels and the spacecraft. When they both come to a stop, the station hasn't lost any kninetic energy or altitude: this method requires zero propellant, so it is 'free'.

Not free, as several people have pointed out.  In terms of the fuel the station needs to expend to maintain its orbit, this flywheel system will provide exactly, precisely, mathematically zero benefit over the simple braking system.

Conserving energy is no big deal.  Ships in space can generate energy for "free" via solar panels, nuclear reactors, or whatever.

The problem here is that the space station has lost momentum, and that's why you need to spend propellant to accelerate the station.  It's exactly zero difference from using a simple braking approach.  If you start with "ship is approaching with high velocity relative to the station", and if you finish with "ship is at rest relative to the station", and if nobody expended any reaction mass anywhere, then the station has lost momentum (by giving it to the ship).

It doesn't matter if you use friction braking, or flywheels, or pulleys, or large numbers of monkeys, or any other clever contraption.  All you have to do is look at "what's the initial momentum of the ship+station system", and compare that with "what's the final momentum of the ship+station system".  The mechanism by which the station transfers momentum to the ship doesn't change the amount of momentum transferred in the slightest.

[MatterBeam]

9 hours ago, Snark said:

Not free, as several people have pointed out.  In terms of the fuel the station needs to expend to maintain its orbit, this flywheel system will provide exactly, precisely, mathematically zero benefit over the simple braking system.

Conserving energy is no big deal.  Ships in space can generate energy for "free" via solar panels, nuclear reactors, or whatever.

The problem here is that the space station has lost momentum, and that's why you need to spend propellant to accelerate the station.  It's exactly zero difference from using a simple braking approach.  If you start with "ship is approaching with high velocity relative to the station", and if you finish with "ship is at rest relative to the station", and if nobody expended any reaction mass anywhere, then the station has lost momentum (by giving it to the ship).

It doesn't matter if you use friction braking, or flywheels, or pulleys, or large numbers of monkeys, or any other clever contraption.  All you have to do is look at "what's the initial momentum of the ship+station system", and compare that with "what's the final momentum of the ship+station system".  The mechanism by which the station transfers momentum to the ship doesn't change the amount of momentum transferred in the slightest.

Let's consider the system as two objects: the payload and tether system, and the flywheel.

The payload is assumed to join its kinetic energy an momentum into the the tether+payload system instantaneously, without losses. In one example I worked out, the payload is 12.95 tons and the tether is 338.4 tons and the pulleys are 93.5 tons for a total of 448.85 tons. 

Payload momentum: 7334*12950 = 94.97MN.m
Payload kinetic energy: 348GJ @7334 m/s

After braking, using conservation of momentum:

Tether+Payload momentum: 94.97MN.m @213.5m/s
Tether+Payload kinetic energy: 10GJ (??)

After braking, using conservation of energy:

Tether+Payload kinetic energy: 348GJ @1245.24m/s
Tether+Payload momentum: 558.9MN.m

We'll use conservation of momentum. We have to handle the 338GJ deficit in the system. This is where the flywheel enters into play.

A flywheel with -94.97MN.m of momentum and 338GJ of kinetic energy would be able to cancel the momentum and energy of the tether+payload system. The momentum will be angular, due to its rotation.

According  to half an hour of iterations on Wolfram Alpha (this equation), 10 flywheels of 10 tons each, with a diameter of 23.1cm and a rim velocity of 822m/s can provide exactly the required amount of angular momentum and kinetic energy to cancel those of the tether+payload system.

After all is done, the momentum and kinetic energy of the tether+payload+flywheel system is zero. Nothing is 'taken' from the station to which all is this connected to.

[Snark]

28 minutes ago, MatterBeam said:

We'll use conservation of momentum. We have to handle the 338GJ deficit in the system.

Confused by your terminology.  You used "momentum" and "GJ" practically in the same breath.  GJ is energy, not momentum.  These are completely different, unrelated things.  This statement makes it sound like you're conflating energy with momentum.

You don't need to handle energy deficits, particularly, because there are lots of ways to deal with that.  Ships in space aren't closed systems, where energy is concerned.  They can gain energy via solar panels, or nuclear reactors, or whatever.  They can dump energy as heat via radiators.  It's simply an engineering problem; the fundamental laws of physics don't really enter into it.

However, you do need to handle a momentum deficit, because momentum is conserved, and there is no way to handle a momentum deficit without either applying an external force (e.g. gravity), or else by expending reaction mass.  Cannot be done.

28 minutes ago, MatterBeam said:

This is where the flywheel enters into play.

A flywheel with -94.97MN.m of momentum and 338GJ of kinetic energy would be able to cancel the momentum and energy of the tether+payload system. The momentum will be angular, due to its rotation.

^ This.  This is where it all falls down, right here, and why the fundamental design you're proposing can't work, at all.

Linear momentum and angular momentum are different things.  Apples and oranges.  They have different units on them.  And you can't convert one to the other with a flywheel or a tether or a pulley or anything else.  Perhaps the fact that they both have the word "momentum" in their names may be what's confusing you, but they really are completely different things.  You might as well ask how many meters in a kilogram.

Essentially what you're proposing is "Let's make a reactionless drive by magically converting between linear and angular momentum", which cannot be done because the laws of physics prevent it.  Period, full stop.

If you like, we could talk through all the nitty-gritty details of exactly how and why it wouldn't work... but there's really no need to, because we already know what the answer's going to be, due to momentum conservation.

[natsirt721]

On 2/3/2017 at 7:55 AM, Snark said:

Linear momentum and angular momentum are different things. 

Man, I thought the death of the Dean Drive in the 50's really nailed this point down but boy was I wrong.