Rotating Spacecraft slingshot

[Whirligig Girl]

So I haven't had time to think this through, and I am almost certainly wrong, but what if you had a big, massive reaction wheel in space that spins up to high acceleration then lets go of the payload at the right time? The slingshot would move, but since it's massive you could correct the orbit more easily with conventional propellents.

SO what are the good/bad of this idea?

[Kryten]

How does the reaction wheel spin up?

[Armchair Rocket Scientist]

So I haven't had time to think this through, and I am almost certainly wrong, but what if you had a big, massive reaction wheel in space that spins up to high acceleration then lets go of the payload at the right time? The slingshot would move, but since it's massive you could correct the orbit more easily with conventional propellents.

SO what are the good/bad of this idea?

Like this?

A Bolo, or rotating tether, is a tether that rotates more than once per orbit and whose endpoints have a significant tip speed (~ 1 – 3 km/s). The maximum speed of the endpoints is limited by the strength of the cable material and the safety factor it is designed for.

The purpose of the Bolo is to either speed up, or slow down, a spacecraft that docks with it without using any of the spacecraft's onboard propellant and to change the spacecraft's orbital flight path. Effectively, the Bolo acts as a reusable upper stage for any spacecraft that docks with it.

The momentum imparted to the spacecraft by the Bolo is not free. In the same way that the Bolo changes the spacecraft's momentum and direction of travel, the Bolo's orbital momentum and rotational momentum is also changed, and this costs energy that must be replaced. The idea is that the replacement energy would come from a more efficient and lower cost source than a chemical rocket motor. Two possible lower cost sources for this replacement energy are an ion propulsion system, or an electrodynamic tether propulsion system that would be part of the Bolo.

[magnemoe]

It you spin the reaction wheel one way the station spins the other.

Better to use an tether, as in long cable connecting the satellite and station so you don't have to spin so fast.

However its no free lunch, you station would loose speed as the satellite gain speed.

Using a many kilometer long cable you don't need the spin but can simply use tidal forces, one idea for space stations was to lower a space shuttle some km below it on a cable and release it.

This would reduce the speed of shuttle and deorbit it while the station was lifted, would work well with the heavy space shuttle.

Main issue is the cable weight, you might get problems with the winding or mechanism and most serious, stretch in cable would having the connector at the end coming back fast.

[peadar1987]

It's completely feasible, and in fact, I wrote my Masters' Dissertation on an aspect of it.

This is a good paper on it: http://www.tethers.com/papers/cislunaraiaapaper.pdf

We could build such a system, and build it relatively easily without any major advances in technology, we just don't currently have any reason to do so! It would be a great way of supplying a moon base though. Spaceplane or rocket goes up on a suborbital trajectory, deposits the payload at one end of the tether as a return payload is captured, then one orbit later, the payload you just brought back is released and boosted into a lunar transfer orbit, while the return payload from the moon is put on a reentry trajectory to stop the tether from losing altitude.

[dryer_lint]

Considering reaction wheels use electricity, wouldn't this be a "propellant-less propulsion system" like that microwave thruster?

[Moesly_Armlis]

docking ports or maybe claws but must kerbal this idea together.

[peadar1987]

Considering reaction wheels use electricity, wouldn't this be a "propellant-less propulsion system" like that microwave thruster?

Not quite. The "propellant" is the tether/station itself. It's pushed the other way when you release the projectile.

[AngelLestat]

Nice to hear that you wrote your master about this.

There is another way to do it as this video show, but instead use a returning payload as energy exchange, This concept use the electrodynamic tether effect to recover the energy lost in the tether.

http://www.nasaspaceflight.com/_docs/MXER%20TAG%202003%20Report.pdf

Edited August 16, 2014 by AngelLestat

[Armchair Rocket Scientist]

Considering reaction wheels use electricity, wouldn't this be a "propellant-less propulsion system" like that microwave thruster?

The issue is, every time the system flings something into orbit you take angular momentum from the tether and transfer it to the spacecraft, so the tether's rotation slows down. To spin it back up, you'd have to increase the speed of the reaction wheel every time you launched, which would quickly saturated. Reaction wheels are good for satellites that need to make tiny, very precise adjustments to their position without using propellant, not so much for something with a whole bunch of momentum. It's better to throw out the reaction wheel and use whatever system you're using to desaturate the wheel to directly boost the tether's speed. Fortunately, because the tether is really big and can spin back up over the course of several hours or days, it can use propellantless or high-efficiency drives that ordinarily would be too low-thrust for orbital launches.

One thing to note: if the tether is changing the payload's velocity by 5 km/s (enough to reach LEO from roughly mach 10) the tether would need to be 1000 km long to get an acceleration of 5 Gs. With a dV of 8 km/s (good for Geostationary Transfer Orbit) it would have to be 2560 km long.

[dbmorpher]

If you can get the spacecraft to a speed equal to that of the spin rate couldn't the tether lift the ship to a higher altitude with no loss to either vessel?

[Rickenbacker]

Sounds like it would work, but there's one small problem. To spin the wheel up to the required speed, you'd need to use just as much energy as you would if you simply put a rocket engine on the payload in the first place... There's no such thing as a free lunch.

[peadar1987]

Sounds like it would work, but there's one small problem. To spin the wheel up to the required speed, you'd need to use just as much energy as you would if you simply put a rocket engine on the payload in the first place... There's no such thing as a free lunch.

This is true, but it is possible to use solar panels to power the motors for rotation. The station can be boosted by interactions with the earth's magnetic field. This means that even though you use the same amount of energy, the tether system can do it without the need to transport propellant into orbit, which could give a significant saving if we ever needed to transport large quantities of stuff to the moon.

[JavaProphet]

My 2 cents: To stop the tangent(?) force from the release of the payload, you would need to release something of equal mass at the same speed in the opposite direction, or on the other side of the circle. To charge the 'wheel', just use a generic motor(one helluva strong one), and reverse the charge direction to slow down instead. As for the amount of energy required, I'll do the math I know. A one ton payload requires two tons to be launched in each direction(you'd need a strong wheel, as so the forces don't rip it in pieces). Say you wanted to launched this one-ton payload at... 10 KM/s relative to the wheel. Since the velocity of one the objects is our angular velocity, we need to get a net torque on the wheel of some large amount(torque is always confusing for me, we need a math guy to analyze this better). As mentioned earlier, the other part of the launching craft would rotate opposite. How do we fix this? I saw in an Artificial Gravity video by HOCGaming a while back, that you can have a wheel on the opposite side in the opposite direction. You'd need the same mass for the payloads. You would likely want the wheels to be slightly tilted back, as so the two forward payloads DO NOT collide, but the two behind do(?). Orbital debris would be an issue regardless.

As for power, in modern technology, perhaps a large solar field? The amount of energy we need I don't feel(could be wrong), is the main issue, but the strength of the spinning wheels. The forces on those things would be twice what you have for torque, just trying to rip those things in half. We'd need some kind of inward force, or just a super-strong material(Carbon Nano-Fiber?) to keep the contraption together.

Edited August 18, 2014 by JavaProphet

[GoSlash27]

Whatever kinetic energy you impart to a payload when you fling it is also imparted in an equal and opposite fashion to the launcher. It will eventually deorbit unless you replace the kinetic energy you imparted to the payload.

Given the fact that no energy transfer is 100% efficient, it stands to reason that it would be more efficient to simply attach your source of kinetic energy to the payload and be done with it.

Best,

-Slashy

[peadar1987]

Well, if time is not an issue, you can pretty much get anywhere in the solar system with a gravity assist off the moon. To get to the moon, you need a delta-V of about 3.1km/s.

With a 100km rotating tether, you can get the necessary delta-V with a tether length of about 100km. Modern materials like kevlar and even UHMWPP are strong enough to deal with the tensile forces. No carbon nanotubes needed.

As for deorbiting the station, with a 100km tether, you can boost the orbit relatively easily by passing a current through the tether. This creates a magnetic field which pushed against the earth's, generating a force. It's not big, but over time it will do the job.

The other option is you just set up a constant transport system. If you have a mining colony on the moon, you can fire cargo "backwards" to reenter earth's atmosphere at the same time you fire your supplies at the moon. This keeps your station in the same place, as there is no net force on the station.

To spin up the station, you just have to apply a force. Get a central hub with some solar panels on it, attach it with an axle to the root of your tethers, spin the hub one way using an electric motor, the tether will spin the other way, but much slower, as it has far more inertia.

[Idobox]

To spin up the station, you just have to apply a force. Get a central hub with some solar panels on it, attach it with an axle to the root of your tethers, spin the hub one way using an electric motor, the tether will spin the other way, but much slower, as it has far more inertia.

Or even simpler, pass current through the tether to generate torque. The current will have to be reversed twice per rotation (otherwise you get thrust), and the center of mass need not to be in the middle of the tether.

If you can't use a conductive tether, you can just put a big electromagnet on the station. It will try to align with Earth magnetic field, and you just need to flip the current around when it is. That's how DC motors work, except they use magnets rather than the Earth magnetic field.

Making things spin is easy if you have current. The difficult part is thrusting.

With a momentum exchange tether, you have a few options. Tether thrust is a real thing, but it's very weak, which would mean long "load" times. Another option is to slow down stuff, for example manned capsules (that way, they won't need so much aerobraking, meaning less stress and heat shields, meaning cheaper rockets), waste (old satellites, poop from space stations), or bags of dust launched from the moon.

Because the moon has no atmosphere, mass drivers are relatively easy to make, and whatever is sent would arrive with some pretty insane velocities to the momentum exchange tether, allowing a single kg of moon stuff to accelerate several kgs of spacecraft from suborbital to orbital velocities. My favourite lunar mass driver is a tower with a tether spinning horizontally. With the low gravity and lack of atmosphere, you could accelerate the spin over hours or days, unravelling the tether as its speed allows it not to touch the ground. And with an appropriate counterweight, the tower would get very little stress.

[peadar1987]

Or even simpler, pass current through the tether to generate torque. The current will have to be reversed twice per rotation (otherwise you get thrust), and the center of mass need not to be in the middle of the tether.

If you can't use a conductive tether, you can just put a big electromagnet on the station. It will try to align with Earth magnetic field, and you just need to flip the current around when it is. That's how DC motors work, except they use magnets rather than the Earth magnetic field.

Making things spin is easy if you have current. The difficult part is thrusting.

With a momentum exchange tether, you have a few options. Tether thrust is a real thing, but it's very weak, which would mean long "load" times. Another option is to slow down stuff, for example manned capsules (that way, they won't need so much aerobraking, meaning less stress and heat shields, meaning cheaper rockets), waste (old satellites, poop from space stations), or bags of dust launched from the moon.

Because the moon has no atmosphere, mass drivers are relatively easy to make, and whatever is sent would arrive with some pretty insane velocities to the momentum exchange tether, allowing a single kg of moon stuff to accelerate several kgs of spacecraft from suborbital to orbital velocities. My favourite lunar mass driver is a tower with a tether spinning horizontally. With the low gravity and lack of atmosphere, you could accelerate the spin over hours or days, unravelling the tether as its speed allows it not to touch the ground. And with an appropriate counterweight, the tower would get very little stress.

It's not really great for manned capsules, as the centripetal acceleration with a 100km tether is approximately 9g, which is extremely uncomfortable/possibly fatal!

[Idobox]

How do you get 9g? With 100km, you would need a speed of 3km/s for the tip.

The station would have an orbital velocity of 7.8km/s, that would mean the speed of the vehicle when it is grabbed from its suborbital trajectory would be around 5km/s.

With a 200km cable, with the same speeds, you would get "only" 4 or 5g, easily survived in horizontal position.

Or with the same 100km cable, but only a 2km/s tip speed, you would get into 4g territory and need 6km/s velocity for the craft.

And that's talking only about putting stuff into orbit.

For deorbiting, you need much less deltaV. By loosing 1km/s from an original 8km/s, you loose 23% of your kinetic energy. By using the 2km/s, 100km cable, you loose 43% of your energy, making reentry so much easier and saving a lot of weight on the heat shield.

The main problem with momentum exchange tethers is that long tethers are heavy, very heavy, and we have no idea how to send such a heavy payload where we need it. A solution might be to send tanks of precursor and to manufacture it in situ.

[peadar1987]

How do you get 9g? With 100km, you would need a speed of 3km/s for the tip.

The station would have an orbital velocity of 7.8km/s, that would mean the speed of the vehicle when it is grabbed from its suborbital trajectory would be around 5km/s.

Centripetal force is given by v^2/r, where v is the tip speed relative to the hub, and r is the distance from tip to hub.

If v=3000m/s (for lunar insertion from LEO), v^2=9,000,000. If r=100,000m, that means that centripetal acceleration must be 90 m/s, or roughly 9g.

Just to be clear, I'm talking about the g-forces induced by spinning around the hub, not the forces required to accelerate the payload from suborbital to orbital velocity.

With a 200km cable, with the same speeds, you would get "only" 4 or 5g, easily survived in horizontal position.

Or with the same 100km cable, but only a 2km/s tip speed, you would get into 4g territory and need 6km/s velocity for the craft.

And that's talking only about putting stuff into orbit.

For deorbiting, you need much less deltaV. By loosing 1km/s from an original 8km/s, you loose 23% of your kinetic energy. By using the 2km/s, 100km cable, you loose 43% of your energy, making reentry so much easier and saving a lot of weight on the heat shield.

The main problem with momentum exchange tethers is that long tethers are heavy, very heavy, and we have no idea how to send such a heavy payload where we need it. A solution might be to send tanks of precursor and to manufacture it in situ.

Designs I've seen place the tether at about 30x the mass of the payload it is planned to deliver. If you want to deliver a 500kg payload, the tether would have a mass of about 15,000kg, which is well within the capabilities of current launch vehicles. A Delta IV Heavy could probably launch the entire thing in one go.

[KerikBalm]

A better idea than using "conventional" propellants to keep your rotating slingshot thing in orbit, would be to use very high ISP propellants, and beamed power, like a VASMIR engine keeping your slingshot in orbit.

It would allow you to "store" the thrust from a High ISP, low thrust device, and periodicaly execute a high thrust maneuver.

But I'd rather have a giant space-gun

[G2-]

Read a really good book on space tethers, definately worth a read if you have access to it.

Space Tethers and Space elevators, Van Pelt, 2009

Fortunately for me I have access you a university library as I'm currently studying for a PhD on space systems.

[Idobox]

Centripetal force is given by v^2/r, where v is the tip speed relative to the hub, and r is the distance from tip to hub.

If v=3000m/s (for lunar insertion from LEO), v^2=9,000,000. If r=100,000m, that means that centripetal acceleration must be 90 m/s, or roughly 9g.

My question was were the exact 9g figure comes from. I showed that 4g is also totally plausible.

The original issue was that 9g was too much for humans, but 9g is not necessary.

[peadar1987]

My question was were the exact 9g figure comes from. I showed that 4g is also totally plausible.

The original issue was that 9g was too much for humans, but 9g is not necessary.

It comes down to the strength of the tether. 100km is achievable with current materials like Spectra or Kevlar. 200km isn't, it would snap under its own weight before you put a payload onto it. A lower speed is certainly a possibility, but then you need to carry propellant to get to the moon, which is the only place I can think of we'd need to send that amount of stuff with a tether system.

[FleshJeb]

Tank Farm Dynamo by David Brin has a practical solution to masslessly generating orbital energy:

http://www.davidbrin.com/tankfarm.htm

If only we had these physics to play with in KSP...