A winding solution to the space elevator power

problem

B. Michel, UCL

o The ‘Reference Space Elevator’ (RSE) is 100.000 Km long with a tapered2 mm2 max cross section.

o It is made from CNT with a 150 GPa strength.

o It costs around 6000 M$, 1/3 dedicated to power the system.

o It will have to overcome many problems due to interference with the earth atmosphere, low orbit objects, and the laser power beams needed to beam energy to the climbers.

The Reference Space ElevatorThe Reference Space Elevator

o A heavier, longer ribbon (150000 Km, constant section)

o The climbers will not climb the ribbon; they will be attached to it.

o The up and down translation is done by winding and unwinding the cable on reels at both ends.

o Energy is provided to the system only at ground level and at the counterweight station

A simple winding solutionA simple winding solution

o Start : cabin at earth level, 35900km Start : cabin at earth level, 35900km cable reeled-in on a spool on the ground. cable reeled-in on a spool on the ground. The CW is at 150000km attached to an The CW is at 150000km attached to an empty spoolempty spool

o Unreel the ground spool, reel-in the cable Unreel the ground spool, reel-in the cable on the CW spool : the cabin climbs….on the CW spool : the cabin climbs….

o End : ground spool empty, cabin at GEO, End : ground spool empty, cabin at GEO, CW down to 73000 km with a big spool CW down to 73000 km with a big spool of cableof cable

How it works (1)How it works (1)

Why do we move the counterweight up and Why do we move the counterweight up and down?down?

To keep the ribbon stress under its To keep the ribbon stress under its maximum allowed value at all times, andmaximum allowed value at all times, and

To keep the centre of gravity farther than To keep the centre of gravity farther than the GEO.the GEO.

When the CW spool is heavier, it has to be When the CW spool is heavier, it has to be moved closer to the ground.moved closer to the ground.

How it works (2)How it works (2)

How do we move the counterweight down?How do we move the counterweight down?

The CW spool is actuated by electric The CW spool is actuated by electric motors and solar panels.motors and solar panels.

As the payload climbs up, the CW lowers As the payload climbs up, the CW lowers itself from 150000 km to half that while itself from 150000 km to half that while its weight increases 20x !its weight increases 20x !

(15 Tons to 313 Tons).(15 Tons to 313 Tons).

15 tons is the minimum weight of the CW, 15 tons is the minimum weight of the CW, used for solar panels, motorised winch used for solar panels, motorised winch and a maintenance station.and a maintenance station.

How it works (3)How it works (3)

While we move up…While we move up………the Earth station unwinds its winch and the Earth station unwinds its winch and

uses the recuperated energy to power the uses the recuperated energy to power the base station.base station.

……the CW spools its winch using the solar the CW spools its winch using the solar panel energy.panel energy.

While we go back to earth…While we go back to earth………the Earth station winds its winch using the Earth station winds its winch using

low cost electricity.low cost electricity.……the CW spool is unreeled and the energy the CW spool is unreeled and the energy

is dissipated by radiator panels.is dissipated by radiator panels.

How it works (4)How it works (4)

Cross section : 2mmCross section : 2mm22

Total cable length : 185,900 KmTotal cable length : 185,900 KmTotal cable mass : 483,600 KgTotal cable mass : 483,600 KgMin CW weight : 15,673 Kg Min CW weight : 15,673 Kg @@150,000Km150,000KmMax CW weight : 313,113 Kg Max CW weight : 313,113 Kg @@71,500Km71,500KmMax stress : 75 GPaMax stress : 75 GPaMass of one ‘lift’ = 2,000 KgMass of one ‘lift’ = 2,000 Kg

Some figuresSome figures

Compared with the R.S.E :Compared with the R.S.E : Heavier cable,Heavier cable, Higher cable stress Higher cable stress Lighter payload Lighter payload Downward journey wastes useful timeDownward journey wastes useful timeButBut No need for energy transferNo need for energy transfer Very simple climbersVery simple climbers

Cable repair easy at Earth (lower part)Cable repair easy at Earth (lower part) Cable repair possible at CW, or at the Cable repair possible at CW, or at the

GEO station (upper part)GEO station (upper part)

First conclusionFirst conclusion

o WindWindo Thunderstorms and lightningThunderstorms and lightningo RadiationRadiationo Atomic oxygenAtomic oxygeno Sulphuric acid Sulphuric acid o LEO satellites and know debrisLEO satellites and know debriso Micrometeors Micrometeors

o Most of the above threads are directed Most of the above threads are directed to the lowest percent(s) of the cable.to the lowest percent(s) of the cable.

Threads to the elevatorThreads to the elevator

o Almost all damage to the cable can be fixed Almost all damage to the cable can be fixed when the cable is on the ground. We can when the cable is on the ground. We can even replace sections of the cable if needed!even replace sections of the cable if needed!

o No more need to beam energy to the No more need to beam energy to the climbersclimbers

o Better climber payload/dead weight ratioBetter climber payload/dead weight ratio

o In case of major damage, we can uncoil fresh In case of major damage, we can uncoil fresh ribbon at the bottom while discarding the top ribbon at the bottom while discarding the top section from the CW.section from the CW.

o The same method can be used to The same method can be used to progressively increase the ribbon section.progressively increase the ribbon section.

Advantages of the winding solutionAdvantages of the winding solution

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Seeing the above conclusions, why not trying to Seeing the above conclusions, why not trying to keep the best of both words?keep the best of both words?

o A tapered cable, relatively lightweightA tapered cable, relatively lightweighto Beam powered climbersBeam powered climbersButButo A winch at the earth station and a winch in the A winch at the earth station and a winch in the

CWCW

Why not an hybrid compromise ?Why not an hybrid compromise ?

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The operating mode will be a compromise too:The operating mode will be a compromise too:

We lock the climber to the cable and use the We lock the climber to the cable and use the winch for the first 2600 Km winch for the first 2600 Km

We play with the CW altitude to keep the We play with the CW altitude to keep the centre of gravity above GEO and the stress in centre of gravity above GEO and the stress in the cable at an acceptable level.the cable at an acceptable level.

The rest of the journey to GEO is classical with The rest of the journey to GEO is classical with energy beamingenergy beaming

When at GEO, we disconnect the climber from When at GEO, we disconnect the climber from the cable for our cable roll-backthe cable for our cable roll-back

The hybrid compromiseThe hybrid compromise

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The lower 1% of the cable is regularly accessible on The lower 1% of the cable is regularly accessible on earth for maintenance (and the major threads are in earth for maintenance (and the major threads are in the lower area)the lower area)

For exceptional repairs in the lower 5%, we can roll-in For exceptional repairs in the lower 5%, we can roll-in slightly more cable on ground if no climber attached slightly more cable on ground if no climber attached and if the CW is winched down.and if the CW is winched down.

Power beaming requirement are lower (75% at Power beaming requirement are lower (75% at 1000Km, 50% at 2600 Km, 30% at 5200 Km)1000Km, 50% at 2600 Km, 30% at 5200 Km)– Smaller PV panelsSmaller PV panels– Better efficiencyBetter efficiency

Power beaming accidents at low altitudes no more Power beaming accidents at low altitudes no more possible.possible.

Power beaming from GEO becomes possiblePower beaming from GEO becomes possible

The hybrid advantagesThe hybrid advantages

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The main parameter is the reel-in/reel-out distanceThe main parameter is the reel-in/reel-out distance We have to balance the reel-in/reel-out distance againstWe have to balance the reel-in/reel-out distance against

– Cable strength and taper valueCable strength and taper value– Additional weight and complexity at the CWAdditional weight and complexity at the CW– Power beaming lower requirementsPower beaming lower requirements– Easy maintenance for the reeled part of the cableEasy maintenance for the reeled part of the cable

For low values of the parameter, A fixed CW without winch For low values of the parameter, A fixed CW without winch could be usedcould be used– Higher cable stressHigher cable stress– Lower maximal payloadLower maximal payload

More simulations are required to find the best compromiseMore simulations are required to find the best compromise

The compromise parametersThe compromise parameters

www.benoitmichel.be

Benoit MICHEL, November 2007

Images copyright Alan Chan & his space elevator visualisation group