A winding solution to the space elevator power problem B. Michel, UCL

o The ‘Reference Space Elevator’ (RSE) is Km long with a tapered 2 mm 2 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 Elevator

oA heavier, longer ribbon ( Km, constant section) oThe climbers will not climb the ribbon; they will be attached to it. oThe up and down translation is done by winding and unwinding the cable on reels at both ends. oEnergy is provided to the system only at ground level and at the counterweight station A simple winding solution

o Start : cabin at earth level, 35900km cable reeled-in on a spool on the ground. The CW is at km attached to an empty spool o Unreel the ground spool, reel-in the cable on the CW spool : the cabin climbs…. o End : ground spool empty, cabin at GEO, CW down to km with a big spool of cable How it works (1)

Why do we move the counterweight up and down? To keep the ribbon stress under its maximum allowed value at all times, and To keep the centre of gravity farther than the GEO. When the CW spool is heavier, it has to be moved closer to the ground. When the CW spool is heavier, it has to be moved closer to the ground. How it works (2)

How do we move the counterweight down? The CW spool is actuated by electric motors and solar panels. As the payload climbs up, the CW lowers itself from km to half that while its weight increases 20x ! (15 Tons to 313 Tons). 15 tons is the minimum weight of the CW, used for solar panels, motorised winch and a maintenance station. How it works (3)

While we move up… …the Earth station unwinds its winch and uses the recuperated energy to power the base station. …the CW spools its winch using the solar panel energy. While we go back to earth… …the Earth station winds its winch using low cost electricity. …the CW spool is unreeled and the energy is dissipated by radiator panels. How it works (4)

Cross section : 2mm 2 Total cable length : 185,900 Km Total cable mass : 483,600 Kg Min CW weight : 15, ,000Km Max CW weight : 313,113 71,500Km Max stress : 75 GPa Mass of one ‘lift’ = 2,000 Kg Some figures

Compared with the R.S.E : Heavier cable, Heavier cable, Higher cable stress Higher cable stress Lighter payload Lighter payload Downward journey wastes useful time Downward journey wastes useful timeBut No need for energy transfer No need for energy transfer Very simple climbers Very simple climbers Cable repair easy at Earth (lower part) Cable repair easy at Earth (lower part) Cable repair possible at CW, or at the GEO station (upper part) Cable repair possible at CW, or at the GEO station (upper part) First conclusion

oWind oThunderstorms and lightning oRadiation oAtomic oxygen oSulphuric acid oLEO satellites and know debris oMicrometeors oMost of the above threads are directed to the lowest percent(s) of the cable. Threads to the elevator

oAlmost all damage to the cable can be fixed when the cable is on the ground. We can even replace sections of the cable if needed! oNo more need to beam energy to the climbers oBetter climber payload/dead weight ratio oIn case of major damage, we can uncoil fresh ribbon at the bottom while discarding the top section from the CW. oThe same method can be used to progressively increase the ribbon section. Advantages of the winding solution

12 Seeing the above conclusions, why not trying to keep the best of both words? oA tapered cable, relatively lightweight oBeam powered climbers But oA winch at the earth station and a winch in the CW Why not an hybrid compromise ?

13 The operating mode will be a compromise too: We lock the climber to the cable and use the winch for the first 2600 Km We lock the climber to the cable and use the winch for the first 2600 Km We play with the CW altitude to keep the centre of gravity above GEO and the stress in the cable at an acceptable level. We play with the CW altitude to keep the centre of gravity above GEO and the stress in the cable at an acceptable level. The rest of the journey to GEO is classical with energy beaming The rest of the journey to GEO is classical with energy beaming When at GEO, we disconnect the climber from the cable for our cable roll-back When at GEO, we disconnect the climber from the cable for our cable roll-back The hybrid compromise

14 The lower 1% of the cable is regularly accessible on earth for maintenance (and the major threads are in the lower area) The lower 1% of the cable is regularly accessible on earth for maintenance (and the major threads are in the lower area) For exceptional repairs in the lower 5%, we can roll-in slightly more cable on ground if no climber attached and if the CW is winched down. For exceptional repairs in the lower 5%, we can roll-in slightly more cable on ground if no climber attached and if the CW is winched down. Power beaming requirement are lower (75% at 1000Km, 50% at 2600 Km, 30% at 5200 Km) Power beaming requirement are lower (75% at 1000Km, 50% at 2600 Km, 30% at 5200 Km) –Smaller PV panels –Better efficiency Power beaming accidents at low altitudes no more possible. Power beaming accidents at low altitudes no more possible. Power beaming from GEO becomes possible Power beaming from GEO becomes possible The hybrid advantages

15 The main parameter is the reel-in/reel-out distance The main parameter is the reel-in/reel-out distance We have to balance the reel-in/reel-out distance against We have to balance the reel-in/reel-out distance against –Cable strength and taper value –Additional weight and complexity at the CW –Power beaming lower requirements –Easy maintenance for the reeled part of the cable For low values of the parameter, A fixed CW without winch could be used For low values of the parameter, A fixed CW without winch could be used –Higher cable stress –Lower maximal payload More simulations are required to find the best compromise The compromise parameters

Benoit MICHEL, November 2007 Images copyright Alan Chan & his space elevator visualisation group