Dynamics & Control - Tether Critical Design Review Anne DeLion 27 February 2001
Tether Type - Multiline • Multi-line Tether • Weight is comparable to a single line tether • High redundancy • The number of lines is proportional to redundancy • Very high survivability • Hoytether structure is less likely to tangle on deployment Survival probability of a failsafe multiline tether versus an equal-mass single line tether Hoyt, R. and Forward, R. “The HoytetherTM: A Failsafe Multiline Space Tether Structure.” www.tethers.com
Material: Spectra 2000 Safety Factor: 5 (minimum) Tether Specifications
Tether Material: Spectra 2000 Cable: 2210 kg/km Safety Factor: 5 (minimum) Hab Tether/Cable Specifications
Hab Deployment & Spin-up • Three-Step Deployment Method • Thrust to spin up the connected vehicle system to the desired initial spin rate • Stop thrust, release vehicles, and conservation of angular momentum will deploy the tether • After tether is completely deployed, thrust to spin the system up to 2 rpm for artificial gravity Note: Thrusting is done on both the Hab and NTR. The thrust is proportional so moments about cm cancel. Also, by doing some thrusting on the NTR less propellent is used.
Hab Deployment & Spin-up Numbers Hab Thrust = ~660 N NTR Thrust = ~250 N • If the tether is cut during the 2rpm spin: Imparted DV = 17.8 m/s
Hab Tether Deployment • Tether Deployment • Thrust = 0 N • Tension is kept on tether by centripetal force • Deployment time = 1 hour • Is arbitrary and can be set longer/shorter • Mechanism to control deployment rate is black-boxed in reel system • Final spin rate after deployment = 0.017 rpm
ERV Deployment & Spin-up • Three-Step Deployment Method • Use same method as the Hab • One significant difference: ERV will initially spin up to .38g. At some point in the mission, the ERV will spin up .02g additional each day for 31 days to finally create 1g
ERV Deployment & Spin-up Numbers ERV Thrust = ~660 N MLV Thrust = ~250 N • If the tether is cut during .38g phase of spin: Imparted DV = 28.8 m/s • If the tether is cut during 1g phase of spin: Imparted DV = 46.7 m/s
Tether Cost/Risk • Manufacturing Cost: $1000/kg • Hab Tether: $255 000 • ERV Tether: $975 000 • Success Probability • Hoytether > 95% • Avoiding catches in tether winding > 95% • Due to greater than 10 N pulling out tether.[Hoyt]
Tether Cost/Risk • Manufacturing Cost: $1000/kg • Hab Tether: $255 000 • ERV Tether: $975 000 • Success Probability • Hoytether > 95% • Avoiding catches in tether winding > 95% • Due to greater than 10 N pulling out tether.[Hoyt] • Mechanical Risk = ??