Magnetic Ballooning through the Solar System

1. Magnetic Ballooning through the Solar System Robert Sheldon UAH Physics Colloquium September 5, 2000

2. Credits • Dennis Gallagher -- MSFC/SD50 Space Plasma Physics Branch (and Vic, Paul, Mark etc.) • Tim Gautier & ALL the MSFC Test Area 300 personnel! • Les Johnson - MSFC Propulsion Directorate • Robert Winglee -The University of Washington (and Tim, Ben, etc.) • Wes Swift - UAH CSPAR • Clark Hawk - UAH/PRC

3. The Rocket Equation • Vexhaust = Isp * g [d/dt(MV) = 0] • dV = Vexhaust* log( final mass / initial mass) • Material Isp Limitation • solid fuel 200-250 mass-starved • LH2/LOX 350-450 mass-starved • Nuclear Thermal 825-925 mass-starved • MHD 2000-5000 energy-starved • ION 3500-10000 energy-starved • Matter-Antimatter ~1,000,000 mass-starved • Photons 30,000,000-¥ both-starved

4. Ad Astra? • To do interstellar flight, we need to approach relativistic speeds, say, 12% c (which doesn’t do much to our clocks) • Isp Mass_rocket/Mass_payload • 2,000,000 6 • 1,000,000 50 • 500,000 2670 • 150,000 264,000,000,000 • Conclusion we aren’t going to do Interstellar travel any time soon

5. Okay, how about Pluto • Voyager=16 years to Pluto. A 1.6 year trip would take dV = 5.8e12m/5e7 s ~100 km/s • Isp M_rocket/M_payload • 100,000 1.1 • 10,000 2.7 • 1,000 22,000 • 400 72,000,000,000 • We aren’t going to use chemical rockets if we want a fast Pluto flyby larger than a pencil eraser.

6. How do solar sails work? • Momentum of photon = E/c, if we reflect the photon, then dp = 2 E/c. At 1 AU, E_sunlight = 1.4 kW/m2 ==> 9mN/m2=9mPa • Then to get to Pluto in 1.6 years, we need 0.004 m/s2 of acceleration. To get this acceleration with sunlight we need a mass loading of <2gm/m2 ! • Mylar materials ~ 6 gm/m2 • Carbon fiber mesh < 5 gm/m2 ( 3/2/2000) • We are getting close!

7. Solar Sails and Space Tethers Robert Sheldon University of Alabama in Huntsville Advanced Propulsion Workshop April 9, 1999

8. WIND FKEEL FRES Jibsail FWIND Keel Mainsail Sea Sailing • Tacking into the wind requires 2 forces acting together: wind & water. • The boats keel acts to convert momentum gained by wind, into a force from the water. • The Keel force also produces a torque.

9. Tacking Torques • Since the Keel is below the water, and the wind above the water, the forces not only add as a resultant, but generate a torque, twisting the sailboat.

10. The America’s Cup • The keel force generates a torque which must be compensated. • Sailors hang off the edge. • Or Australia’s invention: keel winglets. • After 133 years Aussie’s took it. • Moral: the more forces you control, the more maneuverable you are.

11. Solar Sail Forces: Sunlight • Sunlight reflected at an angle exerts a cos2 force. • Depending on symmetry, it is also a torque force. • Maintaining the “tack” angle may require “trimming”

12. “Pole Sitter” Auroral Observer • By placing an inclined sail behind and above the Earth, gravity and “lift” cancel resulting in a stationary “orbit” which can observe the Northern hemisphere and aurora continuously. • A Russian or Canadian “GEO”!

13. Trimmers • Trimmers are usually shown as moveable elements arranged near the edges of the sail. • Changing their angle adjusts the torques on the sail. • These trimmers function as “gyros” on satellites, e.g., HST, Rosat, SPOT...

14. Solar Sails: Magnetic Torque • The magnetic torque force attempts to anti-align the B-field & magnetic dipole. t = m x B = NiAx B • Plugging in for standard: • 10nT IMF B-field, 10kA, 0.9Mm2 = 90N-m • .a=t/I = 0.16mdeg/s2 • = 12 min/45deg adjustment D B i 1 loop = 1 degree of freedom. 2-axes may be required. Spin may do it.

15. Torquers Are Not Propulsion! • If we have gone to all the effort of including magnetic torquers on our sail, then can we use the same equipment for propulsion? • Yes! • (In fact we can use the same equipment for many other tasks: radio receivers, magnetic field sensors, sail “actuators” that can turn the sail into a “rubber mirror”…)

16. Propulsive Forces • Static FxB force • Linear B: current carrying wire in B-field • Quadratic B: dipole-dipole interaction • Dynamic Plasma forces (from a long list) • “Solar wind” sailing • Interaction size is determined by the “standoff distance” between the solar wind mag field & sail. • Tethers w/e- & w/ions • A “solar wind” ion engine • The MHD “solar wind” engine

17. Scooped Twice@APW! • Robert Forward and “mag sail” • Dani Eder’s 1994 Collection of Propulsion Concepts. (takes a superconductor). • Robert Winglee’s “M2P2” or “mini-magnetospheric plasma propulsion” • Use plasma to greatly inflate the magnetic field in the “mag sail” concept and couple to the solar wind with 10X or 1000X the diameter. • A better terminology might be magnetic balloon, though Winglee liked his name better.

18. Principle of Mag Balloons • Solar wind density = 3/cc H+ at 350-800 km/s • H+ Flux thru 1m2/s= 1m2*400km*3e6/m3=1.2e12 • Pressure = 2e-27kg*1.2e12*400km/s = 1nPa • That’s 1/10,000 the pressure of light! • Why would anyone ever bother with Solar Wind? • Because it isn’t pressure, it’s acceleration we are after. If we can make a mag balloon lighter than a solar sail, we may still achieve higher acceleration. • Magnetic fields don’t weigh very much for their size. • Trapped plasma can make an even larger mag field.

19. What it doesn’t look like. Supersonic Solar Wind (350-800 km/s) “Magnetic Wall” 15-30 km radius Electromagnetic- plasma interaction Not mechanical Constant Force Surface Streamlines & Density color contours

20. Winglee’s M2P2 Interplanetary Magnetic Field Bow Shock Plasma Injection Current Sheet: B ~ R-1 Dipole: B ~ R-3

21. M2P2 Capabilities • Inflation is electromagnetic--no mechanical struts • Balloon size depends on ambient pressure, so it expands as it moves away from Sun. ==> Constant force surface • 10-30km diameter = 1-3 N of Solar Wind Force • which is 0.6 MW of Solar Wind Energy • If payload weight were 20-100 kg, it would attain 50-80 km/s over a 3-month acceleration period. • Limiting speed is, of course, solar wind speed. However the balloon expands to some maximum size which limits its force as well.

22. The Plasma Inflator

23. Some Lab shots Nitrogen Plasma Helium Plasma 0.5 mTorr, 350G, 500W 4.0mTorr,350G, 500W

24. What sort of things can go wrong? • Scaling: The nemesis of plasma physics has been scaling up something that works at small scales. The reason is that plasmas have long-range forces. They are not local. • Plasma loss rate: If the plasma is lost too quickly, we end up losing the inflation, and we revert to a magnetic sail or dipole field. • Waves: If the SW plasma can “slip by” without transferring momentum, we lose our thrust.

26. Tim, Dennis & Pump

27. Scaling from 3’ to 30’ Helium Plasma

28. roll footage

29. Some Firsts (to my knowledge) • First artificial magnetosphere constructed in the laboratory and filled with plasma. • Largest space-physics laboratory experiment. • We will soon be blasting it with a Hall thruster--the first full blown solar-wind/mag’sphere experiment ever performed. (Date - middle September, 2000.) • First test-bed for global MHD models.

30. Can balloons compete w/sails? • Let’s be honest. Sails work 10,000 times better than SW balloons. If 30 km is the biggest balloon we can make, then a 300m sail will have equal thrust. At 5gm/m2, that is a mass of 354 kg. With some incremental improvements to sail materials, Winglee’s COTS advantage will be gone. Should we invest in mag balloons when sails are nearly there? • YES! Because there are ways to make balloons even more efficient, and therefore better than sails. • If plasma is opaque to light, even at 0.1% its a 10-fold increase in thrust over SW.

31. Black Plasma • Charged dust, when combined with a plasma, scatters light. At proper conditions, it even forms a “Coulomb crystal”

32. What sort of mass loading? • The dust grains are micron-sized, which is about 1e-15 kg apiece. At 36/mm3, that gives a density of about 36 mg/m3. • Since the volume (and mass) go as r3, while area goes as r2, the mass loading is r dependent ~ 25 mg * r. A 200 meter radius dusty plasma sail would then have the mass-loading of a carbon fiber sail. • Can we make the dust lighter and more reflective? Perhaps buckeyballs with chelated sodium atoms. Or even reflective ions - e.g., transition metal ions.

33. A Mars Mission Scenario

34. Mars Express

35. Hypothetical Balloon • Let’s suppose that we find an opaque plasma material for our balloon that weighs the same as the propellant ~ 100 kg. Then let satellite + propellant + payload =300kg • 30 km diameter with 1% opacity = 91nPa • 64 N / 300 kg = 0.21 m/s2 = 2% of g! • 36 days to Mars • 72 days to Jupiter • 7.4 months to Pluto

36. Conclusions • Magnetic balloons may be the fastest way through the solar system. They offer COTS technology for very fast transport. • The technology may not scale, however we are confident that it works from 1m => 10m • If “opaque” plasma were used, balloons may stay competitive with sails up to the many kilometer scale size. • One could imagine hybrid balloon/sail systems that combine both methods of travel.