SPRAY CAN IN SPACE!

Robert Hutchings Goddard launched the first liquid-fueled rocket on 16 March 1926. Fools wondered what its exhaust would push against if it entered the vacuum of space. Newton had it iced. However, folks were qualitatively correct: rockets are crappy.

Getting from here to there in the radiation-sleeted vacuum of whatever is change of momentum, (mass)(velocity). The bar tab for drinking deeply is exhaust kinetic energy, (mv2)/2. As the momentum of a self-contained rocket changes by a factor of X, the cost of doing business, energy, increases as X2. It is a (very NASA) losing proposition. Nuclear reactors haul radiation shielding mass and complaints from gamma-ray astronomers. Solar panels as external energy input are a sparrow fart in a hurricane. Nobody goes zooming to Mars in Starship One.

NASA management ponders unemployment for being denied hundreds of $billions in fast cash with human meat as the bowsprit. Gotta go somewhere, gotta take tourist snaps and later belch slide shows. Seeing a lot of itself, late June 2011 had NASA six months away from photographing interplanetary garbage in high definition.

Spaceship Dawn (launched 27 September 2007) aimed at protoplanet Vesta (upgraded from asteroid by PR blitz, but dwarf planet way short of spec). Spacecraft Dawn has solar-powered ion thrusters! Solar panels feed the greater fraction of 10 kilowatts into a xenon ionizer (decreasing as the square of its distance from the sun). Heavy ions are accelerated out the rear while charge-compensating electrons are shot out with them. About 10 times the momentum change/mass of chemical fuels is available as a remarkably small dribble, demanding patience. This is sexy if you like nursing home denizens and healthcare reimbursements. The fuel tank is 937 pounds of xenon.

Massage one number: 425 kg, 3237 moles, 72,552 STP liters of gas. Xenon is 8.711 v/v parts-per-million in air. At 80% extraction efficiency that is 11.1 billion liters of air processed at room temperature. Xenon is pricey. Spacecrap Dawn had a $510,000 fill-up if NASA got the bulk discount rate, a cool $1 million for lab grade. Xenon's critical constants are 16.6°C at 847 psi. Keep tankage chilly so xenon stores liquid phase at higher density and lower pressure. Vesta tops at -20°C.

On 1700 hrs UTC 27 June 2011, asteroid 2011 MD buzzed Earth 7500 miles above its surface. Geosynchronous orbit is about 22,200 miles altitude. Why did NASA travel well beyond the orbit of Mars, 220 million miles from the sun? Maybe NASA confused millimeters with miles. They both start with the same sound.

Diversity Atsrofysicals Personager, "Ahm seein' seben hund'd fiddy heah, wid a zera, mils. An' ahm seein' two hund'd twenny heah, wid no exter zera, mils wid stuff. We go wid de close un', two hund'd twenny mils."

She gave a PowerPoint presentation, too. It can't be wrong. Vesta may have lost about 1% of its mass in a mammoth space collision. Debris locally arrived FOB, re kryptonite, as Howardite-Eucrite-Diogenite meteorites. If things go wonky, success is already dry-labbed. Certain lab techs can be lost in an unexpected SUV crash.

The huge brute 9-solid boostered three stage Delta 7925-H rocket massed 232,000 kg. Its fuels' specific impulse ranged from 278 sec for solids to 319 sec for dinitrogen tetroxide/aerozine. It accelerated New Blue Dawn to 11.46 km/sec. 425 kg of xenon suggest a velocity change of 10+ km/sec with specific impulse 3100 sec. Each of its three ion thrusters, one fired at any time, gives 92 mN of thrust or 0.331 ounce-force, the weight of two pieces of 20 lb 8.5x11" copier paper at sea level on a medium humidity day. It runs continuously for more than a decade.

Mercury is a choice ion propellant: 1.53 times the atomic weight of xenon, cheap, abundant, stores densely as liquid, easily ionized. Alas, condensed vapor shorts out electrical connections and dissolves the vehicle by amalgamation. Sulfur hexafluoride is 1.16 times the atomic weight of xenon, etc. Its decomposition products are violently corrosive, including toward ceramics. Since money is no object... buckeyballs! Fullerene soot. C60 MW = 720.642, C70 MW = 840.749, 3.5:1 molar ratio: weighted average MW = 747, 5.69 times the molecular weight of xenon, and stores as a solid.

Acta Astronautica 51(12) 865 (2002)
http://www.cs.cmu.edu/afs/cs/usr/mnr/st/std080

Ion engine current tops at the space charge limit:

(5.402x10-8)[(v/M)1/2](E3/2)/d2 amps/cm2

where "v" is ion charge, "M" ion mass in AMU, "E" the accelerating potential (volts), and "d" the gap (cm). Given constant exhaust velocity and electrode spacing, thrust density scales as (M/v)2. Large mass/charge ratio gives a compact ion engine.

Space probe Dawn of an Era will orbit Vesta, take snaps, then move onto Ceres (a real dwarf planet!). It will orbit Ceres, take snaps, and - if there is still enough remaining fuel to deorbit - go somewhere else and take more snaps. Every square centimeter of everything in the solar system will eventually be mapped. On that magic day... NASA will propose doing it again.


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