Slashdot Mirror
Boeing Readies For First Ever Conjoined Satellite Launch
Zothecula writes Boeing has successfully joined two of its 702SP satellites in a stacked configuration in preparation for a launch scheduled for early 2015. Aside from being the first involving conjoined satellites, the launch will also put the first satellites to enter service boasting an all-electric propulsion system into orbit. "Designed by Boeing Network & Space Systems and its defense and security advanced prototyping arm, Phantom Works, the 702SP (small platform) satellites are an evolution of the company's 702 satellite. Operating in the low- to mid-power ranges of 3 to 9 kW, instead of chemical propulsion, the satellites boast an all-electric propulsion system that Boeing says minimizes the mass of the spacecraft and maximizes payload capacity."
According to wikipedia the 702SP uses xenon.
Yes, ion thrusters. Used to adjust from the original elliptical orbit to the final circular one (and yea, it takes a long time to do that compared to a conventional booster). They've been using these to maintain position for almost 20 years.
good question, the efficiency comes from the speeds achievable
squirting pressurized gas out of a nozzle into space results in an exit velocity of ve = sqrt(C), where C is some clever arrangement of specific heat, R, T, etc (it's been a while since i've study nozzle design)
Basically, the exit velocity is limited by mostly the chemical's temperature; quadrupling the absolute temperature would only double the speed, so a lot of energy would be needed
ions are different (bare with me, I'm not an electrical engineer), because you just need to create them (chemical reaction, heat, etc), and then control them (static field, magnetic field). it's a lot lower power, and a lot greater impulse (over a lot longer time) than what ejecting molecules in to space can provide
And it seems to have a 4.5 kW power input, a specific impulse of 34 kNs/kg (insert an anti-Imperial rant here), and gives a thrust of up to 165 mN, which is fairly decent, Dawn has to do with about a half of that.
Ezekiel 23:20
It has to do with how quickly you can throw the propellant - how much momentum you can impart to it, which in return imparts a certain change in momentum to the rest of the satellite (delta-v). With conventional satellite propulsion, like fuel+oxidizer rockets or monopropellant thrusters, the energy available to impart that momentum is chemically based. That is, the propellants undergo a chemical reaction, get hot and/or change phase into a gas, and nozzles force that gas to exit at some velocity. Details vary with engine and nozzle design, but there are limits on how much thrust you can get each fuel type. Mass in, reaction energy, mass*velocity (momentum) out. Rocket designers measure this "efficiency" with a quantity called specific impulse (measured in units of seconds) For a given mass of fuel, you can pretty quickly calculate what the total delta-v the satellite has available to it.
Ion engines can impart much higher velocities to the "fuel" than chemical rockets, in part because they are using electrical energy (of which there is an arbitrarily large supply) rather than whatever you can get from chemical reactions. Again, the details vary based on the design, but ion engines tend to have specific impulses much higher than chemical rockets. The actual thrust (i.e., total force) from an ion engine tends to be miniscule, but is provided very efficiently, and can be produced for days or weeks at a time.