Comments on: Dawn resumes descent to Ceres

By: Gealon

Gealon — Wed, 29 Jul 2015 18:46:00 +0000

Mmmm. The limited life expectancy of the engines probably has something to do with the electron gun used to balance the charge of the Xenon after it leaves the engine. If memory serves, the engines operate by stripping the electrons off of the Xenon atoms, giving them a positive charge. the atoms are then repelled out of the rear of the engine and a rear facing electron gun returns electrons to the atoms to rebalance their charge so they don’t get attracted back to the spacecraft. The gun is essentially an open ended vacuum tube so I can see it eventually wearing out. Likewise I remember DS1 having trouble with dust getting between its high voltage grids and Hyabusa was plagued with all sorts of problems.

Thanks Edward for the info.

By: Edward

Edward — Mon, 20 Jul 2015 21:22:36 +0000

In reply to Gealon.

Good question. Strange answer:
http://www.nasa.gov/mission_pages/dawn/spacecraft/index_prt.htm
“A total of three ion propulsion engines are required to provide enough thruster lifetime to complete the mission and still have adequate reserve.”

Thruster lifetime means that they were worried that a single thruster would not last long enough to perform the entire mission of visiting two asteroids. Thrusters have a design lifetime, but I am not familiar enough with them to know what wears out over time. The third thruster may or may not have been for redundancy.

Although they provide low levels of thrust (they wouldn’t get you off a planet), ion engines are useful in space because they are fuel efficient. Engineers and rocket scientists call rocket efficiency “specific impulse” (Isp), which is measured in seconds. Although they came up with that unit of efficiency from a weird ratio, you can easily think of it as the number of seconds that a thruster could produce one ounce of thrust while using one ounce of propellant.

Ion engines have an Isp in the thousands of seconds. A launch rocket’s chemical engine has an Isp somewhere around 300 seconds, so the ion engine is somewhere around ten times more efficient than the chemical engine. Since the probe only needs about a tenth as much fuel as if it had used conventional chemical engines, there was more weight available for instruments, or more likely, there was more weight available for enough fuel to visit the second asteroid.

By: Gealon

Gealon — Mon, 20 Jul 2015 18:18:04 +0000

I never could understand why in all of the art and diagrams of Dawn that it had those three engines mounted in three different orientations. Now even more bafflingly I see here that two of them are gimbaled??? Redundancy aside, what is the point of having three engines doing the job of one other then to make the craft more expensive?

If they needed to point the engine to maintain a certain facing of the spacecraft for the instruments while thrusting I can understand the off-axis engines, but then why not simply use a single engine that can gimbal in both directions? It would reduce weight and complexity as well as increase the craft’s performance.

If redundancy is really what predicated using more then one engine, then why not use two like on Cassini? When one engine is thrusting the other is just dead weight, but they went with a design that has two dead engines. This bit of new information has left me even more confused then before.