Comments on: SpaceIL gets $5 million for its lunar lander/rover

By: Edward

Edward — Wed, 21 Nov 2018 20:01:09 +0000

Andi,
I don’t know the engine that Sparrow uses, but generally speaking engines with better efficiency (higher specific impulse) tend to have lower thrust. Ion engines, for example, are very efficient but are very low thrust. With the greater efficiency, they can use less propellant to do the same task, which allowed the Dawn probe to visit both Vesta and Ceres asteroids.

However, with lower thrust, they have more difficulty using the TLI approach, so using multiple apogee-raising thruster burns tends to help, as they can also take advantage of the Oberth effect, which is the added efficiency of using thrusters at perigee.
https://en.wikipedia.org/wiki/Oberth_effect

By: Andi

Andi — Wed, 21 Nov 2018 04:55:31 +0000

Thanks, Edward, that makes it quite clear. I assume the purpose of all this orbit raising is to save fuel over the TLI approach.

By: Edward

Edward — Tue, 20 Nov 2018 00:55:56 +0000

Andi,
This may help explain it:
https://en.wikipedia.org/wiki/SpaceIL#Launch

Once Sparrow is in Earth orbit and separated from the Falcon 9 launcher [as a secondary payload], and after several orbits around Earth, the spacecraft will slowly perform orbit raising. The orbit raising would take 2.5 month before reaching the Moon’s area of influence. Once there, the spacecraft will perform manuevers to be captured in a Lunar orbit, and orbit around the Moon between two weeks and 1 month. In the right orbit around the landing site, it will decelerate until soft-landing on the lunar surface.

Rather than use one translunar injection burn, as Apollo did, Sparrow will use several burns to raise its apogee and will eventually be captured by the Moon.

By: Andi

Andi — Mon, 19 Nov 2018 21:30:12 +0000

” launched in early 2019 and reach the moon two months later “.

Given that Apollo got there in three days, I wonder what the trajectory will be that takes two months.