Beresheet, SpaceIL Israeli Moon mission

[Norm38]

So you’re saying that they burn as long as they efficiently can each orbit, but their engine isn’t powerful enough to get the dV they need in fewer orbits?

[Phil Stooke]

Beresheet is not the first spacecraft to do this.  Chang'e 1 did it, so did Chandrayaan 1 and LADEE, and Chandrayaan 2 will do it.  Maybe the best example is ESA's SMART-1 which used an ion engine with its tiny thrust to make the trip over many months.  A small spacecraft with a small rocket or even an ion engine, can get to the Moon using this method, where the alternative would be to use a larger launch vehicle with its attendant much higher cost.   If you have lots of time and no crew to carry through the radiation belts, it's a good strategy.  The other advantage is that a direct route has to be accurate and may have only a couple of chances to correct its path, but a multi-orbit path has many opportunities for adjustment. 

[smoliarm]

So you’re saying that they burn as long as they efficiently can each orbit, but their engine isn’t powerful enough to get the dV they need in fewer orbits?

IIUC, the reason is: they do not have enough fuel to do Trans Lunar Injection in one burn. Therefore they use Moon's gravity to help: they calculate each intermediate burn so that afterwards, at some apogee the Moon is "in the right place" to add a bit of speed to the spacecraft.

[mcgyver]

So you’re saying that they burn as long as they efficiently can each orbit, but their engine isn’t powerful enough to get the dV they need in fewer orbits?

Being it not powerful enough, it steals power (energy, actually) from planet Earth at each orbit. Slow, but cheap.

[Llian Rhydderch]

The Wikipedia article on the LADEE mission from 2013 has a decent technical summary of the "orbit phasing" approach, and the rationale for using it, along with a very good animation from the blog of the team that guided/specified the astrogation on that mission.  Worth a look!

This would seem to be quite similar in approach, and taking advantage of the lower cost of a rocket/mission that does not put the spacecraft fully on it's final lunar trajectory. 

[Llian Rhydderch]

So you’re saying that they burn as long as they efficiently can each orbit, but their engine isn’t powerful enough to get the dV they need in fewer orbits?

IIUC, the reason is: they do not have enough fuel to do Trans Lunar Injection in one burn. Therefore they use Moon's gravity to help: they calculate each intermediate burn so that afterwards, at some apogee the Moon is "in the right place" to add a bit of speed to the spacecraft.

This is not quite correct.  The Moon will be in the wrong position to assist with gravity during many (but not all) of the highly-elliptical geocentric orbits that the Beresheet spacecraft will take during its first six weeks or so.  The Beresheet orbit durations are much shorter than the 28 day Lunar orbit, so it has to be this way.

[Llian Rhydderch]

So you’re saying that they burn as long as they efficiently can each orbit, but their engine isn’t powerful enough to get the dV they need in fewer orbits?

Being it not powerful enough, it steals power (energy, actually) from planet Earth at each orbit. Slow, but cheap.

Do you have a source for that? 

As long as it is in the highly-elliptical orbit that it is in following release from the Falcon 9 2nd stage (~69,000 km apogee; ~200 km perigee), it does not seem like it is the subsequently gaining more energy from Earth for each subsequent full orbit.  Just holding it's own, on a per orbit bases.

Extra delta-V is coming from the dozen or so engine firings and mid-course corrections that it will undertake in coming weeks.

If I'm wrong, happy to be corrected.  But it seems it'll get no net energy gain from Earth by simply remaining in an elliptical geocentric orbit.

[ppb]

So you’re saying that they burn as long as they efficiently can each orbit, but their engine isn’t powerful enough to get the dV they need in fewer orbits?

Being it not powerful enough, it steals power (energy, actually) from planet Earth at each orbit. Slow, but cheap.

Do you have a source for that? 

As long as it is in the highly-elliptical orbit that it is in following release from the Falcon 9 2nd stage (~69,000 km apogee; ~200 km perigee), it does not seem like it is the subsequently gaining more energy from Earth for each subsequent full orbit.  Just holding it's own, on a per orbit bases.

Extra delta-V is coming from the dozen or so engine firings and mid-course corrections that it will undertake in coming weeks.

If I'm wrong, happy to be corrected.  But it seems it'll get no net energy gain from Earth by simply remaining in an elliptical geocentric orbit.

200 km perigee seems pretty low to stay there for multiple orbits. Do they plan on raising that? Wouldn't take much DV at that high apogee.

[lamid]

Being it not powerful enough, it steals power (energy, actually) from planet Earth at each orbit. Slow, but cheap.

For a gravity assist need to change the sphere of influence (SOI).The Beresheet now has a orbit in SOI Earth.

[lamid]

Space-track.org
44049
BERESHEET
2019-009B
PAYLOAD
ISRA

[Steven Pietrobon]

200 km perigee seems pretty low to stay there for multiple orbits. Do they plan on raising that? Wouldn't take much DV at that high apogee.

Yes. Their first burn is at apogee to raise perigee to 600 km.

http://www.planetary.org/blogs/jason-davis/beresheet-what-to-expect.html

[ppb]

200 km perigee seems pretty low to stay there for multiple orbits. Do they plan on raising that? Wouldn't take much DV at that high apogee.

Yes. Their first burn is at apogee to raise perigee to 600 km.

http://www.planetary.org/blogs/jason-davis/beresheet-what-to-expect.html

Cool--thanks for the link Steven.

[mlindner]

Shouldn't this be in the Commercial Space Flight section? That's the first place I looked for this. This isn't a science mission really.

[FutureSpaceTourist]

Cross-posting:

https://twitter.com/jasonrdavis/status/1098993073281851392

SpaceIL Beresheet update: The optional first apogee burn was not required, SpaceX's Falcon 9 gave the lander a "perfect" trajectory. Apogee Maneuver 2 is scheduled for Sunday at 11:29 UTC.

[Blackstar]

Shouldn't this be in the Commercial Space Flight section? That's the first place I looked for this. This isn't a science mission really.

It's not really a commercial mission either--they took donations to fund it, not advertising or other contracts.

It's really an aspirational mission.

[mcgyver]

So you’re saying that they burn as long as they efficiently can each orbit, but their engine isn’t powerful enough to get the dV they need in fewer orbits?

Being it not powerful enough, it steals power (energy, actually) from planet Earth at each orbit. Slow, but cheap.

Do you have a source for that? 

It's just a guess: if they had available all the needed fuel&power , wouldn't they go straight to the moon in 3 days?
Instead, at each perigee they can perform a gravity assist with Earth to raise apogee; once apogee is high enough, Beresheet enters moon SOI, and then all the road is downhill. :-)
Gravity assist = energy stealing :-)
Maybe with just 100 M$ that's the best you can afford.

[mcgyver]

Gravity assist interactive simulator:
http://galileoandeinstein.physics.virginia.edu/more_stuff/flashlets/Slingshot.htm

[lamid]

But Beresheet does not make use Earth gravity assistance.
He in the perigeum ignites the engine and lifts the apogeum.
It uses the oberth effect.
The gravity assist should fly from SOI Sun to SOI Earth, so that the Earth could drag him.

[mn]

http://www.israelnationalnews.com/News/News.aspx/259508

Engineering team of SpaceIL and IAI complete first maneuver of Israel's first lunar lander spacecraft

Edit to add details

The maneuver was made at a distance of 69,400 km from Earth for 30 seconds and will increase the spacecrafts closest point of approach to Earth to a distance of 600 km

[lamid]

Video the 1st Israeli spacecraft on the moon