SMART-1 used an ion engine of some sort though, I believe. I think Beresheet is using a storable bi-propellent rocket.
Yes: similar trajectory but different engine trajectory. Except not really similar trajectory. Just the principle that if you're willing to have patience, you can change the fuel parameters.
So perhaps the Beresheet crew can promote their mission as: taking as long as you're used to.
Or merely, "...an appropriate orbit for an unmanned spacecraft." A low-energy orbit isn't practical for Apollo because you have other consumables to support human life that would have to be increased to sustain the crew for the longer mission. There's a sweet spot between the required propellant mass and the required consumables mass. Then you have probably-of-failure distributions for all the critical equipment. The longer the mission, the longer the period over which you have to integrate that failure function -- that is, the higher the probability something critical will fail. Again there's a sweet spot between the probability of failure by operating the spacecraft over a longer time and the probability of failure by employing a higher-energy propulsion system.
The trajectory here is closer to the cycler concept than the ever-accelerating SMART-1 orbit. That comparison was pemature. The operational advantage you get from cycling your way hither and yon is that you have a lot of opportunities to observe the developing orbit and adjust it efficiently with the main propulsion burns. In contrast, Apollo required the MCC type burns, which are themselves the sweet spots between how accurately you can observe the orbit versus how efficiently you can alter it. The design and planning portions of aerospace engineering are all about the sweet spots.
Incidentally, how is it that multiple short burns to raise the apogee are more efficient than a single longer one? Is it that the efficiency of the burn is greatest when the spacecraft is closest to perigee, and this efficiency drops off considerably even minutes either side of perigee?
Well, yes for certain concepts of "efficiency." Adding kinetic energy to an elliptical orbit changes its shape. How the shape changes depends on where in the orbit you add it -- the real anomaly of your spacecraft. You know the circularization maneuver; it's applied at apogee, and it has the effect of raising the perigee. At apogee, the spacecraft is at the right altitude but the "wrong" velocity. The velocity is too slow for a circular orbit at that altitude, so you just add velocity while at that altitude. Think of the Beeresheet orbit as the opposite -- it's an elongation maneuver. If you add velocity at perigee, it changes the shape of the orbit by raising the apogee. Or you can think of it as elongating the orbit, changing its eccentricity. The highly-eccentric orbit is the lowest-energy orbit having the desired
altitude at some point (instead of at many or all points). It just takes several cycles to achieve with minimal fuel.
The goal here is to get to the Moon. Or rather, to achieve the farthest distance from the Earth with the least effort. This orbit achieves that by means of a long, skinny orbit whose apogee is at the desired altitude. Once you reach an apogee altitude of 238,000 miles or so, it's just a matter of waiting for the Moon to come along and change the orbital mechanics from a two-body problem to a restricted three-body problem.
It may also have more to do with thermal limits.
I don't know if that's the reason here, but it's often the reason. The point of the LM plume deflectors was to lengthen the duty cycle for thermal reasons.
