Al Yah 3: One way to go
BLOG: Go for Launch
As I showed yesterday, impulsive transfer strategies for the near future can be found to insert the Al Yah 3 satellite from the orbit it is now in after the VA241 launch. (There is an analysis of the launch on spaceflight101) However, the Delta-v cost of any near-term solution would likely exceed the capability of the chemical onboard propulsion system by far. A much cheaper strategy consists in waiting until mid-September 2018.
(Author’s Note: All I am providing here is my personal opinion and some results of mathematical calculations I have performed myself based on publicly available information. These results could have been obtained by anyone with a background in orbital dynamics. Nothing I write here shall be construed as reflecting the official position of my employer. Essentially, what we have here is a mathematical problem: A spacecraft is in a given initial orbit and shall reach a certain target orbit B with a certain amount of delta-v. What I am presenting is a mathematical solution to this problem that I have identified. Now, back to the blog article …)
Firstly, I numerically propagated the current orbit of Al Yah 3 for one year, taking into account all relevant perturbation sources. The evolution of the mean argument of perigee, inclination and perigee altitude is shown below. The argument of perigee goes through 360 deg after 230 days. This corresponds to the calendar date 19 September 2018. Until then, the inclination will also increase somewhat. The perigee altitude continues increasing, so there is no problem with the orbital stability from now until then. This is good news.
On 19 September, two manoeuvres will suffice to insertion into a perfectly nominal geostationary orbit. The first, with a size of 1505 m/s, gets rid of almost all of the inclination and raises the perigee. The second is much smaller at 122 m/s. This lowers the apogee and reaches 0 deg of inclination so the final orbit is equatorial and circular. The total impulsive cost amounts to 1627 m/s.
This is less than 10% more than the 1500 m/s commonly budgeted for geostationary orbit insertion. But of course, it is not realistic to assume this impulsive delta-v value. In reality, manoeuvres are not impulsive but finite, so their efficiency is reduced. Then, especially the first, large manoeuvre will have to be spread out over a sequence of several days. During this time there will be perturbations, so the total cost will increase.
Still, it is likely that with the propellant aboard Al Yah 3, it should be possible to reach a final orbit that is at least very close to the nominal geostationary orbit. If necessary, the ion propulsion system foreseen for stationkeeping during the operational phase will have to mop up the residues. This may cause some problems. The ion propulsion system will have been designed for stationkeeping, so the thrusters may be canted with respect to the in-plane and the out-of-plane directions. Also, their power consumption and thrust level may be rather low because they are supposed to be used while the payload is operating.
Al Yah 3’s current orbit has a period of 13 hours, so the spacecraft passes through the Earth’s radiation belts at least three and often four times per day – twice per orbital revolution. The solar arrays and the electronics are not designed for the radiation loads this will incur – there will be some degradation of the electronics. The solar array panels are still folded up and will stay that way until after the high thrust chemical propulsion system will have been used for the last time, in September.
This means that most of the solar array panels are to some extent shielded from the charged particle flux. The outermost panels will receive the full brunt, though – not just of radiation, but also of the other bit of nastiness that the exosphere has to offer: atomic oxygen. The question is to what extent this will degrade the spacecraft’s functionality, or the lifetime of its payload.
The solution would be to perform part of the perigee raising already now. This should be largely cost-neutral. All perigee raising that is performed now will not have to be performed in the final insertion. Raising the perigee to 2000 km costs 149 m/s, to 4000 km costs 293 m/s.
The raise might not have to be very large. ESA’s SMART-1’s spacecraft was launched in September 2003 and slowly spiralled its way up to the Moon using an ion thruster. It experienced significant radiation damage, but it was found that once the perigee altitude was above 4000 km, there was virtually no longer a problem with radiation. This may not be the case for Al Yah 3 – after all, SMART-1 was designed to survive this harsh environment – but it appears likely that also for Al Yah 3 such a raise in perigee altitude would significantly mitigate the radiation problem.
The downside of a perigee raise is that this will slow down the rate of change of the argument of perigee, so it will take much longer for that angle to drift to a value of 360, where the final orbital insertion can take place. With 4000 km perigee altitude, it might take until June 2019 before the insertion manoeuvres can be applied. Therefore, it appears advisable to find an acceptable compromise for the perigee raise: high enough to reduce the radiation damage but still low enough so that the GEO insertion is not delayed excessively.
Reduced Operational Lifetime?
Assuming that the ion propulsion will have to be enlisted for orbit topping, this will use up some Xenon propellant that was foreseen for stationkeeping. Then, as discussed, there will be some degradation of its electronics, unless the perigee altitude is raised pronto. This means a reduction of orbital lifetime, perhaps by several years. And there is of course inevitably also a belated entry into service, which likely will not take place before October 2018.
A Possible Way Forward
One way to proceed would be as follows:
- Ascertain whether a perigee raise is required to radiation mitigation. If yes, define the altitude to which the perigee must be raised in order to avoid excessive radiation damage. Some damage should be accepted.
- Once the waiting perigee altitude is selected, perform the raising manoeuvre asap.
- Wait until the argument of perigee has drifted to 360 deg. This will happen no sooner than mid-September 2018. It can be later if a perigee raising manoeuvre is performed.
- Perform the final insertion manoeuvres, using up all chemical propellant
- Deploy the solar arrays, commission the stationkeeping thrusters and perform orbit topping (if required).
- Commission the payload and commence normal operations.