New attempt to launch the Crew Dragon on May 30: trajectory

screenshot from the May 27 live webcast

In an earlier post I discussed the SpaceX Crew Dragon Demo-2 launch. Originally slated for 27 May, it was postponed (with the astronauts already seated on board) because of bad weather: Tropical Storm Bertha more north on the US coast was the main culprit.

The new launch attempt will be on May 30 at about 19:22:45 UT (the subminute time comes from Spaceflight Now, not from an official SpaceX or NASA source, so is apocryphal). If that launch is scrapped to, the third backup date is May 31 near 18:59 UT.

As things currently (29 May 21:00 UT) stand, weather prospects are not that good for both these dates either, with currently a 50% chance of a weather violation on the 30th and 40% on the 31st: so perhaps we will see a scrub again.

Click map to enlarge

But in case the launch does happen on 30 May, the map above is the trajectory the Crew Dragon will fly on its first revolution (times on the map are in UT).

Some 23 minutes after launch, the Crew Dragon will pass over Europe, along this trajectory (times are inUT: add one hour to get BST and 2 hours to get CEST):

Click map to enlarge

Note the location of the day/night terminator...only eastern and southeastern Europe has sufficiently dark skies at that moment.

The launch time has shifted considerably forward compared to the May 27 original launch date, by about 1h 10m. As a result, the pass is no longer favourable for NW Europa, as the pass will be before sunset for the UK, and around sunset for coastal Europe.

Only longitudes east of say longitude 13 deg E will have a sufficiently dark sky to see it on the first revolution, so eastern and southeast Europe will have a prime seat this time.

Coastal western Europe and the UK might have, depending on your locality, a theoretical chance to see the second pass 1.5 hours later, near 21:18 UT. For most localities, that will however be a very low elevation pass though, often at a maximum elevation of les sthan 10 degrees.

At the end of this blogpost, I will provide some sky charts for several European localities for both those localities with a chance to see something of the first pass, and those who might theoretically catch the second pass.

The reason that the launch time is 1h 10m earlier on May 30 than on May 27, is that the launch time is instantanious as it is determined by the moment that the orbital plane of the ISS passes over the launch site. This time shifts back by 23m 22s each day, as is clear from this tabel in which I calculated orbital plane crossings over LC-39A (and is visualized in the illustrations below it):

ISS plane crossing over LC-39A:
-------------------------------
Date           UT   
27 May         20:36:52
28 May         20:13:30
29 May         19:50:09
30 May         19:26:47
31 May         19:03:26
-------------------------------

You can also see in the table that the actual launch time is a few minutes before the plane crossing. This has two main reasons.

One is that what is actually of relevance is the position of the orbital plane once the rocket reaches orbital height (a few minutes after launch).

The other is that the Crew Dragon initially is inserted into a ~200 km altitude orbit, which is only half the orbital altitude of the ISS. As a result, the Precession rate of the RAAN is faster than that of the ISS: so launch has to be somewhat earlier or otherwise, over the 19 hour flight, its RAAN would overshoot rather than match that of the ISS upon arrrival at the orbital altitude of the ISS.

The reason May 28 and May 29 were not chosen as backup dates, is because of a second consideration: the ISS has to be within a certain distance window to the launch site in order for the two (Crew Dragon and ISS) to meet up after 19 hours of flight. As it happens, and I am not sure that is deliberate or just a happy coincidence, this also means that on the chosen dates, docking will happen on the night-time side of the Earth (with launch on May 28 or 29 it would have happened on the daytime-side).

Below are a number of sky maps for localities that have a dark enough sky (generally: sun no less than 5 degrees below the horizon) to see the first pass, some 25 minutes (for eastern Europe) after launch near 21:46 CEST. Note that there is a time uncertainty of about 1 minute or so.

TLE's are provided below the maps.

NOTE: if you are not near one of these localities, then Heavens-Above provides you with predictions for your custom location. Please note however that Heavens-Above predictions for the second revolution (the 23:19 CEST pass over Europe) seem to be based on the TLE for the first revolution, resulting in a time difference of about 1 minute with my predictions below.(but also realise there is an uncertainty of 1-2 minutes in the estuimated orbit anyway).

Maps for locations in NW Europe might theoretically be able to see the Crew Dragon on its second revolution, near 23:18 CEST (22:18 BST), some 2 hours after launch. But in most cases this will be very low above the horizon. Please note that the time uncertainty is 1-2 minutes at least!

Here is an estimated TLE for the first revolution:

CREW DRAGON                                      initial orbit
1 70000U 20999A   20151.80474535 -.00003603  11390-4  00000+0 0    04
2 70000  51.6423 075.0039 0122953  45.6251 315.4951 15.99554646    01


And here is an estimated TLE for the second revolution:

CREW DRAGON                                      second revolution
1 70001U 20999A   20151.93029831 -.18507952  12289+0 -23808-1 0    05
2 70001  51.6233 074.5097 0096856  46.3995 314.2887 15.95177824    03