# Shortest ever F9 2nd stage burn for the largest ever F9 payload? (Starlink Mission's SES-2 burn time of 3 seconds)

According to the SpaceX broadcast the first SpaceX Starlink launch (24-May-2019) is notable for reasons including being the heaviest ever Falcon 9 payload to date.

At roughly T+ 46:10 SES-2 initiates and the announcer says the burn is only 3 seconds long!

1. This seems to me to be quite a short 2nd stage burn. Considering that it's the heaviest payload to date, why such a short burn?
2. Is this the shortest-ever Falcon-9 2nd stage burn?

It's done 3 second burns before. An example was Irridium 8, which did the same 3 seconds. I don't have a databaes of all such press kits, but I think it is pretty common for LEO satellites. Deorbit burns might even be shorter too.

• I wonder if the payload masses are at least comparable? – uhoh May 25 at 2:07
• it's within a factor of two at least; Iridium Next 860 kg $\times$ 10 ~ 8,600 kg, Starlink 227 kg $\times$ 60 = 13,620 kg – uhoh May 25 at 2:42
• It seems strange to me that they don't either use a single stage or have the first stage do less and load up the second stage more. Apparently there's something not intuitive about staging, which doesn't surprise me. I have no idea what question to ask to get a better understanding. – bitchaser May 25 at 4:06
• @bitchaser This is just the second burn of the second stage used for final orbit adjustments. The main (first) burn of the second stage was 6 minutes. – asdfex May 25 at 10:10
• @bitchaser Essentially yes. The first burn of first and second stage places the payload in an elliptical orbit with the right perigee. The burn needed for circularization then depends on how elliptical this orbit was. How length of the initial burn influences eccentricity might be a good question on its own. – asdfex May 26 at 9:52

As an addition to @PearsonArtPhoto, let's have a look at the numbers.

The second stage has a thrust of almost 1000 kN and a dry weight of about 5000 kg. Add the payload and some remaining fuel we might be at a total weight of 30 t (conservative). $$\Delta v = \frac{F\cdot t}{m} = \frac{1000~\rm{kN} \cdot 3~\rm{s}}{30~\rm{Mg}} = 100 ~\frac{\rm m}{\rm s}$$

A $$\Delta v$$ of 100 m/s at perigee (the burn takes place 45 minutes after launch) is sufficient to raise the apogee by 350 km in LEO. This seems to be a realistic value for a 550 km target orbit.