# Understanding Flightclub.io simulation of SpaceX's Bangabandhu launch

In this answer I used Flightclub.io's current simulation of the recent Bangabandhu Satellite-1 Mission. The pre-built simulations (you can do your own there as well) are sometimes refined over subsequent days.

But right now there are two things puzzling me.

1. 2nd pass of 2nd stage over Florida

In the screen shot below you can see the 2nd stage passing over Florida a second time. If I understand correctly, at about T+ 27:40 the 2nd stage stared again over Gabon, Africa, and burned for about 1 minute, putting it into GTO. Is this just an incomplete simulation?

2. complex events in timeline

From the Flightclub.io simulation:

Main Engine Ignition    -00:02
MECO-1                   02:31
Second Engine Ignition   02:36
Fairing Separation       03:37
Entry Burn Ignition-1    06:15
Entry Burn Ignition-2    06:19
Entry Burn Cutoff-1      06:24
Entry Burn Cutoff-2      06:25
Landing Burn Ignition    07:43
SECO-1                   08:19
Landing Burn Cutoff      08:29
Restart Ignition         27:38
Restart Shutdown         28:37
Bangabandhu-1 Separation 33:38


In the timeline of the simulation, why are there two Entry Burn Ignitions, and two Entry Burn Cutoffs with each pair separated by a few seconds? In the simulation software, is this a way to approximate a ramp-up and ramp-down of the thrust?

# #1

Checking the simulation config it can be seen that the second burn of the second stage is not completely configured. The Restart Ignition defines 0.7 throttle level but 0 of engines used so the burn does not have any effect on the simulation results.

And looking at the 3D view there is no simulated burn happening over Africa (no red segment and no change in orbit):

Screen from FlightClub.io

# #2

Two burn definitions are used to simulate the 1-3-1 burns used by SpaceX (probably to allow more precision and possibly to lower the stresses on the vehicle)

From the "Flight profile" entries in the simulation configuration it can be seen the number of Merlin 1D engines for the burn.

1. Entry Burn Ignition-1 - 1 engine
2. Entry Burn Ignition-2 - 2 engines
3. Entry Burn Cutoff-1 - 2 engines
4. Entry Burn Cutoff-2 - 1 engine

These pair exactly to produce the 1-3-1 burn with 4s of single (center) engine, 5s of 3 engines total and 1s of remaining center single engine burn.

You can see the config by building a simulation and using the "Bangabandhu-1" from Past missions as a template.

Screen from FlightClub.io

• That's really interesting! Is the simulation configuration something you can link to, or show a screen-shot of? – uhoh May 13 '18 at 8:21
• @uhoh added a description of how to get there and the screen. – jkavalik May 13 '18 at 8:29
• @uhoh added some findings about the second s2 burn too – jkavalik May 13 '18 at 8:44
• @jkavalik I'm the Flight Club dev - you're completely correct on all accounts. On point #1, the Restart Ignition is purposefully left with 0 engines because carrying out the engine restart squashes the resulting plots and makes the results page less meaningful (for example, the y-range in the altitude vs time graph changes from [0, 200] to [0, 36000]). However, the restart guidance and the throttle settings are configured such that by simply setting the number of engines in the ignition event to '1', the restart will be carried out properly, and the apogee will be raised to ~35,000km. – Declan Murphy May 14 '18 at 8:50
• @DeclanMurphy great explanation! thank you and good work. – jkavalik May 14 '18 at 9:13