I've just watched a video of the recent test and one thing I noticed is that comparing to other LES, like Mercury, Dragon, Soyuz, it looks really sluggy - like it has like 1.3-1.5 TWR. For pad abort, the LES should act quickly enough to get the crew a safe distance away before the rocket explodes. For in-flight abort good TWR is even more important, as the main boosters may be firing (and out of control) so it definitely must be higher than TWR of the rocket core without the payload - otherwise it won't even separate!

Is there something I missed about this test? Footage running in slow-motion? Extra-heavy mass simulator? Or do I misunderstand the requirements of LES?

  • $\begingroup$ The ignition at ~12s in the video looks a lot like slow-motion to me. The exhaust and dust billowing seem very slow and the lighting is different. You're absolutely right though - LES typically have very high TWR ~15 for Soyuz $\endgroup$ – Jack Jul 6 '18 at 14:17
  • $\begingroup$ I can't find any official source giving the specs of the test, but there's some good discussion on this NASASpaceflight thread $\endgroup$ – Jack Jul 6 '18 at 15:00
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    $\begingroup$ I think you mean 1.3-1.5 TWR btw - less than 1 means it wouldn't get off the ground at all. $\endgroup$ – Jack Jul 6 '18 at 15:01
  • $\begingroup$ @Jack: Yeah. Fixed. $\endgroup$ – SF. Jul 6 '18 at 15:18

The official video appears to show the ignition of the Launch Escape System in slow-motion and unfortunately doesn't include a time stamp for reference. The motion of the exhaust/dust on ignition and the change in lighting are evidence of this.

Back-of-the-envelope maths suggest the acceleration was ~10g which is backed up here (although the wording of the report makes it seem like little more than a guess) . The news report on ISRO's site has a rundown on some of the launch numbers, but doesn't mention the acceleration.

This pad test announcement has some further information on the ground tracking equipment.

As mentioned, an LES is of limited use if it is unable to outrun the launch vehicle itself or an explosion on the launchpad, so a high TWR is essential.

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  • $\begingroup$ I expect we may hear an official word in the coming days, please feel free to edit as more information arises. $\endgroup$ – Jack Jul 6 '18 at 15:28

The linked footage of the ISRO LES test is in slow motion. Look at how slowly the exhaust flame and smoke plumes propagate compared to Mercury, Dragon, Soyuz, etc LES launches.

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