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Comment by JshWright | Hacker News Reader

There are a few changes with this parachute system. Notably, it uses 4 chutes, rather than 3 (which is the number used by the current cargo Dragon, and how many the Crew Dragon test article used for the pad abort).

All in all though, you're right, this was mostly just a checkbox that needed to get checked off.

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Josh, do you know if is there anywhere that confirms the 204s ISP number for the superdraco thrusters? And what the fuel mass is for that?

EDIT: yes its a typo, I mean 240s

The citation for the wikipedia page (240s ISP) gives an exhaust velocity of 2,300m/s :

https://en.wikipedia.org/wiki/SuperDraco#cite_note-DragonFly... - page 12 of the PDF.

So taking a rough 2300/9.8 gives about 235s

FWIW, Isp is measured in seconds and by multiplying it times gravity, you get the exhaust velocity. I believe this was done to more easily compare rocket efficiency regardless of units of gravity.

It looks like you're doing this calculation:

Isp * Gravity / Gravity

If you're trying to predict the hover duration, that will depend on the delta V of the vehicle. Then, a hover duration would be:

Delta v / Gravity

Not sure what the expected delta V is for the Dragon though.

Yes I read that (and the spaceflight insider article (http://www.spaceflightinsider.com/organizations/space-explor...), all numbers that are derived by third parties.

So while I do not doubt that the numbers for that vehicle and that test are accurate, I keep hoping to find actual data from SpaceX on the performance of the production rocket slated for use in the production Dragon Module.

I understand that SpaceX plays this information close to its vest given the competition with Boeing for Commercial Crew and perhaps Blue Origin should SpaceX ever decide to sell rockets for sub orbital tourist jaunts[1]. Anyway I'm just interested in more official specs so that I can have more confidence in my understanding of their capabilities.

[1] If you think about it, they have all the hardware, an F9r with a Dragon Crew mounted on top, they boost up to 100km, separate, the Dragon gets a nice ballistic arc for that weightless feeling and then both the booster and the capsule go back and land back at the pad for re-use.

I don't know of any source for that. Is 204s a typo of 240s? 204s would be awfully low for a Hydrazine engine (though I suppose it's possible they care a lot more about the TWR and have optimized for that).

Even an Isp of 240 is not all that high. This page:

http://www.astronautix.com/props/n2o4mmh.htm

Lists a bunch of engines that have Isp values in the 300 range.

SpaceX tends to optimize for total operational cost, rather than performance, so I wouldn't be surprised by the tradeoffs they've made to create a reliable and reusable system.

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