How far could a heavily modified SpaceX Dragon capsule fly just by using its SuperDraco engines? By heavily modified i mean unmanned, unpressurized and full of mono propellant.
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2 Answers
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Dragon V2 (Crew Dragon) uses 8 SuperDraco engines with maximum of 73,000 N thrust each. So, in theory, it could produce enough thrust to lift about 50,000 kg off the pad at thrust-to-weight ratio (T/W) of ~ 1.19. You need T/W larger than 1 to lift off the ground, and some more. T/W of ~ 1.2 is low for a launch vehicle and is usually closer to ~ 1.5, but let's run with this number and see where we get if we stretch Crew Dragon and its 8 SuperDraco engines to near senseless theoretical limits;
So we have vehicle's maximum wet mass of 50,000 kg. Its dry mass is, according to linked Wikipedia article, around 4,200 kg. Assuming no modifications that will add dry mass to the vehicle are needed to accommodate that much of NTO/MMH propellants at specific impulse of 240 seconds (yes, a ludicrous notion, but we'll let this one slide too and let's assume that we dropped solar panels, heat shield, nearly everything inside and used saved mass to extend its storage to accommodate that much more fuel - i.e. about three times its nominal volume), we now have its maximum propellant mass it could possibly lift off the ground of 45,800 kg.
We now have everything we need to plug our numbers in the Tsiolkovsky rocket equation, which is good enough as it is for a single stage vehicle:
So our generously (ridiculously?) light and highly modified Crew Dragon filled with 45,800 kg of NTO/MMH reaches a maximum delta-v of about half the change in velocity needed to reach orbit from the ground, and about 76% of the orbital velocity of the ISS if it was deployed in vacuum and didn't lose any of that thrust to atmospheric drag. Note that this also assumes no thrust loss to multiple engine combustion instability and so on.
Assuming all that, it probably could fly close to the Kármán line on a suborbital flight (it would have enough fuel for about 103 seconds of powered flight at full thrust), and reach altitude officially declared as the boundary of outer space, but then again, this is a generous back of an envelope notion and not really achievable in real world. Such paper thin propellant tanks simply wouldn't withstand dynamic pressure and ascent heating during launch, likely not even the acoustic shock during engine ignition.
answered Nov 26, 2015 at 17:23
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$\begingroup$ +1 but I think 50000 kg wet mass is too much of an overestimation. Based on the Wikipedia article for the Dragon V2, the volume is at maximum 14 cubic metres, and with an average density of about 1150 kg/m3 for the propellant you end up being able to fit about 16000 kg inside (plus about 2000 kg in the tanks). That gives you a $\Delta v$ of about 3.9 km/s. $\endgroup$ Nov 26, 2015 at 17:49
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$\begingroup$ @BrianLynch Agreed, it is an overestimation and I've mentioned that several times in my answer, I was just entertaining a possibility of using some incredibly lightweight propellant tanks with the 8 SuperDraco engines (the question does ask about "heavily modified" Dragon, so I only took from it what's needed to loft it as high up as possible). I also assumed linear thrust gain using 8 thrusters, and other things that I'd end up with a spanked bottom if I attempted it in real life :)) $\endgroup$ Nov 26, 2015 at 18:10
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$\begingroup$ WOW man i wasn't really expecting such an in depth answer! thank you so much for pointing out all the reasoning steps! will def look into them more in depth! $\endgroup$– pastulloNov 27, 2015 at 14:42
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We've seen how high a Dragon can reach with its superdraco engines on its pad abort test, and it was about 5000 feet. If you filled it with propellant instead of cargo and passengers it might get a bit higher, but not that much. If you hooked up a big fuel tank to it the engines would not be powerful enough to lift it. It's not going to get anywhere near space on its own.
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$\begingroup$ sorry but i think not only your message is not useful at all but it's also very wrong. In the pad abort test the Dragon used the trusters just to "escape" from a theoretical exploding booster. However, you know very well that Dragon v2 is meant to have landing capabilities which would obviosuly require much, much more fuel than what we have seen in the video. $\endgroup$– pastulloNov 27, 2015 at 14:50
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1$\begingroup$ In a soft landing all a system would need to do is slow down from terminal velocity, the atmosphere does most of the work for you. You don't need that much fuel or thrust to do that. $\endgroup$– GdDNov 27, 2015 at 15:10