Elon Musk is quoted to have said, after the launch of Falcon Heavy, that «it can launch things direct to Pluto or beyond, no stop needed, don’t even need gravity assist or anything». Now, I am not an expert in orbital computation, but this claim seems highly dubious. New horizon took 9 years to reach Pluto. How long would it take for a spacecraft launch by Falcon Heavy without using any gravity assist?
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5$\begingroup$ Long time and/or small probe. That it would be possible does not mean it would be a good idea. $\endgroup$– jkavalikCommented Feb 7, 2018 at 20:16
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2$\begingroup$ New Horizons was launched by an Atlas V 551, which has a payload to GEO of 8.9 tons. FH has 26.7 tons of payload to GEO. That difference translates into a lot of extra delta-V for a small payload. $\endgroup$– HobbesCommented Feb 7, 2018 at 20:22
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2$\begingroup$ @Hobbes Delta v is logarithmic in the ratio of initial to final mass, no? Your figures would result in a log 3 at best, hardly 10% Am I missing something in your reasoning? $\endgroup$– user23096Commented Feb 7, 2018 at 20:31
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3$\begingroup$ 26.7t to GTO, not GEO. $\endgroup$– Russell BorogoveCommented Feb 7, 2018 at 20:49
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1$\begingroup$ @Hobbes 5km/s only gets me an 88-year Hohmann, which I don't think is a useful capability for FH. 12 years needs 10km/s additional ∆v, which isn't happening on kerolox. I think we need a big contribution from a second stage that isn't pushing 26t, as well as a mysterious third stage, and I don't want to work out the whole thing from ground level. $\endgroup$– Russell BorogoveCommented Feb 9, 2018 at 21:40
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1 Answer
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It could have done so, but it is not the only rocket that can do so. The Atlas V that launched New Horizons to Pluto could have taken a direct path, but it would have taken 3 years longer. Falcon Heavy reportedly can lift 3.5 tonnes to Pluto directly, which is about 7 times heavier then New Horizons.
As to how quickly could Falcon Heavy have done that without the gravity assist? The exact numbers are pretty difficult to guess. SpaceX no longer publishes their performance charts like they used to. From NASA's performance estimator, I have the following charts. Note that the performance numbers are suspect, will update them when I have better.
This indicates that while a direct to Pluto mission is possible, it would not have the high energy required. Even with the expendable version, it still doesn't perform as well.
Bottom line is, the lack of a high ISP upper stage really limits the functionality of the Falcon Heavy in carrying cargo beyond Earth orbit. The mission could be done, but it would take longer then with an Atlas V 551 mission, as New Horizons used. For a fully expendable mission, the crossover point between Falcon Heavy and Atlas V 551 seems to be about $60 km^2/s^2$. That won't quite get you to Jupiter. It turns out the two are pretty equivalent in price.
answered Feb 8, 2018 at 11:57
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$\begingroup$ OTOH, couldn't Falcon Heavy take a fairly big probe with a lot of xenon for a ion engine to LEO instead? $\endgroup$– SF.Commented Feb 12, 2018 at 14:12
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1$\begingroup$ Sure, or some other similar other high ISP type system. That doesn't usually work very well for getting to the outer solar system, however. $\endgroup$– PearsonArtPhoto ♦Commented Feb 12, 2018 at 14:22
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$\begingroup$ It seems those graphs were generated by the KSC Performance Calculator: elvperf.ksc.nasa.gov/Pages/Query.aspx According to an NSF post, that calculator uses outdated information (forum.nasaspaceflight.com/…) $\endgroup$– HobbesCommented Feb 12, 2018 at 14:38
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1$\begingroup$ @ORcoder Good point, I've updated accordingly. $\endgroup$– PearsonArtPhoto ♦Commented Feb 16, 2018 at 22:46
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1$\begingroup$ A Falcon Heavy with an expendable core costs $95M and lifts 2.5x as much to GTO as the Atlas V. It's Pluto injection orbit payload is nearly half of the Atlas V's GTO payload. I'm struggling to understand why it can't outperform the Atlas V to Pluto? space.stackexchange.com/questions/25452/… $\endgroup$ Commented Feb 18, 2018 at 0:16