As I understand it, the Merlin 1D engine on the Falcon 9 that is used to land the first stage, is fairly limited in how low it can throttle. This makes landing on the barge more difficult -- a suicide burn of sorts.
How low can the engine throttle?
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$\begingroup$ This was answered in this question, where @geoffc stated: "Merlin 1D can only throttle down to about 70% and that still generates more thrust (about 100Klbs of thrust)" $\endgroup$– Brian Tompsett - 汤莱恩Apr 15, 2015 at 22:46
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1$\begingroup$ So, that would be a good answer here. Some references would be nice. It is a different question though. $\endgroup$– ErikApr 15, 2015 at 22:48
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$\begingroup$ I did not have a reference handy. $\endgroup$– geoffcApr 16, 2015 at 2:13
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Wikipedia and Spaceflight101 both say 70%.
http://en.wikipedia.org/wiki/Merlin_(rocket_engine_family)#Merlin_1D http://www.spaceflight101.com/falcon-9-v11.html
With the other specs given on the Spaceflight 101 page, this implies close to 2.5:1 TWR at touchdown (~15 m/s2 deceleration) on an empty tank.
This tweet from Musk says "~40%" but if you read the responses to the tweet you'll see that's rather ambiguous. If it could actually reach 40% rated thrust that would be a much less dramatic deceleration in the hover-slam.
Someone with more free time than myself could count pixels and frames in the CRS-6 landing video to settle the question (of deceleration rate; you'd need to know the final mass of the stage to get the thrust/throttle figures).
answered Apr 15, 2015 at 23:00
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1$\begingroup$ That makes for a tricky control systems problem. It would be nice if the coverage of the latest attempt would mention this complication. $\endgroup$– ErikApr 15, 2015 at 23:04