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Comment by eddy_chan | Hacker News Reader

Forgive the laymans question but how much would it cost to ferry one extra rocket whose mission is to help it land by giving a larger margin of error when landing on the drone ship. I'm guessing carrying a few extra tonnes of deadweight engine for 99% of the mission is a big deal.

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Since most of the weight of the rocket is fuel and payload, when landing it is very very light so even one Merlin engine, throttled to its minimum is way to much and the rocket has thrust to weight ratio bigger than one, so it cannot hover (the moment it slows down it starts going up again) thus the suicide burn - it has to cancel all of its velocities at the precise point of landing.

But even if that was not the case and it could throttle down enough, suicide burn is the most efficient way of landing a rocket (the least amount of fuel required) as it has very small gravity losses - the faster you slow down, the less you have to fight against gravity, so they would use it in most cases.

The amount of fuel left when going anywhere besides low earth orbit (or the international space station) is so big that they hardly have any to spare - the tyrany of the rocket equation and all that... You can test it all out yourself in Kerbal space program :-)

Even more than the extra mass, they'd have to have a different engine design with its own production line (the second stage engine is slightly different from the first stage engine but IIRC that's mainly just a longer nozzle), control systems and so on. Modern control systems are good enough that doing it this way works.

On the first stage every last little bit of weight counts since it only weighs the rocket down. They already lower their payload to orbit by about 30–40 % when they land the first stage. That stage has to fight most of Earth's gravity and atmosphere so accelerating at all takes a lot of effort. So adding weight to the part that is only needed for accelerating the rocket through the atmosphere lowers the total payload considerably. It's bad on the second stage too, but not that much (but even then, they shed the payload fairings or the Dragon nose cone when they're no longer needed).

Adding another (different) engine also complicates things. Currently they only have a single engine type for the whole rocket (plus thrusters), the one on the second stage having a different nozzle. This greatly reduces complexity and thus cost and risk.

You've got the effect of additional mass on the stages backwards. Additional mass on the first stage is bad, but additional mass on the second stage reduces payload in greater proportion.

Mass on the second stage is on the rocket during both 1st and 2nd stage burns, while you get to leave 1st stage mass behind for the 2nd stage burn.

Good point; my bad.

They've never really had problems with the suicide burn.

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