Despite the deep throttle of the Merlin 1D engine, the weight of the Falcon 9 booster is less than the force of a single Merlin engine firing at minimum throttle (See here). This means that the Falcon 9 booster can't hover, and that the landing burn (sometimes known as a Suicide burn) must be performed with a great amount of precision, as (correct me if I'm wrong) there are no second chances.

I have been unable to find info on the intended throttleability of the Raptor engine, nor have I found info on the expected dry mass of the BFB/super heavy booster. Even so, the fact that it has 31 engines instead of 9 indicates to me that they are much more likely to achieve a TWR of close to unity just by only firing one engine.

On this basis, is it likely that the BFB/Super heavy booster will have an easier time landing than the Falcon 9 booster? Has there been any word from SpaceX about the expected difficulties here?

By "easier", I mean more tolerant of mistakes/is able to abort and attempt another landing if the first attempt is slightly off.

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    $\begingroup$ Note that TWR is not the only reason for a suicide burn. Every second you burn, you are fighting gravity and burning fuel unnecessarily. You want to fight gravity for as short a time as possible, ergo burn as short as possible. $\endgroup$ – Jörg W Mittag Apr 8 '19 at 16:45

Spaceflight 101 gives a throttle range of 20-100% and a sea-level thrust of about 3MN for the Raptor engine, so at minimum thrust it can hover about 60 tons against 1g. Estimates for the dry mass of the superheavy are considerably more than that, closer to 250 tons, so it should have little problem hovering on that level. There is also, though, the question of fuel supply. Every extra kilo of fuel used for landing is one less kilo of fuel that could have been on the Starship payload. That is a pretty strong incentive to use a short fast landing burn and rely on getting it right first time if you can.

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    $\begingroup$ One of Neil Armstrongs main worries about Apollo 11 was the limited hover time. In fact, he used up almost all of it (17 seconds remained) to avoid obstacles on the ground. Since the Starship is expected to land on unprepaired terrain, in those cases one would expect them to reserve fuel for fine tuning the landing spot in the last moments. It's hover time would be important for those landings. $\endgroup$ – Johnny Robinson Apr 8 '19 at 11:24
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    $\begingroup$ That's true, but that would be Starship, not Superheavy. $\endgroup$ – Steve Linton Apr 8 '19 at 11:47
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    $\begingroup$ @JohnnyRobinson, the mythical 17 seconds strikes again. Apollo 11 landed 17 seconds prior to the "bingo fuel" call, which is the point at which they'd need to land in the next 20 seconds or abort. Because of sloshing in the fuel tank, the countdown to "bingo fuel" started early, and they actually had somewhat more than 37 seconds worth of fuel left when they landed. $\endgroup$ – Mark Apr 8 '19 at 23:00

Beyond the thrust vs empty mass of the different vehicles allowing hover, it seems that SpaceX has managed to figure out how to successfully hover slam, so this might be less of an issue.

But other features may help. Being much bigger makes reentry easier, having a larger surface to distribute heating loads.

Additionally, specifically for the Super Heavy the engines that are used for landing, assuming three for control, one for final landing, would allow for much wider spacing apart. Obviously the middle engine will be used, but possibly the outer row engines could be used for additional control.

Being further apart will allow for more control authority when used to manage roll.


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