[ezegolub]

If any of the engines has a TWR above one when landing, even at it's lowest setting, i can't help but wonder why don't they use a smaller engine which can be more effectively used to land.

I'm sure theres a perfectly good reason for this, there's a reason SpaceX has its reputation, but i'm curious as to why.

[jccooper]

Mostly, I think, because they'd prefer to avoid having several types of engine involved. SpaceX is big on cost savings through commonality. The development and tooling and testing etc. for creating a smaller landing-only engine would probably cost darn near as much as for the main engines, so if you can get it done with them, it makes sense to do so.

If they can't get them on the ground reliably with just the Merlins after trying enough, then maybe they'll do something like that. But if it's a simple matter of software and a handful of tests, as it seems may be the case, then they've saved a lot of effort and cost.

The other answer is that the F9 architecture was designed (and flying) before this mode of recovery was "the one". And they'd prefer to not redesign the thing if it can work as-is. So they'll try it and see before spending gobs of money on a re-design. Rockets aren't software (except to the extent they are) and changing things about them is dangerous and expensive.

[dlgeek]

Because the primary job of the engine is to get as massive a payload as they can to orbit, not to land. That's just a bonus.

[sfeng]

For one, developing reliable rocket engines takes a long time, you can't just whip up a new one. The current Merlin has a very large throttling range already when compared to existing engines. The size of the engines they use now is defined by the need to get payload's to space, the landing has to be secondary.

[simonh]

Grasshopper was able to hover, or even perform a powered descent and was probably lighter than an F9 first stage, so I don't think this can't be it. They likely do have an engine configuration capable fo a lower minimum thrust, but they're choosing not to use it presumably because it's not optimal for the primary mission, and they believe it's not necessary for the landing.