I hadn't known that. Is that a center engine?

Intuitively it seems like more would be better so that in the case of that engine failing you could use the other engines to push up and maybe light another engine.

Of course, if there's two engines and one fails, it's going to tilt. Same with 3, though to a lesser degree. 4 engines lessen it even more, etc. etc.

Or maybe they can detect a bad center engine on descent (it doesn't light or flame is detectably not the right) and then go to outer engines. Or drop it in the ocean. Hope there aren't people on that one.

> Intuitively it seems like more would be better so that in the case of that engine failing you could use the other engines to push up and maybe light another engine.

The landing F9 first stage is so light even one engine at minimum throttle has a thrust-to-weight ratio greater than one; they can't throttle it down enough to hover. Three lit engines would send it shooting back up rapidly.

I was under the impression that there's no hold-down or clamping of the booster for this vehicle. The clamps are disengaged around T-10 (maybe T-15?).

The booster is securely clamped to the launch stand. However the starship is only resting on top of the booster - the two parts of the ship are not clamped together.

Launch clamps are released on command of the engine computer - i.e when it detects that sufficient engines are running correctly it commands a clamp release.

Note that Starship Booster's outer engines are actually started using feeds from the launch pad. They need power and pressurized nitrogen from ground equipment to start. The outer engines are not able to restart once off the ground.

So the clamps are required to keep the vehicle in place until all the engines that will start have had a chance to do so. They're also useful if there is an abort during engine start so the vehicle remains secure.

> However the starship is only resting on top of the booster - the two parts of the ship are not clamped together.

That isn't what it looked like to me when it made what, 3 go arounds?

I may be incorrect and they are now using explosive bolts. I've seen those mentioned elsewhere.

However the engines were on the whole time, producing >1G of thrust along the direction of the booster. So that force is going to hold the StarShip and the booster together. But you would expect some lateral force from air friction to push the stack apart at some point.

Might just be the thrust-to-weight being under 1 while they light the engines, but it visibly sits there for a signifcant amount of time, much more than you'd get with a landing suicide burn.

Then what holds the rocket down while the engines are spooling up?

Could possibly this explain why there was so much lateral movement of the booster before it cleared the tower?

That makes sense, but on mars? Full payload, very little atmospheric drag, little atmosphere to keep the blastolith from scrubbing the bottom of the vehicle. I am talking generically since presumably the first stage won't be landing on mars.

Mars is far off enough it's entirely speculative on how they'll handle this. Drop some robot bulldozers first? Specialized landers for landing pad construction crews? Bring the heavy cargo via a separate non-atmospheric craft built in Earth orbit? Hover from height for a while to blow debris off a rocky flat spot?

I don't have the answer, but IMO cheap mass to Earth orbit is the biggie that enables a whole bunch of options.

There was an idea of adding aluminum powder to exhaust and hovering a little over the pad, so that you deposit a solid metal surface where you intend to land.

You missed the moon solution - use engines high off the ground (most of the way up the ship) for landing.

Another idea (not starship specific) is to introduce cement (or similar) into the engine burn to build a landing pad from middair.

Lower pressure could help help the exhaust spread more.