If they're landing on the launch platform, they've already solved the backpressure issue from the rocket exhaust being that close to the rocket body, haven't they? I'm not sure I see how the catcher's mitt helps. Didn't the legs and platform already have to hold the static weight of a fully loaded Heavy plus a fully loaded Starship, and now it just has to hold up a mostly empty Heavy plus whatever delta V or delta H at rocket cutoff... Is that more than the empty/full weight differential?
The problem is that when the rocket shuts off, they usually haven't quite landed right. If they are still moving, the legs have to catch the rocket. If they are still a bit in the air, you'll fall down and have to catch the rocket.
The legs on a Falcon have built in shock absorbers. Here is why. The energy with which you hit the ground is 0.5 m v^2. This has to match force times distance while the legs catch it. If it is perfectly rigid, the force is insane, even when rigid. But if the legs can flex, you can make this work out.
But if we've got a structure that catches you, then the rocket doesn't need to have those shock absorbers. The structure does. And if the longer the distance over which the shock is absorbed, the smoother the landing.
Now in Elon's plans, Starship needs to be able to land on Mars unassisted. So it actually needs legs with shock absorbers. But SuperHeavy only needs to take off from and land on Earth at pre-arranged locations. So he can make it more efficient by not building in any unnecessary shock absorbers.
Side note. This is why seatbelts matter. With a seatbelt your crash is spread over all of the time that the car is crashing. Without it your crash is spread over the time it takes your head to hit the car. When your head stops in 1/10 the distance you have to have 10x the force. It is not hard for this to be the difference between being shaken up and having your brains scrambled.
1. The catcher’s mitt can suspend the rocket far higher above the ground than the landing legs can, reducing backpressure. Even if this is already “solved”, this could make that solution cheaper and easier.
2. Right now, the legs need to be capable of absorbing the shock of the landing. With a dedicated platform, not only can the weight of the shock absorbers be added to the platform rather than the rocket but you can also potentially absorb over a dramatically greater distance and time (increasing the proportion of successful recoveries).
Literal impact (the engineering term of art) is usually the controlling force for these types of structural calculations. The force varies with respect to velocity-squared, and due to the dynamic nature of landing, the forces are not likely to be distributed evenly (high pressures at unpredictable locations), so you end up with the controlling condition needing to be much stronger than perhaps even supporting the (static, evenly distributed) weight of a fully loaded vehicle.
I think they're not landing on the launch platform. With catcher's mitt on a different platform, launch configuration won't have rocket legs, just a platform tower holding up the rocket.
Comments
If they're landing on the launch platform, they've already solved the backpressure issue from the rocket exhaust being that close to the rocket body, haven't they? I'm not sure I see how the catcher's mitt helps. Didn't the legs and platform already have to hold the static weight of a fully loaded Heavy plus a fully loaded Starship, and now it just has to hold up a mostly empty Heavy plus whatever delta V or delta H at rocket cutoff... Is that more than the empty/full weight differential?
The problem is that when the rocket shuts off, they usually haven't quite landed right. If they are still moving, the legs have to catch the rocket. If they are still a bit in the air, you'll fall down and have to catch the rocket.
The legs on a Falcon have built in shock absorbers. Here is why. The energy with which you hit the ground is 0.5 m v^2. This has to match force times distance while the legs catch it. If it is perfectly rigid, the force is insane, even when rigid. But if the legs can flex, you can make this work out.
But if we've got a structure that catches you, then the rocket doesn't need to have those shock absorbers. The structure does. And if the longer the distance over which the shock is absorbed, the smoother the landing.
Now in Elon's plans, Starship needs to be able to land on Mars unassisted. So it actually needs legs with shock absorbers. But SuperHeavy only needs to take off from and land on Earth at pre-arranged locations. So he can make it more efficient by not building in any unnecessary shock absorbers.
Side note. This is why seatbelts matter. With a seatbelt your crash is spread over all of the time that the car is crashing. Without it your crash is spread over the time it takes your head to hit the car. When your head stops in 1/10 the distance you have to have 10x the force. It is not hard for this to be the difference between being shaken up and having your brains scrambled.
Super heavy can hover on reentry, so it will land far softer than a Falcon 9 first stage that is require to do a suicide burn.
This is also true of Starship on Mars since it will be massing over 200 tons.
1. The catcher’s mitt can suspend the rocket far higher above the ground than the landing legs can, reducing backpressure. Even if this is already “solved”, this could make that solution cheaper and easier.
2. Right now, the legs need to be capable of absorbing the shock of the landing. With a dedicated platform, not only can the weight of the shock absorbers be added to the platform rather than the rocket but you can also potentially absorb over a dramatically greater distance and time (increasing the proportion of successful recoveries).
Literal impact (the engineering term of art) is usually the controlling force for these types of structural calculations. The force varies with respect to velocity-squared, and due to the dynamic nature of landing, the forces are not likely to be distributed evenly (high pressures at unpredictable locations), so you end up with the controlling condition needing to be much stronger than perhaps even supporting the (static, evenly distributed) weight of a fully loaded vehicle.
I think they're not landing on the launch platform. With catcher's mitt on a different platform, launch configuration won't have rocket legs, just a platform tower holding up the rocket.
what makes you think the "catchers mitt" can't/won't be another couple meters up?