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The chopsticks actually don't catch the booster by the grid fins, there are little struts that stick out from the rocket that don't stick out nearly as far. I
by rockemsockem 2y ago
The chopsticks actually don't catch the booster by the grid fins, there are little struts that stick out from the rocket that don't stick out nearly as far.
I thought the same thing before the first catch, if you go look at the catch footage you can see the booster resting those on the chopsticks.
- carabiner 2y agoSo the struts (plus supporting structure) are lighter than the legs? Why is that?
- ceejayoz 2y agoThey’re much smaller and don’t have to move/deploy.
- krisoft 2y agoThe legs would need to be much longer (because you can’t push the engine nozzles all the way into the ground and still hope for good things.) Longer structure means more mass, but also larger torque which need to be handled with the support of the structure. Legs need to move to deploy. The struts are just there, static things are much simpler. Simpler things weigh less. Legs need to contain shock absorbers. With the struct solution the shock absorber is in the chopsticks. It doesn’t matter how much the shock absorber weighs when you don’t need to carry it up with you.
- adolph 2y agoIt was pretty neat how Bezos explained to Everyday Astronaut how they were using 6 legs so the legs weren’t as long. On the whole, pad catch is the way to go for non-expeditionary vehicles. For orbital uses each booster basically becomes a pyrotechnic elevator.
- avmich 2y agoIt would be good to see the numeric analysis of variants here. Legged landings are surely possible - say, with longer legs (twice as long as struts?), possibly static (legs are always deployed, even when launching), with shock absorbers which aren't that heavy... Would be good to see good and bad qualities next to each other.
- rockemsockem 2y agoI think it all just comes down to weight. If you can trim mass on the vehicle then you should do it.
- deleted 2y ago[deleted]
- mhio 2y agoThe pins/struts are a 2 point system that double as the booster lift points in general operations. The booster mostly hangs in tension which the existing tank structure can support. I would guess they share some of the structure beefiness with the grid fins. Legs require at least 4 points, probably more. Shock absorption hardware, ability to unfurl to an acceptable width. Require reinforcement (cross bracing) near the base of the tanks to handle the loads pushing inwards toward the center of the tanks.
- fooker 2y agoLegs require at least 3, not 4 points.
- avmich 2y agoYou can technically imagine two legs with really wide feet, allowing some perpendicular stability. I wonder if one-leg lander could be imagined. 3-legged landing scheme was used in Surveyors, first American automatic Moon landers, and was surely considered for Appolo LEMs, but rejected. So there could be additional, secondary reasons when choosing the number of legs.
- timschmidt 2y agoThe cost of one additional leg is pretty inexpensive for the redundancy it provides for the other three.
- chipsa 2y ago4 legs have no additional redundancy over 3. One leg failing will still result in the booster tipping over. They do push the maximum angle of tipping before your CG is no longer supported out farther though.
- kevin_thibedeau 2y agoUseful for when you land on the rim of a crater like Apollo 12.
- addaon 2y ago> So the struts (plus supporting structure) are lighter than the legs? Why is that? Besides the other answers you've received, the lugs hold the booster from (near) the top. This means that the body of the booster is in tension during and after landing. Legs, on the other hand, support the landing load and weight after loading in compression. The booster is basically a thin-shelled tube, which is limited in compression strength (for a given wall thickness) by buckling; in tension, the strength approaches the strength of the material, so less additional reinforcement is needed in the structure to support landing loads.
- avmich 2y agoThe tanks can surely be pressurized at landing, which greatly helps to avoid buckling.
- IX-103 2y agoPressurized with what? They've already used their fuel for the landing. They can't put anything else in the tanks without worrying about contamination for the next flight.
- avmich 2y agoPressurization gases? The fuel goes from tanks to engines (engine pumps) because tanks are under pressure, right? Even if the liquids are spent - they are rarely spent in full - the gases remain.
- r2_pilot 2y agoHelium is a common pressure/purge gas in the fuel/oxidizer tanks.
- agrajag 2y agoThey autogenously pressurize the tanks - they heat up the cryogenic propellants with the engines and use some of the gas to pressurize the tanks. In Starship’s case it’s methane and oxygen.
- 2y ago
- bunabhucan 2y agoThe "struts" are needed anyway to lift/move the rocket using cranes, they aren't optional.
- eichin 2y agoOne of the things that came up in one of the livestreams was that some of the changes to the starship heat shielding were to test a couple of different spots for those struts - because the booster doesn't do orbital-speed reentry, starship itself does, so to catch that, you need to avoid burning off the struts...