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Could an aeronautics person kindly explain why it's so hard to get this thing to land as it's supposed to? I'm not being judgmental at all here; I am saying tha

Could an aeronautics person kindly explain why it's so hard to get this thing to land as it's supposed to? I'm not being judgmental at all here; I am saying that as a software engineer with no aeronautics experience, I simply don't understand the problem. Assuming all of the sensors indicating tilt, altitude, etc. are accurate, the software portion coordinating where and how much thrust needs to deployed to keep it level and descending at an appropriate speed should be relatively straightforward to write. And yet, with at least dozens of undoubtedly brilliant people working on this very problem, the result is the world's most expensive fireworks show.

narag 12y ago
I wonder if the thin cylinder shape is to blame and if it would be easier to land if it sported a stabler shape. Can aerodynamics be compatible with something like a tetraedron mesh?
- pjscott 12y ago
  It actually has a pretty low center of mass. Most of its bulk is empty fuel tank, and the engines are on the bottom.
vladimirralev 12y ago
Not aeronautics, but control systems engineer and I've seen the explanation. The rocket engine they use is too powerful to hover and move towards the destination in a slow adjustment loop. And there is no throttle control on the engine, 70% minimum which is still too powerful. In order to land you turn the engine off and wait to pick up some vertical speed, then in the last N seconds you turn the engine on to kill the speed you picked up from the freefall just before you hit the ground. Best described as a hover-slam. If your position during the freefall is off, you can't just keep the engine running to hover around and fix it (like they do in their dev rocket), it would fly up quickly if they do. In this video they still have lots of fuel in the rocket so they were trying to hover a little, however in the real landing presumably the rocket will be empty thus the hover-slam will be harder.
- downandout 12y ago
  Thank you. I assume there are legitimate reasons for not having throttle control, but perhaps they should look into adding it. The hover-slam seems risky.
- grecy 12y ago
  Thanks, that's very helpful. > The rocket engine they use is too powerful to hover and move towards the destination in a slow adjustment loop. And there is no throttle control on the engine, 70% minimum which is still too powerful I've seen this a lot, and I've also read about how many times each engine can be re-lit. Can you talk a bit about why the engines have a limited number of times they can be lit? If there were no limit on that, I'd want to just turn the engine on and off a bunch to get the right amount of thrust to hover.
  - NamTaf 12y ago
    One limiting factor of reignition is the mechanism for lighting it. Some for example use a very combustable fuel that ignites in the presence of air to then ignite the main fuel flow. Depending on what mechanism the Merlin uses, this could limit their number of reignitions.
  - shasheene 12y ago
    While there's a limit in the amount of (highly toxic) TEA/TEB hypergolic fuel that's carried in the ignition system I'm not sure it's the bottleneck. The limited number of times the Merlin 1D can be relit is probably referring to the re-usability aspect (from memory they tested "cycling" the engine something like 40 times in qualification testing, but it's a little unclear exactly what a cycle entails). It's the associated "coking" of the engine with hot and sticky carbon/soot from the incomplete combustion of the kerosene that's been speculated to be a major hinderance to the long-term engines survivability. Other than consumables, it's probably the high (and variable) latency in the switching time that restarting the engine in quick succession that would make it problematic, in addition to the engine damaging aspects you mentioned. Hover-slamming a first-stage with a Merlin 1D has been heavily tested by F9R-Dev1/2 - it's a completely workable solution that minimizes engine restarts over the flight profile to 3 (boostback, re-entry burn, hoverslam). While the switching the engine may be made to be workable solution which can effectively throttle the engines between 0% and 70%, a hoverslam with a vehicle with thrust/weight ratio > 1 gives less stress on the entire (~$40 million F9R first stage) vehicle to be reused.
Patrick_Devine 12y ago
Elon mentioned the problem was that they ran out of hydraulic fluid about 30 seconds too soon. The rocket had to try to right itself using only engine gimbaling instead of using the waffle paddles, which didn't work out so well. The great thing is they've already fixed the problem for the next flight.

narag 12y ago

I wonder if the thin cylinder shape is to blame and if it would be easier to land if it sported a stabler shape. Can aerodynamics be compatible with something like a tetraedron mesh?

pjscott 12y ago
It actually has a pretty low center of mass. Most of its bulk is empty fuel tank, and the engines are on the bottom.

vladimirralev 12y ago

Not aeronautics, but control systems engineer and I've seen the explanation. The rocket engine they use is too powerful to hover and move towards the destination in a slow adjustment loop. And there is no throttle control on the engine, 70% minimum which is still too powerful. In order to land you turn the engine off and wait to pick up some vertical speed, then in the last N seconds you turn the engine on to kill the speed you picked up from the freefall just before you hit the ground. Best described as a hover-slam. If your position during the freefall is off, you can't just keep the engine running to hover around and fix it (like they do in their dev rocket), it would fly up quickly if they do. In this video they still have lots of fuel in the rocket so they were trying to hover a little, however in the real landing presumably the rocket will be empty thus the hover-slam will be harder.

downandout 12y ago
Thank you. I assume there are legitimate reasons for not having throttle control, but perhaps they should look into adding it. The hover-slam seems risky.
grecy 12y ago
Thanks, that's very helpful. > The rocket engine they use is too powerful to hover and move towards the destination in a slow adjustment loop. And there is no throttle control on the engine, 70% minimum which is still too powerful I've seen this a lot, and I've also read about how many times each engine can be re-lit. Can you talk a bit about why the engines have a limited number of times they can be lit? If there were no limit on that, I'd want to just turn the engine on and off a bunch to get the right amount of thrust to hover.
- NamTaf 12y ago
  One limiting factor of reignition is the mechanism for lighting it. Some for example use a very combustable fuel that ignites in the presence of air to then ignite the main fuel flow. Depending on what mechanism the Merlin uses, this could limit their number of reignitions.
- shasheene 12y ago
  While there's a limit in the amount of (highly toxic) TEA/TEB hypergolic fuel that's carried in the ignition system I'm not sure it's the bottleneck. The limited number of times the Merlin 1D can be relit is probably referring to the re-usability aspect (from memory they tested "cycling" the engine something like 40 times in qualification testing, but it's a little unclear exactly what a cycle entails). It's the associated "coking" of the engine with hot and sticky carbon/soot from the incomplete combustion of the kerosene that's been speculated to be a major hinderance to the long-term engines survivability. Other than consumables, it's probably the high (and variable) latency in the switching time that restarting the engine in quick succession that would make it problematic, in addition to the engine damaging aspects you mentioned. Hover-slamming a first-stage with a Merlin 1D has been heavily tested by F9R-Dev1/2 - it's a completely workable solution that minimizes engine restarts over the flight profile to 3 (boostback, re-entry burn, hoverslam). While the switching the engine may be made to be workable solution which can effectively throttle the engines between 0% and 70%, a hoverslam with a vehicle with thrust/weight ratio > 1 gives less stress on the entire (~$40 million F9R first stage) vehicle to be reused.

Patrick_Devine 12y ago

Elon mentioned the problem was that they ran out of hydraulic fluid about 30 seconds too soon. The rocket had to try to right itself using only engine gimbaling instead of using the waffle paddles, which didn't work out so well. The great thing is they've already fixed the problem for the next flight.