General

Falcon 9 Rocket question

edited November -1 in General
Any physicists out there: If the Falcon 9 rocket was spinning like a top as it was landing, would it's angular momentum make it easier to land upright?
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Comments

  • edited November -1
    I'm not a physicist, but I can tell you the best place to ask about this kind of thing is the Nasa Spaceflight Forums, where people with a wide range of expertise discuss every tiny detail of space missions, and where you can find 3 sub-fora dedicated to SpaceX.
    This question belongs in http://forum.nasaspaceflight.com/index.php?board=66.0
    It may have been already answered though, so have a lurk first.
    Cheers.
  • edited November -1
    An early Falcon recovery failed because it was spinning, and forcing the liquid fuel to the walls instead of feeding it to the engines.
  • edited November -1
    I earned an M.S. in physics as a young man.
    Spinning would stabilize the Falcon, but would make a successful landing on a barge much more difficult. Keep in mind that a successful landing ends with the rocket stationary, sitting on the barge. With the rocket spinning as it approached the barge, they would need a mechanism to support it upright on the barge while the spinning is gradually slowed to a stop.

    For comparison, NASA's missions taking astronauts to the moon's surface didn't use spinning vehicles to 'land' on the moon.

    My interpretation of the Falcon crash is that it wasn't even close to a successful landing. Not only was the rocket spinning, it wasn't close to vertical when it reached the barge. And it arrived traveling much too fast.

    Sorry, Elon and SpaceX: This landing attempt wasn't close to successful. I'd you 'stick' such landings. Hopefully, "soon".
  • edited November -1
    They've proven they can controlled land several times now. What was left unproved was navigating from space down to a tiny dot in the ocean using x-wing fins and controlled burns. They've now proved they can do that. They're closer than you think, Frownin'.
  • edited November -1
    Thays right frownin. Despite coming in too fast they were centered. They say they ran out of hydraulics for the control fins. We will see how it goes next time.

    Re spin, that would make attitude changes very difficult- ie too much stability- so, no.
  • edited November -1
    The video of the crash showed the cylinder coming down at about a 30-45 degree angle (at first)and with a good burn going on. The vertical component of that thrust was a fraction of what it otherwise would have been, leaving open the possibility that it might have been prefect if it hadn't run out of hydraulic fluid.

    Elon didn't comment on anything but the hydraulic fluid, so I think that's all it was.
  • edited November -1
    @ EmperorTytus | January 17, 2015
    @ DTsea | January 17, 2015

    <i> They say they ran out of hydraulics for the control fins.

    IMO, the booster just plain crashed. No amount of hydraulic fluid could have enabled the control fins to control it given that it was spinning and descending too fast at point of contact with the barge. Note: My opinions have occasionally been proven wrong, but not often.

    <i>Re spin, that would make attitude changes very difficult- ie too much stability- so, no.

    Absolutely NO. Unless the "control fins" were designed to control a spinning booster. Anyone want to take me up on a $100 bet on this?

    evsisson | January 17, 2015

    <i>The video of the crash showed the cylinder coming down at about a 30-45 degree angle (at first)and with a good burn going on. The vertical component of that thrust was a fraction of what it otherwise would have been, leaving open the possibility that it might have been prefect if it hadn't run out of hydraulic fluid.

    I presume that you think the "control fins" were designed to control a spinning booster. Right?

    <i>Elon didn't comment on anything but the hydraulic fluid, so I think that's all it was.

    Yes, that's what he said. I'd like to believe he always tells the truth and nothing but the truth. When I saw the video, I was shocked. I had expected something close to a successful landing. What I saw was no such thing. To me, it's a bit disappointing.
  • edited November -1
    evsisson observed correctly.
    Because of the angle, the rocket basically tripped over its own landing leg at touchdown, and when the engine pack ripped away from the bottom of the rocket, explosion from the residual fuel "launched" the body of the rocket overboard, never to be seen again.
    That explains why there was so little debris spotted on the barge when it was towed back into port.
    Even if the rocket had been upright, it looked like it was at the far edge of the landing pad which still could have been an issue.
  • edited November -1
    @Grinnin

    <i>IMO, the booster just plain crashed. No amount of hydraulic fluid could have enabled the control fins to control it given that it was spinning and descending too fast at point of contact with the barge.</i>

    Are you saying the rocket was spinning when it crashed? It doesn't look that way in the video.
  • edited November -1
    The small, porous fins that unfold at the top of the rocket provide just enough drag in the atmosphere to keep the rocket upright when falling back down, They can be angled to arrest any spin. They can also be configured to provide a minor amount of directional control. The rest is up to the rocket pack subset of engines when fired. Even then, the rocket motors do not have a lot of directional control on their gimbals, and can only make small corrections. When launching spacecraft into orbit, a little correction goes a long way.
    The hydraulic system for the fins is an open system, which means that after the fluid runs through the hydraulic motors on the fins, it is exhausted overboard. Hence, loss of fluid.
    Elon mentioned that installing a closed hydraulic system would add more weight than they wanted.
    Running out of hydraulic fluid prematurely caused the fins to free float or fold back against the rocket (Not sure how SpaceX is doing this.)
    In any event, running out of hydraulic fluid caused loss of stability which tilted the rocket farther than the engine could recover when it fired for the final time.
  • edited November -1
    @ Grinnin'

    "I presume that you think the "control fins" were designed to control a spinning booster. Right?"

    No, I assumed the booster was NOT spinning, and I didn't even consider whether the fins could control spin (although I imagine they could be designed to do so). I saw in the video what seemed to be the legs of the booster, and they didn't appear to be moving around the cylinder.

    In the video, as the booster first appeared at the top of the frame, it was more upright than it was when it hit the deck. I thought the fins were supposed to control that tilting movement. I feel that if the hydraulic fluid had not run out, the tilting would have been corrected well before it came into view, and the enhanced (downwardly directed) thrust would have slowed it considerably more.

    I didn't think the booster was dropping all that fast. I was also wondering if the legs might have had shock absorbers on them, but that's just a guess. Was the video shown in real time? I didn't think to look for a timer. If it was slow motion, then yes, it might have been going too fast, though I don't know why it would have been, since it's already been tested for landing on a solid surface.

    I know next to nothing about how the engine was expected to react to such a situation, and I don't have the data I'd need to figure out the answers, so I can't assume that my gut reaction is anything close to the truth.
  • edited November -1
    @ Grinnin'

    <i>I presume that you think the "control fins" were designed to control a spinning booster. Right?</i>

    In the following video, you can clearly see the fins being used to initiate a spin on the way down and then arresting the spin just before touchdown.



    It looks to me like spin control is one of the primary functions of these fins as well as vertical stabilization.
  • edited November -1
    Thanks Atlanta I was going to look for the grasshopper video :)
    Yes they control spin.

    SpaceX was designing it to use small stabilising rockets originally, but I presume using these has less of a fuel requirement. Losing their use will have had a few significant effects - though I hope SpaceX starts to look at their automatic landing system "plan B" if these cut out again (even if plan B is "don't hit the barge"
  • edited November -1
    Grinnin';
    Completely out to lunch. The spinning-crash Falcon was a single rocket early design, and this is the F9 composed of 9 cores. It was not spinning, just off-line.
  • edited November -1
    It was a spectacular video. The ship is pretty sturdy it seems as that rocket looks huge and was coming in fast, yet mostly bounced off. No hydraulic fluid would mess up many things and if that was the problem, along with a tweak or two, then next time maybe!
  • edited November -1
    The landing attempt was highly successful to my mind. To even get close enough to crash is amazing. The fins actually do most of their work at high speeds when there is enough drag for them to work. They also help keep the top up, and point the rockets against the trajectory so they can slow it down.
  • edited November -1
    We don't know how high it was when it ran out of fluid. I'd say that if it wasn't within a hundred meters of touchdown, it's orientation in the video is no surprise. It's amazing the thing made it to the platform at all. I expect Elon to dramatically increase the likelihood of a successful landing for the next attempt. He openly gave it a 50% chance last time. Can't wait for the next attempt!
  • edited November -1
    @ AtlantaCourier | January 17, 2015

    <i>@ Grinnin'

    <i>I presume that you think the "control fins" were designed to control a spinning booster. Right?

    <i>... see the fins being used to initiate a spin on the way down and then arresting the spin just before touchdown.

    <i>

    Great video, which I had not seen before.
    I think your interpretation of this is correct.

    <i>It looks to me like spin control is one of the primary functions of these fins as well as vertical stabilization.

    My understanding of the crash video is subject to nontrivial error caused by the poor quality of the video. I thought I saw it spinning. I know it wasn't close to vertical when it hit. And it wasn't descending on a vertical path. Combining these observations, I concluded that the crash wasn't a near-success.

    Certainly, I hope SpaceX is able to do better the next time.
    I definitely believe that, with enough effort and practice, they should be able to pull of this astounding feat. And I look forward to celebrating their success in recovering a Falcon booster as it returns to sea level in a real Space Station resupply mission.

    Go SpaceX
  • edited November -1
    The previous 2 water landings also had it coming down at an angle. The engines were simultaneously slowing its descent and horizontal velocity, and in the second it became upright.
  • edited November -1
    Spin is undesireable. It's a liquid-fueled rocket, and spin forces the fluid away from the engine intakes.

    No spin in the video, just excess tilt.
  • edited November -1
    Success: Aiming at the landing platform. (If this was unsuccessful, then it would have landed in the absolutely huge ocean around the landing platform.

    Not a Success: Safely landing the rocket.
  • edited November -1
    Anyone viewing this is unsuccessful has a significantly distorted understanding of the process. They took a rocket, burning fuel, from space ... steered it from 50 miles up and hit a postage stamp in the middle of the ocean. Frankly, I don't see how they will ever be successful as it was just coming in too fast, fins or no fins. Still, this is an AMAZING achievement, which we all get to witness first hand. Not quite as exciting as landing on the moon, but in the grand scheme of things, significantly more important.

    Yes, Elon is a visionary ... a "dreamer" even ... and you have to take a lot of what he says with a grain of salt. However, you have to give him kudos for even trying such a thing and the fact is that Space X can launch resupply missions $200 MILLION less than what UT can do today ... if they can nail the landing and actually re-use these things, it will cost even less.

    This is a win, no matter how you choose to measure it. It might take 10 more shots to get it right or prove that it can't be done. Still, you've got to be awed just a little bit, no?
  • edited November -1
    @ holidayday | January 20, 2015

    <i>Success: Aiming at the landing platform.

    <i>Not a Success: Safely landing the rocket.

    @cpmarino | January 20, 2015

    I may be wrong, but IMO the purpose of the Falcon 9 booster aiming "at the landing platform" was to actually land on the barge in a condition allowing SpaceX to reuse the booster. If you disagree, please provide a link to the source of the real purpose.

    BTW, I hope and wish for SpaceX to pull off a successful Falcon 9 booster recovery ASAP. Furthermore, I'd guess that they will do exactly that some time this year.

    Go SpaceX!
  • edited November -1
    @Grinnin: Yes, clearly, the objective was to land the thing, standing upright, and bring it back home. I do understand that.

    Hitting the damned platform was a win, no matter how you wish to measure it, given the odds against such, the incredible complexities involved in doing so and the fact that it's never been done before.

    All I'm saying is that you have to see the big picture. Nobody, Elon included, thought they'd do it on the first try. I doubt they'll do it on the second or third either, but, eventually, they will.

    So, no, the ultimate goal of landing and re-using was not successful, but, c'mon man, aren't you at least a little impressed that they actually hit the freakin' thing?

    I mean, the Wright Brothers are largely credited with the first successful flight ... they were like 6 feet off the ground and "flew" for something like 1 minute and 900 feet ... doesn't diminish the accomplishment and look what it led to. Maybe I get too excited by stuff like this but ... you gotta admire the effort and the balls to even try it in the first place.
  • edited November -1
    @ cpmarino | January 20, 2015

    I said that this attempt failed to meet its goal.

    You reacted as if I had made some outrageous claim.

    Can we please just wait until SpaceX lands one of their boosters on a barge before we <b>celebrate this astounding accomplishment?
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