Discussion in 'General' started by Chasbro, Sep 28, 2019.

  1. OGs750

    OGs750 Well-Known Member

    There are small thrusters in the nose, but the main motive force comes from the engines. They’re full hard over when they start the landing burn and that helps turn the vehicle. That and there’s LOx in the nose that’s used for ballast during decent (you can see the nose frost up some) and that LOx is dumped to feed the engines during landing which also helps the vehicle rotate. Problem is that requires a transfer tube to run down the entire length of the vehicle and there are a lot of thermal losses there with the potential that the LOx becomes gaseous before hitting the pumps which may have led to the header pressure thing Elon is mentioning (I’m taking a guess hereI haven’t seen his tweet).

    Edit: he mentioned fuel header, so throw my idea out the window.
    I was thinking the same thing! It crawled off the pad. I had to remind myself how fucking huge starship is.
  2. OGs750

    OGs750 Well-Known Member

    I have to admit that I have a little bit of insider knowledge ;)
    CRA_Fizzer likes this.
  3. mpusch

    mpusch Well-Known Member



    Though the engine cut out was mainly just a symptom of the headers. That's encouraging and gives raptor a really solid grade!
  4. drop

    drop Well-Known Member

    Was a beautiful thing. But question I have, because I'm a wee bit lost.

    I was under the impression this test, was to test, from free falling on its side, to upright and landing.

    Part of flying is landing.
    Yes success as far as coming back vertical from horizontal was amazing to watch. But how can it be called a successful flight when it ended in such an explosion.

    And it looked to me like the 3rd engine never retired.

    I'm seeing some say low fuel. I'm not up to date on some of the terms and whatnot you mention.
  5. mpusch

    mpusch Well-Known Member

    I had a whole explanation typed out and it got deleted. Fun.

    So basically there were several novel events that needed testing or data. The raptor is still a development engine and this was by far the longest its ran doing actual work. Been used on a test stand extensively and a couple short hops, but one of the engines powered for over 4 minutes.

    The aero was also one of the areas Elon talked about needing data on. Spacex is great at simulating airflow and such, but it's always good to get real info. Nothing has flown like this before that I know of.

    The landing burn uses extra tanks called header tanks. If you just used the big main tanks the liquids would be sloshing around and not usable. The relight to using the headers and flipping the ship is a crazy maneuver. That worked!

    Something went wrong with the pressure from the headers eventually though, causing the engines to not get enough fuel. These are very finely tuned machines, and if you don't get the fuel you need things go wrong. One engine shut down and the other was in some step of doing the same.

    I don't think they were trying to light all three at the end. You have to manage the thrust because the engines can only throttle so low.

    This was a win because it was a proof of concept for the overall architecture of the rocket and they got tons of data to refine the next try. Btw, the next rocket is already assembled and probably ready to make its own attempt in a few weeks or couple months. There's literally more than a handful in various stages of assembly after that too!

    It's not a big deal to lose one as long as you learn from it. Big win for them today.
    HPPT, CRA_Fizzer and motion like this.
  6. motion


    Thanks for that!

    Will a full payload BFR (my preferred name) have only 3 raptors?

    I read somewhere.... that someone.... possibly EM, stated that it may take 100 of these test flights before they're ready for humans. I wonder how much each test costs? Sounds spendy. Major kudos for SpaceX for putting their balls on the line to push the envelope.
  7. Monsterdood

    Monsterdood Well-Known Member

    If I’ve learned anything from motorcycle racing, it’s that they needed some tank foam. The MR12 probably sloshed around with that last moment righting maneuver and the fuel pump probably got some air bubbles in it. The old fuel sloshing challenge no doubt. Awesome flight though.
  8. drop

    drop Well-Known Member

    Thank you so much. This is exactly what I wanted to know. So the header is a tank that stores fuel.

    Is it possible that when the maneuver, from horizontal to vertical, caused the slosh as you spoke of up there? And why can't it be used of sloshing?

    And I now understand you can only throttle down so much to control thrust. And even at idle, it would possibly to much to land the vehicle. Hence 2 engines.

    3 things impress me.
    Planes, trains and space. That's it.
  9. mpusch

    mpusch Well-Known Member

    So this is the fun part :D What we watched today was the upper stage. The first stage (super heavy) is a bit bigger than what we watched today. Meaning that the whole ship will be more than twice the size.

    The first stage will have somewhere around *28* raptors.

    As of current plans, starship will have 6 raptors. 3 of which look like the ones we saw today, and 3 that have a bigger engine bell that are more efficient in the vacuum of space.
  10. motion


    Amazing. That's a lot of engines.

    I was struck by how small the raptors are. Wondering why so many and so small?
  11. mpusch

    mpusch Well-Known Member

    The whole point of header tanks is that they're small. They can be used while flipping around because they're full. Don't have to worry about slosh if there's no room for a gas. Once you light the engines and start using that fuel you will replace the space the liquid used up with some sort of gas. However, since the engines are now firing you will have a settling force that pushes the liquid fuel to the bottom - just like gravity! So that's OK!

    Slosh is bad because it would send gas bubbles through the engines, which would basically cause it to tear itself apart.
  12. mpusch

    mpusch Well-Known Member

    They aren't actually that small. Here's a pic of it to scale, with a merlin engine (falcon 9) and a person.

    The number of engines and how big to make them is something spacex went through a number of revisions on. As I understand it, this size and number allows for economies of scale building a large number of them as well as engine out capabilities. If you make very few, large engines and lose one, you're probably screwed. Lose a small one when you have a bunch and you can keep going. Falcon actually did this earlier this year shutting down an engine on the way up but still completing the primary mission.

    Why *exactly* this size and number is way above my pay grade. Tons of different factors to take into consideration.

    Attached Files:

    motion likes this.
  13. motion


    Thanks for that again... I hope you and @OGs750 stick around here. I love this stuff.

    I'm gonna order a badass chinese model rocket - the biggest one I can buy and hopefully illegal and stuff - and blast it off in my backyard.
    mpusch likes this.
  14. OGs750

    OGs750 Well-Known Member

    Not sure about that 100 number, but it only took 85 flights of F9 to fly crew.
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  15. Monsterdood

    Monsterdood Well-Known Member

    Here's a guy working on a self landing model rocket in the spirit of Space-X. He even sells a kit with a stabilizing computer controller and a 3D printed thrust vectoring mechanism.
    JBall and motion like this.
  16. motion


    Good stuff, Chris.
  17. motion


  18. OGs750

    OGs750 Well-Known Member

    There's no doubt in my mind we would have stuck the landing if the engines stayed lit. We have a lot of experience landing high aspect ratio objects on their end and that experience showed yesterday.
    mpusch, NemesisR6 and CRA_Fizzer like this.
  19. mpusch

    mpusch Well-Known Member

    Out of all the really fake looking videos I've seen of Spacex over the years, that video has to rank towards the top. Absolutely incredible shot.
    NemesisR6 likes this.
  20. motion


    I think it gets trippier each time I watch it :D

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