I still don't understand. Those waffle irons are not needed at low speed. They did grasshopper landing without them right? Or perhaps they interfere if they are present and not working correctly? Either way, why did the rocket hit "hard" as Elon said? Rocket velocity shouldn't have anything to do with the waffles right?
I've been thinking someone forgot to include the height of the GPS in some code somewhere, and this whole hydraulic issue while true, is a handy way to not publicly disclose a simple mistake. Don't wanna look like the mars probe that crashed due to unit conversion issues...
So really, how does the hydralic issue affect the impact velocity?
First, "hard" landing doesn't necessarily mean "high speed". It's a generic term for unsuccessful landing. It could be incorrect attitude landing (e.g. tilted) or anything else. In this case it looks like it was exactly that: landed tilted or with significant horizontal speed. If it was high vertical speed, we would've seen big scrapes on the platform's surface, and there's none.
Contrary to popular belief, the fins are _extremely_ effective even at low speeds. First, they have enormous surface area. Secondly, they have huge momentum around the center of mass, which is at the very bottom of the rocket when it's empty. So, even a small force generated by the fins generates huge momentum and can help position the rocket in the desired attitude.
It looks like the control system was commanding the fins to move and was expecting the attitude to change, but they didn't move anymore. Perhaps at the very end of the flight the gimbals on the engine didn't have enough authority to orient the rocket vertically and/or arrest its horizontal speed.
I think it's amazing achievement for the very first attempt. Congrats SpaceX and I'm jealous like hell for what you've been able to achieve.
> I think it's amazing achievement for the very first attempt.
People don't understand this enough. They took the stage of a rocket from supersonic speeds to a tiny barge in the ocean. Even though they didn't complete the objective, they were successful in showing that the theory holds up.
On top of what you've said I don't think enough people get the scale of this task. This is decelerating, balancing, and maneuvering a 14 story tall object, from supersonic speed at the edge of space, onto a tiny barge in the ocean. It's mind boggling when you look at the scope of what they are pulling off.
I saw this same remark during the particle experiments at the LHC. If they discovered the Higgs, then we learn that our guess was right. If they can't find any evidence of the Higgs, then we learn that we might not be right. In any case, we learn things.
I liked to apply that to the Rosetta mission as well. The happy-path goal was to land a craft on a comet. If everything went 120% better than expected, then we would get a large amount of data about the surface of comets. If things went the way they did in actuality, then we get a large amount of data about comets. In either case, we learn a hell of a lot more than we did having not done the mission, regardless of it's outcome.
Science and knowledge isn't binary. Sure, you can say you either know something or you don't, but knowing something has a range of n through infinity. "Failure" gives you the option to learn more.
>> I think it's amazing achievement for the very first attempt.
I completely agree. I was just confused by the term "hard landing" and the effectiveness of the grid fins at low speed. It didn't make complete sense to me. As for my "stupid mistake" comment, I guess running out of fluid is also as simple as it gets if that's all that went wrong, and they are owning up to that.
That said, this is only a "first attempt" at putting it all together. I had fairly high confidence in them. Remember, they hovered and landed in Texas. Their first controlled re-entry failed, the second made it to the sea with onboad camera, the third was seen hovering at low speed from a plane. They had no reason to think this wouldn't work - which is not the same as having high confidence that it will work ;-) Putting it all together often leads to interesting things...
I'm still impressed and look forward to the next one.
Oddly, if it was a 'stupid' mistake I'm pretty sure they would own up to it.
But lets look at what the these things do. During descent they provide control authority over roll, pitch, and yaw of the lower stage. Even though the center of gravity is near the bottom the rocket, the system is trying to keep it vertical. It is travelling down range and needs to go from moving sideways, to coming straight down. Earlier versions of Grasshopper used hypergolic thrusters on the sides but they were scrapped in favor of the fins system. So once you run out of hydraulic fluid, you're limited to the ability to gimbal the Merlin engines for controlling pitch and yaw.
If you watch the video you can see that the fins are used all the way to the point where it lands.
So my guess is they knew they were going to run out of hydraulic fuel and brought the rocket in "hot" (at a higher than planned approach speed) and didn't manage to get close enough before the fins stopped working.
If that really was the only thing that didn't work correctly, then we'll know on the 29th.
There weren't any 'early versions' of Grasshopper. Grasshopper was a single test vehicle (and it didn't use hypergolics, it just relied on gimbaling the single Merlin it had).
The first stage has cold-gas thrusters on board for attitude control, but has never used hypergolics, AFAIK. The only hypergolic engines SpaceX uses are the Draco and SuperDraco, which were never on any first stage cores.
You are correct, a long time ago when reading about the Draco I was under the impression they were used on the early Falcon 9 as thrusters, but according to this : http://www.spacex.com/news/2013/04/04/draco-thrusters they were only used on the upper stage and on the Dragon.
If the grid fins were stuck in an unhelpful position, then the engine would have to compensate much more significantly, reducing the trust available in the 'slowing down' vector.
It's also likely that 'hit hard' just means it crashed. Looking at the pictures of the ASDS, it's clear the rocket came down off-center. Perhaps it was at the appropriate velocity, but because it landed on top of a bunch of support equipment, it fell over and exploded.
If you can't orient your rocket correctly you're going to have a hell of a time getting it to slow down to the correct speed while also maintaining the correct position for touchdown.
I'm not sure how the waffles help with that, but I'll believe the article if they say it's used for steering/rotating.
Right, I believe the grid fins are the primary roll control method (the other being friction and secondarily tilting the ship and making a precession around center of mass). If this force was lost while the control loop was expecting it to work things can go bad I guess.
Also attitude control. The stage isn't going to be much of a glider no matter what you do with it, but at terminal velocity, it has nontrivial lift and drag, and changing those will change the trajectory.
"Needed"? The rocket has maneuverability without them, sure. Just as your car has maneuverability without power steering, and your car has braking capability without ABS. But the grid fins increase the maneuverability a great deal at low cost, which is why they are used. They make it far more likely that the rocket can successfully hit a landing spot during a re-entry. So even though they add some weight and add some cost and add some complexity to the system they still increase the chances of recovering the multi-million dollar rocket hardware, which is a big win.
And the hydraulic fluid running out affects the impact velocity because the terminal trajectory of the rocket was controlled based on the assumption of having the grid fins working. When they stopped working they put the vehicle in a situation that was no longer controllable given the remaining capabilities of the rocket. If the power steering goes out on your car in the middle of a turn that's much worse than if you didn't have power steering at all, and could result in a crash.
> They did grasshopper landing without them right?
As someone who doesn't know the slightest bit about space technology, I was a bit confused about the fact that they successfully landed another rocket. If that's the case why is the landing of Flacon 9 so significant? Is it like a bigger rocket or something?
The Grasshopper was SpaceX's testbed rocket that did the "easy" version of this - taking off straight up (to a maximum altitude of 744m) and then landing right back at its launch pad. I believe one of them also did some sideways maneuvers too.
It was a bit smaller-scale (two-thirds the height) of a Falcon first stage, and I believe also had a less complicated engine. More importantly, though, given the failure mode of this rocket, it had no mission other than taking off a bit and then landing - it wasn't integrated into a rocket that was intended to get to the edge of space, decelerate from hypersonic speeds, and then do the complicated landing.
Grasshopper and the Falcon 9R Dev were the two rockets of spaceX which landed back successfully. Those were limited altitude test flight, this one were the expended first stage of a revenue flight. This rocket went higher, and faster before separation and performing a boost back burn. Might seem as just a quantitative difference, but the different aerodynamical regimes (vacum, hypersonic, supersonic, transonic) provides a real challenge.
It's also worth noting that the engines were lit 100% during those previous tests, while they have to be relight during this one. That comes with it's own set of complications of course.
Hovering a rocket under controlled conditions and actually recovering a rocket stage that was used operationally are fairly different. It seems as though they've put most of the pieces together in terms of proving they can reuse the first stages, but they haven't actually done that operationally, and there are still many doubters (even in this very thread). Actually bringing the first stage back is one step closer to proving they can reuse the stages, which would be a tremendous step forward in spaceflight.
Similarly, Apollo 8 and Apollo 10 proved fairly conclusively that NASA could land humans on the moon, but it was actually doing so on Apollo 11 that was so incredibly important.
I've been thinking someone forgot to include the height of the GPS in some code somewhere, and this whole hydraulic issue while true, is a handy way to not publicly disclose a simple mistake. Don't wanna look like the mars probe that crashed due to unit conversion issues...
So really, how does the hydralic issue affect the impact velocity?