Indeed. It takes me months to do these videos, they are released at a slow pace.
Quote from: Davidthefat on 05/11/2018 08:08 pmWhat makes you think that OF ratios aren't trimmed during flight? Just curious where you got your information.[L2 link] https://forum.nasaspaceflight.com/index.php?topic=45594.msg1818987#msg1818987
What makes you think that OF ratios aren't trimmed during flight? Just curious where you got your information.
Here's a plot of some of the S1 parameters for the Bangabandhu launch. They still throttle down for Max-Q, but there is a gradual reduction in throttle for each phase, giving constant thrust for each phase of the S1 burn. This would limit the thrust loading not just on the Octaweb, but the rocket as a whole.Edit: Perhaps constant (SL max) thrust is necessary to maintain a factor of safety of 1.4 for CCP?
Quote from: OneSpeed on 05/12/2018 02:32 amHere's a plot of some of the S1 parameters for the Bangabandhu launch. They still throttle down for Max-Q, but there is a gradual reduction in throttle for each phase, giving constant thrust for each phase of the S1 burn. This would limit the thrust loading not just on the Octaweb, but the rocket as a whole.Edit: Perhaps constant (SL max) thrust is necessary to maintain a factor of safety of 1.4 for CCP?Okay, compressive stress in the Octaweb is a limit. Wow.Quick check, if the tank ullage is at 30 psi, that's 2.1 meganewtons axially, which is quite a bit less than the liftoff 7.6 meganewtons. Looks like the first stage tank is under significant axial compressive load, at least near MECO. I would not have expected that either.Does anyone have real values for the first stage ullage pressure?
Quote from: Comga on 07/30/2018 08:24 pmJust did a countIridium 7 launch makes it 500 Merlin engines flown since the last in-flight engine failure on CRS-1.This includes the 9 ground lit Merlins and the MVac on the second stage.This excludes the engines on CRS-7 that didn't get to burn for their full profile, or wasn't lit in the case of the MVac. That's quite a record and a statistically significant history. Might be better to skip the F9 after CRS-7 since that was the last of the M1Cs.
Just did a countIridium 7 launch makes it 500 Merlin engines flown since the last in-flight engine failure on CRS-1.This includes the 9 ground lit Merlins and the MVac on the second stage.This excludes the engines on CRS-7 that didn't get to burn for their full profile, or wasn't lit in the case of the MVac. That's quite a record and a statistically significant history.
Quick check, if the tank ullage is at 30 psi, that's 2.1 meganewtons axially, which is quite a bit less than the liftoff 7.6 meganewtons .....Does anyone have real values for the first stage ullage pressure?
As far as we know, there has never been a failure of a Merlin 1D, sea-level or vacuum, during the boost phase of any flight. There have been some problems with landing burns, but that was uncharted territory at first. There may have been failures at McGregor, but those would not be public knowledge.Agreeing with Comga here: any way you want to look at it, the Merlin 1D has been a remarkably reliable engine. Tom Mueller and his team can be justifiably proud of what they have done.
Quote from: rpapo on 07/31/2018 10:20 amAs far as we know, there has never been a failure of a Merlin 1D, sea-level or vacuum, during the boost phase of any flight. There have been some problems with landing burns, but that was uncharted territory at first. There may have been failures at McGregor, but those would not be public knowledge.Agreeing with Comga here: any way you want to look at it, the Merlin 1D has been a remarkably reliable engine. Tom Mueller and his team can be justifiably proud of what they have done.Agreed. I do wonder what they have done differently to others that had resulting in this spectacular engine in such a short timeframe. Is it that they have rapid turnaround? Are they taking more `risks`? Has returning the engines from flight regimes driven change? Are they simply given a decent length of rope by Musk? Do they simply employ better people? Is design technology and simulation simply so much better now? All of the above and more?Interesting.
Quote from: JamesH65 on 07/31/2018 11:33 amQuote from: rpapo on 07/31/2018 10:20 amAs far as we know, there has never been a failure of a Merlin 1D, sea-level or vacuum, during the boost phase of any flight. There have been some problems with landing burns, but that was uncharted territory at first. There may have been failures at McGregor, but those would not be public knowledge.Agreeing with Comga here: any way you want to look at it, the Merlin 1D has been a remarkably reliable engine. Tom Mueller and his team can be justifiably proud of what they have done.Agreed. I do wonder what they have done differently to others that had resulting in this spectacular engine in such a short timeframe. Is it that they have rapid turnaround? Are they taking more `risks`? Has returning the engines from flight regimes driven change? Are they simply given a decent length of rope by Musk? Do they simply employ better people? Is design technology and simulation simply so much better now? All of the above and more?Interesting.I’ll add my 2 cents on 2 ideas why the Merlin is such a strong performer.1) It’s using the newest technology in design and materials.2) They’ve kept it small. Which has given them great T/W and smaller components like the turbo pumps and combustion chambers should be easier to control and build than larger engines.
My 2 cents: Maybe it is because they designed the Merlins from the start with reuse in mind and as so those engines have a higher margin in their design than one meant to be thrown away, so their reliability is a byproduct of their design goals.
Unlike the established companies SpaceX flies more too. The other guys can work 10 years on an engine and never leave the ground.
NASA and the traditional contractors use a different quality assurance philosophy. They use qualification test articles in combination with extensive documentation of how the test articles were produced, on the premise that subsequent articles produced in the same way (and with documentation to that effect) are qualified to fly without representative preflight testing.SpaceX tests every engine every time, which would seem to reduce the need for such extensive documentation and adherence to qualified designs/procedures. But this is uncomfortable for NASA in the context of commercial crew.For example, SpaceX apparently sees some "undesirable anomalies" with Merlin engines on the test stand. From SpaceX's perspective, maybe they just don't fly the particular engines that manifest the anomaly. They test all the engines and weed out any of them that aren't up to snuff. But because of the way that NASA normally operates, they are probably concerned that SpaceX may be unable to reliably build engines that are guaranteed to meet requirements on the test stand.If NASA made the same requirement for, let's say, Intel microprocessors used in their computer systems, Intel would say no, that's not the way the microprocessor industry works... We test everything, there's a certain yield of functional chips, they don't have uniform performance, and we "bin" them into different SKUs and price points accordingly.
One of the things that we did with the Merlin 1D was; he kept complaining— I talked earlier about how expensive the engine was. [inaudible] [I said,] “[the] only way is to get rid of all these valves. Because that’s what’s really driving the complexity and cost.” And how can you do that? And I said, “Well, on smaller engines, we’d go face-shutoff, but nobody’s done it on a really large engine. It’ll be really difficult.” And he said, “We need to do face-shutoff. Explain how that works?” So I drew it up, did some, you know, sketches, and said “here’s what we’d do,” and he said “That’s what we need to do.” And I advised him against it; I said it’s going to be too hard to do, and it’s not going to save that much. But he made the decision that we were going to do face-shutoff.So we went and developed that engine; and it was hard. We blew up a lot of hardware. And we tried probably tried a hundred different combinations to make it work; but we made it work. I still have the original sketch I did; I think it was— what was it, Christmas 2011, when I did that sketch? And it’s changed quite a bit from that original sketch, but it was pretty scary for me, knowing how that hardware worked, but by going face-shutoff, we got rid of the main valves, we got rid of the sequencing computer; basically, you spin the pumps and pressure comes up, the pressure opens the main injector, lets the oxygen go first, and then the fuel comes in. So all you gotta time is the ignitor fluid. So if you have the ignitor fluid going, it’ll light, and it’s not going to hard start. That got rid of the problem we had where you have two valves; the oxygen valve and the fuel valve. The oxygen valve is very cold and very stiff; it doesn’t want to move. And it’s the one you want open first. If you relieve the fuel, it’s what’s called a hard start. In fact, we have an old saying that says, “[inaudible][When you start a rocket engine, a thousand things could happen, and only one of those is good]“, and by having sequencing correctly, you can get rid of about 900 of those bad things, we made these engine very reliable, got rid of a lot of mass, and got rid of a lot of costs. And it was the right thing to do.And now we have the lowest-cost, most reliable engines in the world. And it was basically because of that decision, to go to do that. So that’s one of the examples of Elon just really pushing— he always says we need to push to the limits of physics.