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from AIAA's Daily Launch email newsletter for Mon 11-16-20:
SpaceX Tests Starship SN8
SPACE (11/13) reported that “SpaceX fired the engines on its latest Starship prototype for the third time” Thursday at SpaceX’s South Texas facility. SpaceX founder and CEO Elon Musk said afterward, “We lost vehicle pneumatics. Reason unknown at present.” Musk added, “Burst disk worked, so vehicle appears to be ok. We’ll have to swap out at least one of the engines.” SpaceX is planning “a 9-mile-high (15 kilometers) test flight in the near future,” though “Thursday night’s setback will delay SN8’s big leap.”
Loss of vehicle pneumatics is a system failure of some kind. Burst disk loss means it overpressurized for some reason.
Having to swap out an engine is troubling. I wonder if that is related to the troubles (nature still undefined publicly) that they had a month or two ago with Merlins.
They have to resolve these successfully before even thinking about flying.
GW
Last edited by GW Johnson (2020-11-16 09:00:57)
GW Johnson
McGregor, Texas
"There is nothing as expensive as a dead crew, especially one dead from a bad management decision"
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It now appears the problems were a lot worse than first perceived. Something bad happened to one or more engines during the static fire tests, ultimately leading to a near-explosion of the oxygen header tank. The burst disk relieved it before it could overpressure and explode.
Up to now, the Raptor engines have seemed reliable. Now this is question. Coupled with the Merlin troubles they had just prior to the satellite and crew Dragon launches, it makes you wonder if something rather systemic has not gone wrong.
One place they need to look, and they may not want to admit to this need, is workforce burnout. When you work people as hard as they do, people get tired and screwups happen. It is inevitable. And they have been pushing their workforce very hard for some years now.
That strain is why they generally do not hire anybody over about 45 years old. Only the young can take that kind of strain. And they cannot take it forever.
GW
GW Johnson
McGregor, Texas
"There is nothing as expensive as a dead crew, especially one dead from a bad management decision"
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GW,
I'm worried that they're simply iterating through different designs, hoping something will finally work the way they want it to. If resources are unlimited, then brute force (iterative design) is a reasonably good way of resolving technical issues. If not, and I contend that resources are not unlimited, then a re-evaluation of the design process is in order.
At the end of the day, they're fabricating a welded stainless steel pressure vessel. That is a task that any competent structural engineer and welding outfit should be able to accomplish relatively quickly, despite the size of the pressure vessel. The fact that they've blown up or nearly blown up a half dozen different pressure vessels of similar design is indicative of a design and/or fabrication issue. Do some tests, gather performance data, and then design a sufficiently durable vessel with generous performance margins. That's how the Russians do it and it works quite well when cost rather than absolute bleeding edge performance is the primary design criteria. They still don't have proper welding jigs and tooling. This is sheet metal fabrication, fer cryin out loud. Everybody who produces sheet metal structures for a living uses proper jigs and tooling.
Someone needs to conduct a safety stand down before they get someone killed. No other corporation or government agency is in any danger of ever catching up to them at this point. Do it right the first time and be done with the task.
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Hi Kbd512:
I quite agree with you.
This is a pressure vessel designed by folks without real pressure vessel design experience, plus they are using one dome as a thrust takeout structure, too. Both are mistakes, which is why they have had so much trouble with it.
If you think this is bad, wait till you see what high-q entry airloads do to it. Or even high-gimbal-angle thrust loads.
I can't say such is unexpected, as no one has ever done this type of a vehicle design before. The Merlin-powered Falcon/Dragon design is more like earlier designs, and it has years of experience behind it. That shows in the success rates.
Although in the early days with Falcon-1, they nearly went bankrupt failing. That is where they are now with Starship.
Learning how to do this stuff right is a long and painful process, which I have witnessed with many entities, including NASA. The more technically arrogant, the longer it takes, too.
GW
GW Johnson
McGregor, Texas
"There is nothing as expensive as a dead crew, especially one dead from a bad management decision"
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GW,
Yeah, I really don't understand the rush to fabricate something of such poor quality or unsuitable design as to be unworkable for a practical vehicle test article. Was any reason given for ignoring conventional wisdom on pressure vessel / tank dome design?
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To get ahead of the sls launch with humans as then the only thing its go it a large passenger taxi to orbit until its got the much needed fuel tanker system up and running to make any larger steps towards the moon or mars....
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Kbd512:
Well, it's not about them knowingly doing it wrong. It's about just not knowing that they're doing it wrong. The higher the pressure they need to reach, the more important it is do the pressure vessel job right.
Sort of the same thing happened with the early Falcon-1 failures. They knew rocket engines, but they knew little about flying supersonic vehicles, especially supersonic staging, with the "drafting" effect. They nearly went bankrupt learning that lesson. (There is a very compelling reason weapons release speeds for underwing stores are limited to 485 knots indicated.)
"Right" means pressure shell membranes are round-with-radii, not conical, and you NEVER apply point loads to them. The welds and shape transitions are stress concentrators enough.
Now, they do have a lot of experience getting first stages to survive entry for landing and recovery. You'll notice that the only way feasible to do this, is to point the thing rear-end-first into the slipstream, and fire an engine or engines (in supersonic retropropulsion, no less) to control speed. Letting it get sideways crushes the stage from the entry airloads applied to one side of the tankage, acting against the inertia.
Entry with Starship will have ABSOLUTELY NOTHING to do with their prior experience, because (as Spacex itself reports) the entry AOA will be 60 degrees. That's almost dead broadside, and the belly flop maneuver for Earth landings (irrelevant for Mars) actually is dead broadside at 90 degree AOA.
You are looking at airload dynamic pressures during entry that fall in the 1000-5000 psf range, applied to one side of the tankage. During the final belly-flop descent, these pressures still fall in the 100-1000 psf range way up high where speed is still supersonic.
They have zero actual experience with that, only computer code results. And we all saw how unreliable THAT can be, with Challenger and Columbia, and more recently with the 737-MAX. It's a question of what you analyze versus what actually is. Many examples. In the fatigue crack world, the DeHavilland Comet comes to mind.
My own expectation is the first few flight test Starships will undergo mid-air breakups. The tankage will crush before the aerosurfaces can even be ripped away. In their designs, I don't see the big ring frames needed to stop that one-sided crush.
It ain't like submarine pressure hull work, either, because of the extreme asymmetry of the load application. And the heat.
Spacenut:
SLS Block 1 is 170 metric tons to LEO for about $2B per launch, as currently projected. Max crew in the Orion capsule 7. That's near $300M per seat, or $11-12M/ton cargo.
Starship/Superheavy 100+ (probably around 120-150) metric tons to LEO for likely something in the neighborhood of $200-300M per launch. Max human passenger load up to around 100. That's around $2-3M per seat, or $2-3M/ton for cargo.
That makes Starship/Superheavy the far better ride to LEO, whether for passengers or for cargo. No doubt about it.
Neither has flown yet. We'll see if Spacex can pull off figuring out how to get Starship to survive entry and landing. That will have to happen, before anyone even begins to think about going anywhere outside LEO with refilling.
I do hope Spacex people look at this forum. It's discussions like this, pointing out foreseeable troubles, that could save them a lot of grief and cost.
GW
Last edited by GW Johnson (2020-11-18 13:36:39)
GW Johnson
McGregor, Texas
"There is nothing as expensive as a dead crew, especially one dead from a bad management decision"
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GW,
If the aero loads truly will be 5klb/ft^2, then those propellant tanks will need serious reinforcement on the inside that I haven't seen in the prototype photos as of yet. Steel is well known for yielding or undergoing permanent deformation at a much lower loading before ultimate failure occurs.
Didn't the Russians make the MiG-25 and MiG-31 out of arc-welded stainless steel?
Maybe someone from SpaceX should ask them about how they did that since they still make them.
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Looks to me like the sls to starship says that the sls has a 70 m ton area that could be converted to house the 100 passengers but that is not what Nasa is striving to do as that would make the rocket only as capable as starship in that it would need refueling in order to be able to do anything with it as well. Yes, the sls seat cost would still be more than a starship per seat cost even after the redo of the design. Its not until Block 2 is built that the tonnage goes up...
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The other way out of this apparent quandary is to forego the skydiver suicide dive. Use traditional retro-propulsion. Reduce the effective payload and add more fuel. Some of the payload should become structural members and the monocoque becomes semi-monocoque. Recall the Atlas I had structural issues.
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For Oldfart1939 re #835
Thanks for adding this perspective on the problem ...
It will be interesting to see if anyone looks seriously at that alternative. The tradeoff is a ** lot ** more flights to achieve the delivery objectives, but if ALL hardware is reusable and there is minimal service between flights, it might prove superior in the long run.
***
The Yahoo news feed included an item teasing that Elon gave a presentation to a virtual conference recently in which he advocated use of space tugs to move satellites around rather than expendable rockets.
Hopefully someone in the forum will post a link to the full article.
Following up on your suggestion .... space tugs would need fuel to serve multiple customers.
(th)
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I'm guessing that Spacex is counting on a lower effective ballistic coefficient during early entry to get the vehicle slowed higher up, so that dynamic pressure will trend toward the lower end of the ranges I quoted. This lower effective ballistic coefficient comes about from the large dimensions and low inert mass and low landing propellant load. You pay for this by higher early gees, and perhaps limits on return payload.
What I don't see is the experience in test flights yet, to confirm or deny the computer predictions they are so obviously relying upon. That's what SN-8 and subsequent are supposed to get for them. But based on others' test flight experiences (such as Convair with Atlas I decades ago, thank you Oldfart1939), I think Spacex is in for some very rude surprises.
Also decades ago, when I was a graduate student, I worked in a Mach 5 wind tunnel testing space shuttle nose shapes before that design finally "gelled". We found some rude surprises concerning AOA limits during entry that were independent of nose shape. These were confirmed by large scale hot tests in AEDC's high-$ wind tunnel. Unpredicted by the prior database or the primitive CFD codes of the time, but quite real, these rude surprises were.
Murphy's Law is a very real thing.
GW
GW Johnson
McGregor, Texas
"There is nothing as expensive as a dead crew, especially one dead from a bad management decision"
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GW: Murphy was an optimist.
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There are a few things I feel more compitent in such as control systems, and some things not so compitent, typically things that GW excells in. But I will comment, while already indicating a awareness of my lack of certanty about many things.
TESSLArati has an article titled "SpaceX's Elon Musk hints at "notable" Starship Changes, explains static fire anomaly".
When it was said that the pneumatic control system had gone down, I had an idea that the engine(s) had not been controlled well due to the fault. But I was woried that the pressure/thrust puck had been compromised. It seems that this was the case per at least one engine, and how much damage to the thrust puck is a question still.
The cause was said to have been high velocity chards of pad materials, damaging the controll pneumatics/hydraulics. Their response apparently will be armor and water cooling for some parts of the controll systems pneumatic or hydraulic systems.
But this is a test article. So, if they have only one concern about using it to get test information. I am sure they prefer not to have a "RUD" on the launch pad itself, and probably do not want falling materials to damage the launch site. If they feel confident about the probabilities, then they may repair and continue. Of course they must want a completion all the way to a landing, but SN8 is probably already obsolete, and they have other improved devices in prepairation already, so, I guess the way forward is to get what they can out of SN8 as a test article.
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My understanding is that Starship is capable of hypersonic gliding, but I don't expect that for the first test flight. However at hypersonic speeds it might generate enough lift to stay in thinner atmosphere to bleed off speed. This has been said to allow for a thinner heat shield.
The rest of it, where the concern for the thing breaking up, is beyond my proficiency level to understand. I do understand that there is what I call the feminine dimension of the circumference of the shell, and the masculine dimension of the 'a' to 'b' bottom to top flexing of the end to end tube. This is how I think. I don't care to discuss at this time why.
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I have considered the "Low Hanging Fruit" for Starship. Obviously this would be to deploy Starlink, and rideshare other devices to orbit.
Another thing I consider is that without orbital refueling, microgravity research/production could occur on such a ship. It would not be necessary to make it crew rated, as other systems to get humans to LEO exist/may exist.
And beyond that, building on works as I understand them from GW, I anticipate linking two Starships in LEO, to build a temporary synthetic gavity device. Conect two tail to tail and spin for 1/3 or 1/6 g.
One ship might do animal and human research, and another might do human biological research, and also research into manufacturing in partial gravitational fields.
Since it is intended that Starships must eventually be able to have very extended life support, then this as well could test those aspects.
And none of this would require refueling, I think.
Done.
I am starting to ask seriously "Why do I do this?"
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GW,
I still think "nose up" horizontal attitude gliding using aerodynamic lift and vertical landing using retro-propulsion is best. There was nothing fundamentally wrong with the STS orbiter design for achieving excellent cross range and lower aerodynamic pressure loadings. STS was far from optimal for many other reasons, but the aerodynamic design of the orbiter itself wasn't one of them. Starship really needs to be able to "glide" like our Space Shuttles did. Granted, they did more "falling out of the sky like a brick" than "gliding" in the traditional sense of the word, but it worked well enough. If Starship could land horizontally using 4-poster gimbaling descent engines like the Harrier fighter jet, imagine how easy passenger and cargo extraction would be and how fantastic stability would be when compared to a vertical landing. Lifting body design is the right way and it always has been, even in the nearly useless but still problematic Martian atmosphere.
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Kbd512:
Well, they are planning on nose-up gliding, with lift, during entry. That is very similar to the space shuttle, which entered nose-up between AOA 20 deg and 40 deg (very strictly limited, too). Spacex wants to use AOA 60 deg, but the initial portion is downlifting (inverted). They roll to uplift at high AOA pretty much near peak gees and peak heating.
I am guessing that peak gees and peaking heating occur somewhere around where the shuttle saw this, which would be Mach 12-ish at around 150,000 ft. That corresponds to 280-290 psf dynamic pressure, which is less than I thought it would be.
They should come out of hypersonics somewhere around 120,000-130,000 feet, at around Mach 3. That would be dynamic pressure near 40-60 psf. Their simulations indicate a supersonic AOA-increase (with simultaneous deck angle decrease) to 90 deg as the trajectory bends vertically downward. That's the "belly-flop" maneuver. They ought to be near Mach 1 dead broadside in deep stall at around 100,000 feet, which is dynamic pressure near 16-17 psf. All those q's are lower numbers than I expected.
The "belly flop" takes you to around 10,000 feet (3 km), at which point the vectored thrust and the aerosurfaces flip the ship tail first for landing. Their simulations show a low altitude descent rate near 62 m/s, which would be near Mach 0.19. At 10,000 feet, that would be a dynamic pressure near 35-40 psf. Again, lower than I expected to see.
The Mars entry sequence looks pretty much identical, to end-of-hypersonics at about Mach 3, which is only about 5 km (16,000 ft) altitude on Mars! They stay at about 60 deg AOA, expecting the air lift forces at low altitude to be enough to reverse the supersonic descent into a decelerating supersonic climb to around Mach 1-1.5 at 10 km altitude. Personally, I think they will have to fire up the engines in order to make that decelerating climb, but they don't think they need to do that, until just after the 10 km peak. We'll see.
From there, it's flip tail-first to land from just supersonic near that peak. Real nail-biter, that sequence. But that is inherent with that near-vacuum of an atmosphere on Mars. Surface pressure conditions on Mars resemble those at 110,000 feet here at home.
One of my concerns about the Spacex high-AOA entry attitude derives from the very-strict shuttle entry AOA limits. As a grad student, I helped find these initially in the Mach 5 tunnel at UT Austin (confirmed at AEDC), and they are relatively independent of the exact nose shape! Below 20 deg, the shuttle windscreen saw direct wind blast coming over the nose.
That windscreen would survive only about a second of that 12,000 K direct wind blast. The flow had to separate and jump over the windscreen and cabin roof, in order to eliminate the scrubbing, and so drastically lower the heat transfer coefficients. Same driving temperature, but the coefficient is more than an order of magnitude smaller, which is why the heating rate is also more than an order of magnitude smaller.
Above 40 deg, the lateral separation-line vortices each side of the nose were strong enough to pull the separated jet of over-the-nose dorsal direct wind blast back down into contact with the dorsal surface of the nose, thereby again putting direct wind blast on the windscreen. Same 1 second survival time applied.
The windscreen would only survive entry at those AOA's between 20 and 40 deg, with the separated flow jet jumping over the windscreen and cabin roof. THAT is why the shuttle AOA limits were so strict during entry! Columbia broke up when she lost that wing near peak heating over the Texas-New Mexico border. That loss of attitude control violated the limits for the windscreen, which failed and let the wind blast blast rip the cabin roof off, all before the bird could even turn broadside and break up.
That cabin winscreen and roof failure also ripped Columbia's 4 flight deck occupants out from under their seat belts, in pieces, which is why cooked-but-not-burned body parts came down between Dallas and Tyler. The 3 mid-deck occupants survived until the tumbling cabin section crushed from broadside max-q airloads at about Mach 1 and 10,000-15,000 feet, a few seconds from impact near Tyler.
Starship as planned for its operational form has lots of windows on the nose dorsal surface. As I said, nose shape seems to be irrelevant to the fluid flow pattern. There will be lateral separation-line vortices on this Starship bird, and it is rather likely they will pull the over-the-nose jet of wind blast down, to scrub the windows pretty hard, at high AOA. A second or two to loss of windows, then loss of vehicle. Not a pretty prospect.
That flow pattern was not predictable with CFD in 1972 when I helped to experimentally find out about that phenomenon in the wind tunnel. It might or might not be reliably predicted by the much-better CFD codes today. But one thing I do know for sure about CFD code predictions: you MUST verify in experimental tests, before you dare trust computer results! That is still just as true today as it was in 1972. And too many still fail to heed that warning!
To the best of my knowledge, Spacex has not run hot-flow hypersonic wind tunnel tests at AEDC, the only place where you can do such tests on a big model.
And THAT risky place is where they currently are.
GW
Last edited by GW Johnson (2020-11-19 16:28:07)
GW Johnson
McGregor, Texas
"There is nothing as expensive as a dead crew, especially one dead from a bad management decision"
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Looking at how wrinkled the shell is for the dive maneuver I am thinking that the belly pressure will cause shape distortion to occur as it goes through its landing entry profile let alone the tail flip to direct it into the upright position for the verticle landing. Any tiles on the under belly would also deform and possibly come off as the pressure increases during the entry landing.
Also there would be a problem with the massive mass of the starships structure bearing down of the first stage BFR booster in that with the same given building practice would tend to bulge under its reading for flight.
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GW,
I was attempting to suggest that performing abrupt aerobatic maneuvers aren't a particularly good idea for a heat-soaked paper thin stainless steel cryogen tank larger in diameter than any operational jet aircraft, despite having a minor fraction of the structural mass to volume ratio of those cargo aircraft fuselages dedicated to resisting aero loads. Perhaps the tank can survive a few of those maneuvers, but if it's even slightly outside of the performance envelop or imperfectly fabricated, then that thermally soaked structure may very well yield. Would sufficient internal pressurization in the tanks prevent the structure from yielding under load? That must be at least part of the plan, given what the tanks presently look like on the inside.
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The usual analogy used for the monocoque construction is description of a can of beer or of soda pop. When full and sealed, it can easily support the weight of a man standing on the top. When empty, we squash them flat for the recyclers. Could be that the high test pressures they are using may be the plan for structural integrity of Starship?
Last edited by Oldfart1939 (2020-11-21 01:45:39)
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Kbd512 & Oldfart1939:
I think you are quite right to suggest pressurization as a means to improve resistance to applied external loads. I just don't think there's a lot of pressurization-based increase in the effective strength available with this design, because the thing is only testable to a single-digit number of bars of gage pressure, then it explodes.
Every little bit helps, but that's not much, and it may fail at even lower pressure, after suffering some entry heating. It's the poor pressure vessel design practices that prevent higher pressurization. Three of the most important are: (1) End domes must be spherical segments joined to the cylinder with a thick ring to compensate for the stress strain mismatch between dome and cylinder. (2) You do NOT apply point loads to an end dome, or else you MUST make it very thick and heavy. (3) If there are point loads (like engine thrusts) applied near a lightweight end dome, use a truss to take them to the ring instead. Spacex violated all 3 of these.
Spacenut:
The Young's modulus for stainless is in the same ballpark as the other steels, near 30 million psi. Aluminum alloys are near 10 million psi. The composites are under 10 million psi, some well under. Carbon-epoxy is the highest I know of, but also suffers from low elongation and hidden-damage effects. The toughest from a damage standpoint is kevlar-vinyl ester. But it's rather flexible, not stiff at all.
It is young's modulus and the effects of part dimensions that create stiffness, in turn limiting deflection under load. A measure of stiffness is EI, where E is Young's modulus and I is the moment of inertia for the part in the direction and at the location of that applied load.
GW
Last edited by GW Johnson (2020-11-21 09:46:30)
GW Johnson
McGregor, Texas
"There is nothing as expensive as a dead crew, especially one dead from a bad management decision"
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Has anyone else here ever heard of continuous fiber ceramic matrix composites (CMCs like SiC/SiC)? Most of these materials have densities around that of Aluminum, but much higher temperature resistance, and fatigue cycles prior to failure can range into the millions at 80% of the material's strength. Even though they may not be able to flex as much as stainless prior to failure, they can be considerably stiffer and tolerate temperatures that 304 and Inconel cannot.
Read the article in the link below to understand how gas turbine and rocket engine manufacturers are using these materials in highly stressed components subjected to hot oxidizing environments:
Commercialization of CMCs and developments for next-gen performance
Take note of how well the CNT fiber does in resisting extreme temperatures in this CMC.
Here's an article from NASA about improvements to an existing CMC product:
Silicon Carbide (SiC) Fiber-Reinforced SiC Matrix Composites
I think CMCs are the kind of material technology required to provide the combination of mechanical and thermal properties necessary for the propellant tanks and thrust structure while minimizing heat shielding mass. A comparatively thinner and therefore lighter heat shield would enable Starship to come much closer to the original dry mass targets set when SpaceX intended to use conventional CFRP and a thicker heat shield to insulate the composite from reentry temperatures.
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Sorry, I meant to get back to you sooner.
These materials seem now to be a lot closer to "standard" than just a few years ago. Being used by engine makers does that. I don't know for sure, but I'd bet one or more of these appears in Mil Hndbk 5 soon.
Such appearance is not a perfect guarantee of adequate properties, but it is a good indication. The beta-phase titaniums are in there, but still age unacceptably at room temperature.
GW
GW Johnson
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"There is nothing as expensive as a dead crew, especially one dead from a bad management decision"
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GW must of heard this as the Starship SN8 successfully completes final testing ahead of flight
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Can't see anything about th e recent 500 feet successful Starship cylinder flight. Any thoughts?
Let's Go to Mars...Google on: Fast Track to Mars blogspot.com
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Saw that they were going to attempt the flight today but did not see any news yet so will google it to add to this posting. Could not watch the event as its on a blocked site at work....
Test flight of SpaceX’s Starship aborted at last second
The goal was to shoot Starship to an altitude of eight miles (12.5 kilometers) — the highest yet — and then bring it back to a vertical landing.
But an automatic engine abort occurred with just 1.3 seconds remaining in the countdown...
SpaceX already has conducted five Starship test flights, but these earlier, simpler models have gone no higher than 490 feet (150 meters.) The stainless steel version on the launch pad Tuesday was the first to feature a nose cone, body flaps and three Raptor engines.
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