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Looking more closely at SpaceX landing legs. They appear to be nothing but metal posts. No shock absorber. Something as big as a Starship coming down that fast? Onto concrete? Even a relatively gentle landing will produce a great deal of shock. Any wonder why SN10 exploded?
GW Johnson is an engineer. Perhaps he could comment on this. All large airline aircraft have rubber tires and shock absorbers built into the legs. Landing without any form of shock absorber will produce a kaboom. An Earth shattering kaboom.
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Speaking of landing legs ... here is a bit of an update about the failure of SN10 ...
https://www.msn.com/en-us/news/technolo … ip-sn10-de
tonated-after-landing/ar-BB1esRWN?ocid=msedgdhp
According to Musk, some issues that were already being worked out for the next prototype likely contributed to SN10’s landing issues. To be clear, the spacecraft did “land,” but it did so without a bit too rough for its landing legs to handle. The prototype apparently crushed its own legs when it landed, and Musk thinks he knows why.
“SN10 engine was low on thrust due (probably) to partial helium ingestion from fuel header tank. Impact of 10m/s crushed legs & part of skirt,” Musk explained in an exchange on Twitter. He then went on to explain why the helium ingestion was an issue to begin with: “If autogenous pressurization had been used, CH4 bubbles would most likely have reverted to liquid. Helium in header was used to prevent ullage collapse from slosh, which happened in prior flight. My fault for approving. Sounded good at the time.”
The prior flight Musk is referencing here was SN8, which also ended in an expl
(th)
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I have flown in aircraft that land softly, and at times quite hard. Weather happens, and weather can cause off optimal landing. Shock absorbers in landing gear are necessary. Landing gear for SN10 crushed, implying it's crushable. But that still transfers a lot of shock to the thin stainless steel tank. That can cause buckling or a crack. To quote Marvin the Martian: "Earth shattering kaboom!"
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Some interesting (unofficial) design concepts for Starship interior.
Let's Go to Mars...Google on: Fast Track to Mars blogspot.com
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Interesting video. Musk seems confident they've identified the cause of the SN10 landing & explosion problem.
https://www.youtube.com/watch?v=5i8d0PNCJJc
Hadn't heard about the helium before...any thoughts?
Last edited by louis (2021-03-11 20:47:39)
Let's Go to Mars...Google on: Fast Track to Mars blogspot.com
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Yea, I saw that this morning. But I still feel the legs should have some sort of shock absorber.
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Good update from SpaceXcentric:
Let's Go to Mars...Google on: Fast Track to Mars blogspot.com
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Robert--I recall seeing something about a crushable portion in these new legs. A one-time use expedient. Some of the more fanciful comments by Elon are about catching the Starship instead of using landing legs. But--not here yet, and not "now."
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Felix of WAI seems to thinking Elon is deadly serious about the rocket-catcher. But that would only work with perfectly flat concrete landing pads. He will still need legs to land on the Moon or Mars.
Robert--I recall seeing something about a crushable portion in these new legs. A one-time use expedient. Some of the more fanciful comments by Elon are about catching the Starship instead of using landing legs. But--not here yet, and not "now."
Let's Go to Mars...Google on: Fast Track to Mars blogspot.com
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Hi guys:
I just did a 2021 reverse-engineering estimate for the Starship/Superheavy vehicle to Earth orbit, and a 2021 look at using Starship for direct lunar nearside landings. The Earth orbit thing is already posted over at "exrocketman", and the lunar mission thing soon will be.
This year's estimates use a bit more sophisticated modeling of the drag and gravity losses, plus a bit more sophistication in the engine thrust and specific impulse management.
I got a bit of an increase in estimated payload deliverable to a 300 km circular eastward orbit over my 2020 estimate, at about 170 metric tons deliverable vs just under 150. And I got that increase while requiring Starship be able to conduct an abort landing on Earth while still carrying that undelivered payload! That raises landing thrust requirements significantly, by the way. Inadequate landing thrust is a fatal flaw we have already seen in the prototype tests.
Last time I looked at the lunar mission, I found it completely infeasible, if unrefueled on the moon. It had to be refueled from fly-along tankers that do not land. It was somewhat less infeasible if conducted from an elliptical Earth orbit 300 x 1400 km altitudes. That apogee is considered to be the "base" of the Van Allen radiation belts. But this was still infeasible without fly-along tankers.
This time I looked at higher-apogee elliptical orbits well into the Van Allen radiation belts, plus a rather sophisticated (but also risky) engine management scheme at the moon. I found practical payload delivery is possible if you allow deep penetration into the Van Allen radiation hazard during refueling on-orbit before departure.
What that means is that cargo carried this way to the moon must be radiation-hard, and any crew (or passengers) must have a substantial radiation shelter. It also means the on-orbit refueling tanker problem is very severe, as very little "payload" in the form of transferable propellant can be ferried up to an orbit that elongated. Why? Perigee velocities are significantly-near Earth escape. Which is what reduced the lunar mission delta-vee to something feasible.
I have yet to look closely at the tanker problem, but at least now I have only 3 orbits to investigate: 300 km circular, 300x7000 km elliptic, and 300x10,000 km elliptic (all altitudes, not radii). The moon mission can be done at 75 tons delivered, with zero return, from the 7000 km apogee orbit. The system seems capable of delivering 59 tons to the moon, with 32 tons returned to Earth, from the 10,000 km apogee orbit (for which the tanker problem is even worse).
The departure burn is made at perigee, not apogee. This unrefueled lunar mission is much more demanding than the one-way Mars mission, because the sum of the outbound and return delta-vees significantly exceeds 8 km/s unless you extremize the elliptic orbit elongation. The Mars mission is closer to only 6 km/s one-way, even from circular.
I probably will not re-look at the Mars mission, what I did in 2020 is probably about as good as it gets, given what little we know about Starship and Superheavy so far. I probably will look again at the tanker problem. Musk has said the dedicated tanker design will carry more propellant in extra tanks, as its payload. But, he also said the initial "tankers" are just "other Starships", meaning cargo or crew designs. They would be flown at zero payload, so that the unused propellant can be transferred, excepting only what is needed to land empty.
Bear in mind that these evaluations are of the POTENTIAL performance in the system design. That design is still very immature, and it has some very serious unresolved problems, not the least of which is a totally inadequate landing leg design for rough field operations with a tall, narrow vehicle, onto low-strength soils. Others we have recently seen are propellant slosh/ullage troubles, and propellant leaks leading to engine bay fires.
There will be more problems, we just haven't seen them yet. Trust me, I know. I used to do this kind of work for a living.
Spacex certainly has their work cut out for them, resolving these (and many other) problems. But the potential I have identified does seem to make the resolution efforts worthwhile.
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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Great work, GW!
I think that SpaceX hasn't come to grips entirely with the rough landing area problem. This is going to be worse on the Moon than on Mars, in my estimation. They seem to be in the stage of not knowing or realizing what they don't know, but aren't letting that slow them down to a snail's pace. They not only need the landing leg design to be realistic, but also have sufficient ground clearance for the Raptors bell nozzles to have clearance.
SpaceX has adequately addressed the effects of rocket exhaust on lunar regolith by using the "puller versus pusher" concept, similar to the early Hermann Oberth and von Braun "Repulsor" designs, as well as early Goddard rockets. Blasting small rocks into weird orbits is NOT a good idea!
Any way, we live in an exciting time for space nerds.
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I think there is a consensus here at least that of the three - Earth, Moon and Mars - a rough landing on the Moon would be the most demanding. I only mention Earth as I presume Space X would be carrying out test rough landings on Earth.
Great work, GW!
I think that SpaceX hasn't come to grips entirely with the rough landing area problem. This is going to be worse on the Moon than on Mars, in my estimation. They seem to be in the stage of not knowing or realizing what they don't know, but aren't letting that slow them down to a snail's pace. They not only need the landing leg design to be realistic, but also have sufficient ground clearance for the Raptors bell nozzles to have clearance.
SpaceX has adequately addressed the effects of rocket exhaust on lunar regolith by using the "puller versus pusher" concept, similar to the early Hermann Oberth and von Braun "Repulsor" designs, as well as early Goddard rockets. Blasting small rocks into weird orbits is NOT a good idea!
Any way, we live in an exciting time for space nerds.
Let's Go to Mars...Google on: Fast Track to Mars blogspot.com
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Latest video from Felix at WAI incorporating some nice aerial shots of the developing Boca Chica site.
https://www.youtube.com/watch?v=urDwwEZufiY
Last edited by louis (2021-03-16 15:44:22)
Let's Go to Mars...Google on: Fast Track to Mars blogspot.com
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Felix produces high quality videos on Starship development. His latest is no exception.
I didn't realise though that Space X have a target date for orbital flight of 1 July. Anyone else know about that?
Let's Go to Mars...Google on: Fast Track to Mars blogspot.com
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https://www.nasaspaceflight.com/2021/03 … ht-summer/
Note of it for a change in design to happen for it.
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Following upon what I said in post 985 above, I have been working on the Spacex tanker issue. Specifically, the departure orbit requirements and approaches to refilling, for lunar landing missions. Those are considerably more demanding than one-way Mars missions, because there is no propellant manufacture on the moon.
I could not make lunar nearside landings feasible unless I used elongated elliptical departure orbits, with apogee altitudes well into the Van Allen radiation belts. I sort-of bounded the problem with 75 tons to the moon and 0 tons returned, from a 300 x 7000 km altitude departure ellipse, and 59 tons to the moon with 32 tons returned, from a 300 x 10,000 km altitude departure ellipse. The base of the Van Allen belts outside the South Atlantic Anomaly is considered to be about 1400 km altitude. Radiation exposure before lunar departure is a very serious issue for these kinds of missions.
I looked at dedicated tanker designs, and at using ordinary Starships as tankers by flying at zero payload to have excess propellant left over. They're not that different in capability, as it turns out. The ordinary Starships can deliver about 192 metric tons of usable propellant to low circular orbit, and the dedicated tanker with extra tankage volume about 230 tons. All that assumes these designs can be made to work right at only 120 tons inert vehicle mass.
Tanker capacity to the higher-energy elliptic departure orbits is around 5-10 times less. For refueling a max-payload Starship in low circular orbit, I found 6 dedicated tankers and 7 ordinary tankers, because the Starship arrives there dry of all propellant except an emergency landing reserve of 21 tons, out of a capacity of 1200 tons. The number of tankers needed to fly directly to the elliptic departure orbit and refuel the Starship there fell between 15 and 21 flights, depending upon which orbit, and which tanker version.
The next idea I pursued was to fly at only the lunar payload to low circular, and fully refill there. Then move to the departure orbit, and top-up from tankers sent straight there. Again, tanker capacity to the elliptical orbits was low, leading to around a dozen refueling flights. Or more.
The next idea was to refill both the mission Starship and one tanker in low circular, where tanker capacities are high. Then move the refilled Starship and that one refilled tanker to the elliptic departure orbit. There, the tanker tops-up the Starship, holding back only its landing reserve. I was afraid it might take two, but preliminary results seem to indicate I can get away with just one extra tanker that way. Which looks sort of like 7 dedicated tankers or 8 ordinary tankers to run one of these lunar missions.
There is very, very definitely a distinct advantage to maximizing the number of operations done in low orbit. The "tyranny of the rocket equation" has definitely struck again.
GW
Last edited by GW Johnson (2021-03-20 12:11:13)
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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Interesting stuff!
How difficult would it be to simply sacrifice a Starship on the lunar surface and use something similar to the Apollo lunar ascent vehicle to
rendezvous with a lunar orbit Starship? We know the LAM was used very successfully on - what? - 7 occasions. The designs are out there. Shouldn't be that difficult to put together.
Wasn't Musk saying though that the Starship could get to the Moon and back on one tank from launch?
Following upon what I said in post 985 above, I have been working on the Spacex tanker issue. Specifically, the departure orbit requirements and approaches to refilling, for lunar landing missions. Those are considerably more demanding than one-way Mars missions, because there is no propellant manufacture on the moon.
I could not make lunar nearside landings feasible unless I used elongated elliptical departure orbits, with apogee altitudes well into the Van Allen radiation belts. I sort-of bounded the problem with 75 tons to the moon and 0 tons returned, from a 300 x 7000 km altitude departure ellipse, and 59 tons to the moon with 32 tons returned, from a 300 x 10,000 km altitude departure ellipse. The base of the Van Allen belts outside the South Atlantic Anomaly is considered to be about 1400 km altitude. Radiation exposure before lunar departure is a very serious issue for these kinds of missions.
I looked at dedicated tanker designs, and at using ordinary Starships as tankers by flying at zero payload to have excess propellant left over. They're not that different in capability, as it turns out. The ordinary Starships can deliver about 192 metric tons of usable propellant to low circular orbit, and the dedicated tanker with extra tankage volume about 230 tons. All that assumes these designs can be made to work right at only 120 tons inert vehicle mass.
Tanker capacity to the higher-energy elliptic departure orbits is around 5-10 times less. For refueling a max-payload Starship in low circular orbit, I found 6 dedicated tankers and 7 ordinary tankers, because the Starship arrives there dry of all propellant except an emergency landing reserve of 21 tons, out of a capacity of 1200 tons. The number of tankers needed to fly directly to the elliptic departure orbit and refuel the Starship there fell between 15 and 21 flights, depending upon which orbit, and which tanker version.
The next idea I pursued was to fly at only the lunar payload to low circular, and fully refill there. Then move to the departure orbit, and top-up from tankers sent straight there. Again, tanker capacity to the elliptical orbits was low, leading to around a dozen refueling flights. Or more.
The next idea was to refill both the mission Starship and one tanker in low circular, where tanker capacities are high. Then move the refilled Starship and that one refilled tanker to the elliptic departure orbit. There, the tanker tops-up the Starship, holding back only its landing reserve. I was afraid it might take two, but preliminary results seem to indicate I can get away with just one extra tanker that way. Which looks sort of like 7 dedicated tankers or 8 ordinary tankers to run one of these lunar missions.
There is very, very definitely a distinct advantage to maximizing the number of operations done in low orbit. The "tyranny of the rocket equation" has definitely struck again.
GW
Let's Go to Mars...Google on: Fast Track to Mars blogspot.com
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"Wasn't Musk saying though that the Starship could get to the Moon and back on one tank from launch?"
No, he never said that.
Orbital mechanics says that an unrefueled Starship lunar landing mission has a total delta-vee in the 8-9 km/s range from Earth departure orbit, while the one-way voyage to Mars has a total delta-vee nearer 5-6 km/s from Earth departure orbit.
The rocket equation says the mass ratio for ~6 km/s is one whale of a lot easier to achieve than one for 8+ km/s. It ain't linear, it's exponential. That would be at engine Isp's in the vacuum Raptor range.
Musk said a Starship could get to the surface of the moon and back, unrefueled, from an Earth departure orbit (and he said it was elliptical, not circular). He didn't say exactly how, or with what payload. I found ways to make that Musk statement true, using the best numbers we have for the expected production designs.
I cannot be that far off from what Spacex themselves are calculating. Remember, I used to do this same sort of thing or a living, about a quarter century ago. I do really know how to figure this stuff.
Lunar payloads are smaller than Mars payloads, and the number of tankers is really high, unless you pay very careful attention to exactly how you go about executing the mission.
The only ways I could find to do this lunar landing mission at all, will penetrate quite deeply into the Van Allen radiation belts, before departure from an elongated elliptical departure orbit. Cargo MUST therefore be radiation-hard, and any crew or passengers WILL require a substantial and effective radiation shelter! There is NO way around that requirement!
But it can be done, and I found a way to do it with only one (!!!!) more tanker flight than is required to fully refill the mission Starship in low circular Earth orbit, not double or triple. I did that without any computer software or orbit or trajectory programs on any computers. I thought that accomplishment in itself was quite the remarkable finding!
BTW, that says NOTHING about tall skinny landing stability, or the bearing pressure on soft soils. Those, and many other fatal problems we have already begun to see in prototype test flights, remain to be solved, before any such missions will ever fly!
GW
Last edited by GW Johnson (2021-03-20 18:16:26)
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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Hi GW -
Wasn't querying your calculations - just puzzled as I'd read the Musk (mis)quote. Thanks for the clarification.
I think they should sacrifice 2 or 3 Starships on the surface. to establish a base, and develop a small lander-ascent vehicle to create the link between the surface and lunar orbit, which I presume will cut down dramatically on the need for tanker loading.
I see the lunar base as mostly an opportunity for tourism with some associated science. It's not going to be an independent civilisation as the Mars community will ultimately become.
Ultimately, Space X might be better advised to create hydrogen engines for their lunar rockets. We know there is plenty of water on the moon but processing carbon could be a real problem from what I've read.
"Wasn't Musk saying though that the Starship could get to the Moon and back on one tank from launch?"
No, he never said that.
Orbital mechanics says that an unrefueled Starship lunar landing mission has a total delta-vee in the 8-9 km/s range from Earth departure orbit, while the one-way voyage to Mars has a total delta-vee nearer 5-6 km/s from Earth departure orbit.
The rocket equation says the mass ratio for ~6 km/s is one whale of a lot easier to achieve than one for 8+ km/s. It ain't linear, it's exponential. That would be at engine Isp's in the vacuum Raptor range.
Musk said a Starship could get to the surface of the moon and back, unrefueled, from an Earth departure orbit (and he said it was elliptical, not circular). He didn't say exactly how, or with what payload. I found ways to make that Musk statement true, using the best numbers we have for the expected production designs.
I cannot be that far off from what Spacex themselves are calculating. Remember, I used to do this same sort of thing or a living, about a quarter century ago. I do really know how to figure this stuff.
Lunar payloads are smaller than Mars payloads, and the number of tankers is really high, unless you pay very careful attention to exactly how you go about executing the mission.
The only ways I could find to do this lunar landing mission at all, will penetrate quite deeply into the Van Allen radiation belts, before departure from an elongated elliptical departure orbit. Cargo MUST therefore be radiation-hard, and any crew or passengers WILL require a substantial and effective radiation shelter! There is NO way around that requirement!
But it can be done, and I found a way to do it with only one (!!!!) more tanker flight than is required to fully refill the mission Starship in low circular Earth orbit, not double or triple. I did that without any computer software or orbit or trajectory programs on any computers. I thought that accomplishment in itself was quite the remarkable finding!
BTW, that says NOTHING about tall skinny landing stability, or the bearing pressure on soft soils. Those, and many other fatal problems we have already begun to see in prototype test flights, remain to be solved, before any such missions will ever fly!
GW
Let's Go to Mars...Google on: Fast Track to Mars blogspot.com
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Reading that a need of 8 tankers to send one moon mission at just under 1 million for fuel plus 2 million per launch operations cost with no booster or starship losses as its going to cost them 200 million plus for either makes for a big cost and lots of timing to have errors in or with for it to work.
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Hi Spacenut:
Hope the weather is thawing where you are. It's only a month past the deep freeze here in Texas, and spring has definitely sprung.
I'm still getting my tanker studies together and published on "exrocketman". I've pretty much done 3. Two are up on the site, with one just today. I'm still polishing the third. These support the lunar mission study, where I identified some "sort-of" bounds on the payload sent, payload returned, and the necessary orbits for those bounds. It turns out elliptic orbit apogee and payload are correlated.
The first tanker study looked at sending the lunar mission Starship and all its tankers straight to the elongated elliptic departure orbit. I found tanker deliverable-propellant capacity drastically reduced by that. Something like 20 tankers are required to refuel the mission starship. The exact number depends on the lunar payload and which orbit you are using.
The second tanker study looked at refueling the mission Starship in low circular orbit, then moving it to the elliptic departure orbit, and topping-off its tanks a second time with more tankers sent to that orbit. That substantially reduced the number of tankers down to nearer a dozen, depending upon payload and the exact orbit. The trouble was again the low tanker capacity to the higher-energy orbit. This was a bigger effect than the reduced elliptic top-off requirement after refilling in low circular.
The third study I have not published yet; still polishing the write-up. What I did there was send the mission Starship and all the tankers to low circular, and fully refill the mission vehicle there. I also partly-refill one tanker in low circular, where all the tanker capacities are high. These two vehicles then move to the elliptic departure orbit, with a modest top-off requirement for the mission Starship. There is just enough propellant aboard the tanker to top-off the mission starship without compromising the landing reserve aboard the tanker.
That is how I pulled off enabling the lunar mission from elliptic orbit with only one more tanker than would be required to fully refill a Starship in low circular orbit. Key was sending all the tankers to low circular and doing the bulk of the transfer operations there, where tanker capacities were nearer 200 tons than 40-ish. That's driven by the "tyranny of the rocket equation", which is a way of saying that mass ratios depend exponentially upon velocity ratios, so it bites you really quickly.
Let me get that third study polished-up, so I can post it on "exrocketman". I'm pretty sure the engineers at Spacex reached pretty much the same conclusions that I did. The numbers just don't lie, only people can lie to themselves.
But, as I told Louis, there are a whole host of fatal problems to solve with the Starship/Superheavy design, before any of these missions can ever be flown. Some nobody knows yet, many are now evident. What I identified was the potential of the system, and how sensitive it is to exactly how you go about using it. That potential is attractive enough to warrant solving all those otherwise-fatal problems. All new space system designs have these fatal problems. The trick is figuring out whether it is worthwhile to solve them, or not.
I have no idea what the real price per launch will be for Starship/Superheavy. Spacex knows better than me, but it is too early for them to be sure, either. The cost of solving all those fatal problems will have to be amortized into that launch price, one way or another. I'd be surprised and pleased if it worked out to something on the order of $10M per launch. At 8 flights per moon mission, that'd be something like $80M to send 60-70 tons to the moon. That's something like $12-14M/ton. Not bad at all, really. Not compared to SLS, that's for sure.
GW
Last edited by GW Johnson (2021-03-21 09:57:58)
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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This is a textbook example of how mission planning should be done! The Moon is a great white Goddess that has allure to many space nerds. Dr. Zubrin warns of this potential distraction in his books; we DON'T need to "return to the Moon first."
Onward to Mars!
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The trouble is the expendable to reuse comparison costs which only a counts for the later if you have any kaboom
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https://www.youtube.com/watch?v=9kYMSpzCIu4
Latest video from SpaceXcentric.
Let's Go to Mars...Google on: Fast Track to Mars blogspot.com
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Update on the SN 11: successful static fire today, Monday 22 March 2021.
Flight is imminent soon.
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