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Starship test
Looks as if it went very well apart from the final landing, where it
came down too fast. Here's hoping SN9 will nail it. Sylvia. |
#2
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Starship test
JF Mezei wrote:
Elon said engines performed great. I watched it and when one engine went out, there was a big fire in the area inside the skirt and lots of smoke out the rocket, but it was short lived. Then another engine shutdown with less impact but lots of smoke. I would have expected engine shutdown to be "clean". If you look at the engine bells you can see that just before the first one shuts down the other bells move to prepare for the shutdown... Likewise just before the second shuts down the one that will keep on running is clearly prepared for the other Raptor's upcoming shutdown. All evidence suggests that they used three Raptors for initial take-off and then did controlled shutdowns to keep speed and max altitude under control. And presumably to collect data on shutdown and restart during flight, that was part of the goal of SN8, collect as much data as possible. Also note that the first engine to shut down (IE least running time) is the first to re-lit after the bellyflop, followed by the second one to shut down. This proves the Raptors didn't have problems earlier, they clearly balanced the amount of run-time on them! Apparently, the landing engine lacked fuel pressure in header tank I would assume this is simple to fix. When the second engine relight during descent we see brief flashes of green in the exhaust from the first. A little while later the second Raptor one shuts down again, considering the weight vs thrust this was likely also as planned. Then near the end the exhaust from the remaining Raptor's exhaust goes solid green. Last time we saw that green exhaust it was from "engine rich combusion", this can easily result from insufficient fuel and/or oxidiser (either hot O2 rich exhaust or insufficient cooling), people were already speculating about fuel/oxidiser issues before Elon confirmed it. It may well not be "lack of fuel pressure", it could just as easily be an effect of the bellyflop, perhaps the fuel sloshed and thus starved it temporarily... Or something else, SpaceX probably has lots of data on it now. It may well be something that only happened because they weren't flying higher, though 12.5 vs 15km is unlikely to have mattered. If this was in fact the direct cause I suspect 30+km flight profile would give the fuel WAY more time to settle down before it's needed for the landing burn! And the normal profil is obviously far higher than that. And while it was clearly moving to fast when it came down it wasn't moving THAT much too fast, suggesting the plan was likely to land on one engine. Despote the fireworks to celebrate landing, the concept of free fall on your belly and then rotate at end and land vertical was proven, and I think this is HUGE. Of course, remains to be seen how the vehicle performs with tiles. Based on the NASAspaceflight guys, the 3 engines aren't enough to launch a fully fueled Starship. This was likely the reason they had to scale back from 15 to 12.5km because it would have required too much fuel at liftoff and not enough power. All evidence says SpaceX had to quickly shut down one and then later a second Raptors to avoid vastly overshooting the height target and/or the speed targets they set themselves, this also suggests (but doesn't prove) that they probably also ran the Raptors at or near the lowest possible thrust too. I consider it far more likely that they decided that 12.5km gave them more than sufficient safety margin for the bellyflop and there was no reason to spend more on fuel, especially since there was a decent chance it would RUD long before it reached that altitude anyway! Assuming this is true, does this mean that going forward, they will need to use not only the 3 sea level engines but also additional engines (either sea level or vacuum). I think it's VERY unlikely that SN8 had anywhere near the amount of fuel you seem to think. Eventually, starship will need to launch from sea level and get to orbital speed so it can test heat shield for re-entry. Extra weight, and different aerodynamics. WOuld it be fair to state that for any launch of Starship from sea level, we'll never see an outfitted nosecone cargo or pax) because such will only happen with super heavy because Starship doesn't have the "oumph" t lift off meaningfully with cargo? (thsi question ovviously depends on wheynert vaccum engines could be used to help at sea level). It's been explained to you multiple times before here. Yes, Starship probably CAN do SSTO from Earth without payload which is really convenient for their future testing regime but outside testing they won't be doing that because it makes no sense (on Earth). A lot depends on the final dry weight, if they get that down it might even be able to loft small payloads. However for not that much more (SuperHeavy is much simpler in many ways) they can greatly increase the payload (1-2 orders of magnitude) which is why everyone expects the SH+SS combination going to be used for all non-test orbital Earth launches. On Mars OTOH Starship is a decent SSTO, much lower energy requirement and much less dense atmosphere helps. It's almost like someone has thought this through during planning... Would be inconvenient if they had to bring SuperHeavies to Mars. And yes, Elon has stated that the Raptor Vacuum engines CAN run at ground level, the expansion ratio is higher than the ground engines but not high enough to cause dangerous instabilities - mostly because there wouldn't be space for the required engine bells then. |
#3
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Starship test
On 2020-12-10 2:18, JF Mezei wrote:
Based on the NASAspaceflight guys, the 3 engines aren't enough to launch a fully fueled Starship. This was likely the reason they had to scale back from 15 to 12.5km because it would have required too much fuel at liftoff and not enough power. Another likely explanation I've seen is that they wanted to avoid high-altitude winds at the transition from nose-up to belly-flop. I don't think they would need to fully fuel the Starship just to reach 15 km altitude at low speed. |
#4
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Starship test
Torbjorn Lindgren wrote:
JF Mezei wrote: Apparently, the landing engine lacked fuel pressure in header tank I would assume this is simple to fix. When the second engine relight during descent we see brief flashes of green in the exhaust from the first. A little while later the second Raptor one shuts down again, considering the weight vs thrust this was likely also as planned. Then near the end the exhaust from the remaining Raptor's exhaust goes solid green. Looking at Scott Manleys video with just the relight and shutdown, I now think the plan was indeed not to relight the third engine (Raptor SN42!), it's carefully moved out to give the other Raptors more gimbal range, just like on the shutdowns earlier. However, it doesn't look like this is done for the shutdown of the second Raptor and the exhaust of the first Raptor goes green very shortly afterwards (it was just flashes before). Combined this suggests the second raptor likely shut down due to fuel starvation which also affected the remaining engine. If it had been an engine failure it might have tried relit SN42 if it had time but not much it can due when fuel starved. Other sources think they saw a fuel leak, IIRC Starship use autogenous pressurisation so this could for example be the pipes for this up to the header tank, I would expect this to result in the low fuel pressure Elon mentioned though sloshing is still a possible candidate, people aren't always right about what they think they see. It may well be something that only happened because they weren't flying higher, though 12.5 vs 15km is unlikely to have mattered. If this was in fact the direct cause I suspect 30+km flight profile would give the fuel WAY more time to settle down before it's needed for the landing burn! And the normal profil is obviously far higher than that. Based on the rim lines it did have quite a bit of fuel onboard, I suspect the flight profile "only" hit 12.5km due to massive gravity losses caused by the low speeds - which they caused by throttling back and shutting down engines. Did it go supersonic, there was no telemetry so it's hard to tell but I know there was some speculations on livestreams that they might have done it this way to avoid that for this test. I expect we'll see more detailed third-party analysis later today including estimates on height and speed, this may confirm if that was what they did. And while it was clearly moving to fast when it came down it wasn't moving THAT much too fast, suggesting the plan was likely to land on one engine. Based on the information above this may still be the case, Falcon 9 has some profiles where it uses 3 engines and then 1 for the final touchdown. OTOH, if you can land on multiple engines it's more efficient. Assuming this is true, does this mean that going forward, they will need to use not only the 3 sea level engines but also additional engines (either sea level or vacuum). I think it's unlikely that SN8 had anywhere near the amount of fuel you seem to think. As mentioned above it likely had much more fuel than I originally though, I considered the height/speed but forgot about the runtime and how gravity losses can kick in. I thought I should acknowledge this mistake. |
#5
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Starship test
Torbjorn Lindgren explained :
Torbjorn Lindgren wrote: JF Mezei wrote: Apparently, the landing engine lacked fuel pressure in header tank I would assume this is simple to fix. When the second engine relight during descent we see brief flashes of green in the exhaust from the first. A little while later the second Raptor one shuts down again, considering the weight vs thrust this was likely also as planned. Then near the end the exhaust from the remaining Raptor's exhaust goes solid green. Looking at Scott Manleys video with just the relight and shutdown, I now think the plan was indeed not to relight the third engine (Raptor SN42!), it's carefully moved out to give the other Raptors more gimbal range, just like on the shutdowns earlier. However, it doesn't look like this is done for the shutdown of the second Raptor and the exhaust of the first Raptor goes green very shortly afterwards (it was just flashes before). Combined this suggests the second raptor likely shut down due to fuel starvation which also affected the remaining engine. If it had been an engine failure it might have tried relit SN42 if it had time but not much it can due when fuel starved. Other sources think they saw a fuel leak, IIRC Starship use autogenous pressurisation so this could for example be the pipes for this up to the header tank, I would expect this to result in the low fuel pressure Elon mentioned though sloshing is still a possible candidate, people aren't always right about what they think they see. It may well be something that only happened because they weren't flying higher, though 12.5 vs 15km is unlikely to have mattered. If this was in fact the direct cause I suspect 30+km flight profile would give the fuel WAY more time to settle down before it's needed for the landing burn! And the normal profil is obviously far higher than that. Based on the rim rime? lines it did have quite a bit of fuel onboard, I suspect the flight profile "only" hit 12.5km due to massive gravity losses caused by the low speeds - which they caused by throttling back and shutting down engines. Did it go supersonic, there was no telemetry so it's hard to tell but I know there was some speculations on livestreams that they might have done it this way to avoid that for this test. The NSF crew wasn't expecting supersonic even before flight, and speculation is the apogee target was lowered to avoid issues with winds aloft. I expect we'll see more detailed third-party analysis later today including estimates on height and speed, this may confirm if that was what they did. The NSF crew caught the WB-57 in the area on Tuesday, but sadly the plane had issues the next morning and didn't fly on Wednesday. And while it was clearly moving to fast when it came down it wasn't moving THAT much too fast, suggesting the plan was likely to land on one engine. Based on the information above this may still be the case, Falcon 9 has some profiles where it uses 3 engines and then 1 for the final touchdown. OTOH, if you can land on multiple engines it's more efficient. Assuming this is true, does this mean that going forward, they will need to use not only the 3 sea level engines but also additional engines (either sea level or vacuum). I think it's unlikely that SN8 had anywhere near the amount of fuel you seem to think. As mentioned above it likely had much more fuel than I originally though, I considered the height/speed but forgot about the runtime and how gravity losses can kick in. I thought I should acknowledge this mistake. In the NSF streams, it sure looked like the craft was crabbing out to sea before the third engine shutdown for apogee. That's got to take a lot of fuel. /dps -- The presence of this syntax results from the fact that SQLite is really a Tcl extension that has escaped into the wild. http://www.sqlite.org/lang_expr.html |
#6
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Starship test
On Dec/9/2020 at 19:18, JF Mezei wrote :
Despote the fireworks to celebrate landing, I like your phrase. Yes the flight was worthy of celebrations. Alain Fournier |
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