So the CRS mission today they flew a short MVac and for the second time the announcer didnt bother to explain Why its shorter. following the Standard script seems to be a priority.
so Why is the MVac Shorter?
Posted Jun 5, 2023, 9:08 PM UTC
The engine contains some expensive alloys, so the shorter version is cheaper. Yes, the performance is worse, but that's an acceptable tradeoff for some missions.
To be specific, the nozzle is made out of niobium which is relatively expensive stuff. It's used because that part of the engine is radiatively cooled. Niobium is excellent at radiating heat which is why it glows so brightly after ignition.
Shortening the nozzle reduces the efficiency (specific impulse) of the engine through cosine losses, but if not all of the performance is needed, it's one way to save money on a part that can't be reused. It also reduces the mass of the second stage which mitigates the loss of efficiency to some degree. I would guess that the shorter nozzle can be produced faster as well which is good if you want to launch 100 times in a year.
Shortening nozzle reduces ISP, and most of that is not a cosine loss, it simply that you're using less of the pressure produced by the exhaust.
AFAIR proper de Laval nozzle has no cosine losses regardless if it's long or short. But the simple fact is that the force pushing the rocket forward is produced by the pressure of the exhaust pushing against engine and nozzle walls. The closer to the nozzle end, the lower the pressure, but I'm vacuum of space the effect is always positive[*]. So if you cut the nozzle shorter, the force coming from the now mission part is missing, too.
*] - down in the atmosphere, especially at sea level the last part of too long of a nozzle produces negative (sucking) net force, i.e. the ambient atmosphere acting on the outside of the nozzle wall pushes it inwards and backwards stronger than the exhaust does in the opposite direction. This whole situation is called an overexpanded engine or an overexpanded nozzle.
Yep, that does make sense now that I think about it.
Don’t think of the shorter nozzles as a separate faster production line, its merely using nozzles with cracks in the end that have been cut back to good material. Rather than get 5 good 5 scrap you get 5 good 3 short (I expect shortening only as much as needed to get to good material so each short nozzle is a different length so they can match them to various payloads) and 2 scrapped.
Reason I suspect this is the manually shorter F9 nozzle extension from the early F9 flights where the nozzle extension got trimmed by a guy with a pair of tin snips.
I really don't expect cracks appear that frequently.
Yes, one engine was cut as you described. They had worked out that they would still have enough performance.
But the chances of that happen often are very slim. If it happened often they would have found and fixed the problem long before now.
At least in the past, nozzle extension rejects seem to have been very frequent.
https://www.reddit.com/r/SpaceXLounge/comments/12mtk6d/comment/jgcl6lx/
When visiting the factory in 2016, I saw a large (double-digit) # of mVac nozzles on the side of the building that were being discarded.
(In 2016, there wasn't even a double digit number of Mvacs used: 8 launches and AMOS-6.)
Do you have any sources for this?
I'm not sure I buy this, as I don't think they would trim it back that much if so. When they used tin snips originally, they took a few inches off. They've taken off a majority of this one.
Now, there might be a secondary process in which they can trim some scrap, but I highly suspect these are made to this length.
That being said, we should both be more clear that we are just speculating.
The cost of raw materials is very very rarely a dominant one in aerospace manufacturing.
In this case workmanship (workers time spent), occupying time on a stand (machine and facilities time spent) and then whole inspection process (worker and machine time spent) likely dominate.
Moreover it's opportunity cost. If nozzle is a pacing part of upper stage manufacturing, the cost is all the other stuff not done. And you could only manufacture so many of large nozzles without hiring and training more people and acquiring more expensive equipment. And to make matters worse at relatively low volume[*] of ~90 per year this would be a quantum leap, because you can't hire half a worker or procure half a manufacturing stand. You either have one stand or two. You can man the other stand just for one shift instead of two, but that's mostly it.
*] - 90 is very high volume as space business go, but comparing with stuff like cars (which as Elon noted are not necessarily less complex than rocket engines) which go like 5000 to 10000 a week, it's a low volume.
It's quite amazing that the F9 is so overkill for a cargo run to the station that they can literally cut corners on upper stage propulsion.
You'd have thought rockets are all about squeezing out every last bit of performance.
I guess when you reuse them the cost comes down so much that efficiency doesn't become a big concern.
See starship- fully reusable, structurally strong AF stainless steel construction with fixed grid fins, and even then it has a capacity of 150T to orbit with a habitable volume equal to the ISS.
obviously Dragon Cargo needs a stretch.
Fyi, I highly recommend watching NSF (NASA Space Flight) channel on YT for launches, as they are MUCH more informative, usually better and more camera angles, with tons of cool stats to learn. They do LIVE coverage of all the launches. (they explained the stubby nozzle in yesterday's launch)
i do fairly often, but mostly for starship.
generic F9 launches just pop up as a link in space launch now app and i click through to SpaceX coverage and its pretty rare I'd have a question about those.
Acronyms, initialisms, abbreviations, contractions, and other phrases which expand to something larger, that I've seen in this thread:
|Fewer Letters|More Letters| |-------|---------|---| |ASDS|Autonomous Spaceport Drone Ship (landing platform)| |CRS|Commercial Resupply Services contract with NASA| |Isp|Specific impulse (as explained by Scott Manley on YouTube)| | |Internet Service Provider| |M1dVac|Merlin 1 kerolox rocket engine, revision D (2013), vacuum optimized, 934kN| |NSF|NasaSpaceFlight forum| | |National Science Foundation| |QA|Quality Assurance/Assessment| |RTLS|Return to Launch Site|
|Jargon|Definition| |-------|---------|---| |kerolox|Portmanteau: kerosene fuel, liquid oxygen oxidizer|
NOTE: Decronym's continued operation may be affected by API pricing changes coming to Reddit in July 2023; comments will be blank June 12th-14th, in solidarity with the r/Save3rdPartyApps protest campaign.
^(Decronym is a community product of r/SpaceX, implemented )^by ^request
^(6 acronyms in this thread; )^(the most compressed thread commented on today)^( has 26 acronyms.)
^([Thread #11541 for this sub, first seen 6th Jun 2023, 15:38])
^[FAQ] ^([Full list]) ^[Contact] ^([Source code])
The basic answer is, it's cheaper to produce. It does impact performance but as nothing from 2nd stage is recovered, making it as cheap as possible but still able to complete the mission is a good thing.
We have seen SpaceX optimize launch profiles, Merlin & landings over the years, all this gives some headroom and seems to have made it possible to simply not need all that performance on every mission today, cutting cost.
I would imagine this extra headroom is the reason they also going for RTLS landings for Crew Dragon missions now.
Actually the correct answer is they scrap less, its not that its that much cheaper to produce and material wise its not that expensive ~50USD a KG considering the nozzle is basically sheet metal hot worked on a mandrel, not really any different from a bowl its expensive sure, but not that expensive either. If I had to guess the material cost of a nozzle extension is only a $5-10 thousand USD.
What eats you alive is that if the nozzle has cracks at the exit, is that it would peel open like a banana. So historically all the full length nozzles with cracks near the exit were scrapped. But as spaceX is showing and actually shower during one of the earliest F9 flights a shorter nozzle works fine for flights not at F9 performance limit.
Shit the first time a shortened nozzle extension flew on an F9 it was cut off by hand with a pair or manual tin snips after the rocket was stacked! I’m actually surprised it took them this long to fly shorter nozzles.
I think this is the same as the 'binning' technique they use in semiconductors.
Making semiconductors is very tricky work and not every chip you make will come out of the factory fully functional. So Intel will make an 8-core chip then test which cores work, if all 8 work they'll brand it as an 8-core chip but if one or two are broken they'll disable them and brand it as a 6-core chip for a lower cost.
This lets them make an 8-core chip and a 6-core chip from a single manufacturing process and the 'rejects' are still viable products. So SpaceX aims to make a full size nozzle but if they find a crack near the fragile rim they trim it shorter and brand it as a short-nozzle. They've turned the rejects into a new class of product.
As with chips there's probably another class of reject with really big cracks that cannot be salvaged, just like a chip where 7 cores are broken and it's not worth packaging a single-core variant of their cutting-edge chips. Except when SpaceX have a Liberty Bell-sized crack they probably melt it down and reuse the metal in a new attempt, I don't think you can melt down a cpu to reuse the material.
Why is this downvoted? This is likely as much a reason as, if not more important than, raw materials costs alone. Unless you count fuel, raw materials are seldom a major expense compared to (the time and labor of) manufacturing. Undoubtedly they save money by using less, but it's probably not significant enough by itself to warrant the production, certification, and use of the shorter nozzle.
I will add that the actual price of the C-103 niobium-hafnium-titanium alloy used on the nozzle extensions will be significantly more than the ~$45-50/kg for ferroniobium or Nb2O5. One could order that on Alibaba ^(yes, I know...) for no more than a few hundred dollars per kg--supposedly as little as $98/kg. A higher purity, presuming they need that, and good QA from a reputable supplier should be more expensive. On the other hand, they should get a better deal buying in bulk.
In any case though, it's nothing like gold or platinum. Even if the alloy costs them nearly $1k/kg, SpaceX isn't saving that much just by using maybe ~50-100 kg, if that, less per ~$10 million second stage. (This is nothing lile the $6 milliom fairings.) The extensions are very thin and the short nozzles just use less niobium alloy, not none. Also, this might have been the difference between ASDS and RTLS for the CRS mission, now that they have demonstrated RTLS with crew. Operating ships is expensive, too. That would have eaten into the net savings from the shorter nozzle.
It's presented as a fact, but it's unlikely to be based on internal knowledge.
Raw materials cost is rarely dominant in aerospace manufacturing. It's almost invariably labor and facilities costs which dominate.
And of course if the nozzle were cheap and even 50% were scrapped it would still be cheap. OTOH if work time spent plus the time mandrel is occupied and can't be used for another work, then the whole inspection flow causes it to be expensive then it's already expensive.
Yep. Elon has gone on record before saying that the raw materials account for less than 1% of the vehicles build cost.
I don't follow what you are trying to say.
So you think the raw materials costs are likely more important here?
Yet you also agree with us they are not?
If the nozzle is scrapped, the metal can be recycled or sold as relatively valuable scrap. The modest materials cost is not even a total loss. The resources put into manufacturing the bad nozzle are a total loss--that is unless they can find a way to still use them.
This is an interesting theory, but it could be just that it takes a long time to produce a nozzle, and most of the cost is not the base material, but time the manufacturing occupies the mandrel and workers working it. And then the whole inspection (whole nozzle must be checked for flaws which means imaging it by x-ray or other non destructive method and then processing the images of the whole ~20m² of the thing, which likely includes manual review of anything highlighted by automation).
In short a smaller nozzle = less expensive work time spent on it.
That is the price of the ore. Refined high purity metal is about $400/kg
It's the same thought process behind chipmakers disabling cores. They have a maybe 8 core processor design, but there would be errors on the wafer over some cores. Why trash the whole chip when they can disable maybe two cores and sell those as 6-core? Why send the engine back when you can snip off the end and fly it as a lower-performance version?
Thanks, less scrapped items make sense vs less material, and of course less nozzle, less time. So maybe a short nets out to a 1/4 price nozzle.
So given this, what is the savings for the second stage? It might be mostly labor cost.
that makes sense.