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Starship is meant to imitate (to a degree) the affordability of passenger jets. This applies to Earth-to-Earth travel as well as space travel.

Jetliners started with 4 engines, but have since migrated to using 2, regardless of size. Where the number of engines might have been increased for an earlier generation of wide-body aircraft, now, as with the 777X, companies simply make larger engines for every new design. This twinjet configuration is chosen for saving costs, as each engine requires separate service, paperwork, and certificates.

Why does the same not apply to Starship?

Mehdi Abbassi
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Abdullah is not an Amalekite
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    I suspect you have answered your own question here - available tech forces us closer to the 'six turning four burning' of the B-36 https://en.wikipedia.org/wiki/Convair_B-36_Peacemaker than 777X – GremlinWranger Aug 20 '21 at 13:41
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    The simple answer is that rocket flight has extremely little in common with airflight. For one, the amortized cost of rocket per flight vs fuel costs are not at all comparable to the amortized cost of a 777 vs fuel (and it's much less analogous when looking at F9). But also the flight profiles and stresses introduced are entirely different as well. – eps Aug 20 '21 at 22:59
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    Simiarly, a F1 racecar has a much different engine and shape and has little in common to a highly efficient 4 cylinder honda. – eps Aug 20 '21 at 23:11
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    Aircraft companies don't really make significantly larger engines for each new generation, they generally build smaller airplanes. For instance, the 777X you reference is considerably smaller than the 4-engined 747, while the larger than 747 Airbus A380 uses 4 engines. – jamesqf Aug 21 '21 at 00:12
  • Would it even be feasible from a technology standpoint to simply scale up the Raptor engines? The Saturn V had 35MN of thrust using 5 engines. Super Heavy Booster has 70MN using 29 reusable engines. – Michael Aug 21 '21 at 06:25
  • @Michael upscaling everything without other modifications – certainly would not work (combustion instability etc.). But pretty sure thanks to CFD, 3D-printed parts etc., SpaceX would indeed be able to develop a larger version with much less R&D cost than the original one took. And very possibly they will do that eventually, though for now the priority is clearly on getting the thing to orbit and back with the engines they have. – leftaroundabout Aug 21 '21 at 16:31
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    Scaling up rocket engines doesn't work very well. The bigger the engine the more instability issues you have. An unstable burn tends to be extremely dramatic. – Loren Pechtel Aug 22 '21 at 01:29
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    Don't know much about airplanes, but it would be possible that they only went from 4 to 2 engines because the engines got safer over time, and an engine failure far less likely. – MaxD Aug 22 '21 at 03:42
  • Until recently, space rockets were in the habit of throwing engines away after every use. Coming from that perspective, having a few more engines - but reusing them - probably seems like a great idea. – NPSF3000 Aug 22 '21 at 04:50

6 Answers6

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Because with current technology, the greatest part of expense in building a rocket motor is not the individual construction, but the research needed in the design of it. And it is simpler, easier and cheaper to design a rocket engine of moderate size, than a colossal monster of an engine (like the F1 that Saturn V used)

Even with airliners, the HUGE turbofan engines are not selected because they are cheap to manufacture, the very opposite is true. A single General Electric GE9X as used by the Boeing 777 costs 44.5 million dollars. Each! Whereas each engine on a 747 only costs about 13 million, yet produces 60% as much thrust each.
The huge engines on a 777 are selected because they offer better fuel economy than multiple smaller engines, and slightly less maintenance cost.

Unfortunately, rocket engines are nowhere near the maturity of development that airliner turbofan engines have.

CuteKItty_pleaseStopBArking
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    I wonder how that relates to Musk's claims, that design is easy, production is difficult. – SF. Aug 20 '21 at 17:25
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    @SF. mass production is difficult, producing an individual engine isn't so hard. The individual engines are actually pretty simple and easy to build in comparison to jet engines. SpaceX's Raptor is estimated to have oxygen preburner temperatures of ~800 K, compared to ~2300 K for a turbojet's combustor. Pressures are high and fluids are dense, so the turbines and pumps are much smaller, and there's a huge flow of propellant for cooling. You see some carefully chosen metallurgy, but not the single-crystal superalloy turbine blades with integrated cooling channels you see in jet engines. – Christopher James Huff Aug 20 '21 at 18:25
  • @ChristopherJamesHuff: Terminology note: modern airliners use turbofan engines, with most of the power being used to spin the bypass fan. Turbojet means no bypass at all, with all the thrust coming from the very high velocity exhaust air. This is much less efficient; even fighter jets use low-bypass turbofans these days. What is the difference between turbojet and turbofan engines?. I think combustion chamber temperatures are basically similar for turbofan vs. jet, though, so your point is still relevant. – Peter Cordes Aug 21 '21 at 12:45
  • The really large fan diameter of modern high-bypass turbofans is part of why we need such strong materials for the fan blades: because of the huge centripetal force needed to hold them on. (The fan is the big round part at the front that you can see when looking at a jet engine.) Even though the fan blades aren't exposed to combustion temps. (So that's a separate challenge from the turbine blades you mentioned that have to feed the combustion chamber and extract work from the exhaust.) – Peter Cordes Aug 21 '21 at 12:53
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    I was specifically referring to turbojets as being more similar to a rocket engine's turbopumps (particularly in a full-flow engine like Raptor) and to avoid confusion with the large but low-temperature fans of turbofans. – Christopher James Huff Aug 21 '21 at 13:06
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    @ChristopherJamesHuff “The...engines are...pretty simple...in comparison to jet engines” – I don't think you can put it this way. The challenges are just different. Corrosion, polymerisation and combustion instability are much more problematic for rockets, not to mention the issue with cryogenic temperatures. – leftaroundabout Aug 21 '21 at 16:22
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    Turbojet engines also operate at Thrust-to-weight ratios that are way, way WAY WAY less than those for rocket engines. A GE90 engine has a TWR of 6, a Merlin 1Dvac has a TWR that is 30 times better. (mostly because the turbojet's focus is not TWR, but Fuel economy, while the rocket is all about Thrust and ISP) – CuteKItty_pleaseStopBArking Aug 21 '21 at 17:37
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    Musk and OP are using "design" in two different ways. Musk means actual design of the actual rocket. OP is including manufacturing design in the term "design". Production is difficult--that's why it's better to spread that cost of figuring out how to actually build the engine over 500 engines instead of 50. Why spent all that work designing tooling just to only fart out a handful of engines? – Anton Hengst Aug 22 '21 at 01:05
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Economy of scale, strictly. SpaceX focuses heavily on streamlining and automation of production of these engines. High up-front cost, but low unit cost per engine once the process is perfected.

The larger the engines, the higher the up-front cost would be, as problems of combustion stability, cooling, material durability and so on crop up; this is a well-known problem of massive engines, one that buried Soviets' plan of manned moon landing too.

Then there is the problem of production errors - e.g. faulty 3D prints, problems that become apparent in tests after print completion. Let's say the 3D printer has 1 in 10 chance it will glitch once on given day, producing a fault that ruins the currently printed engine. If the engine is smaller, and takes 1 day to complete the print, one in 10 engines will be faulty, 10% work time of the printer wasted, 10% of production lost. Now increase the engine size, so it takes 2 days to print. Same chance of a glitch ruining it, but now two days are wasted, one engine in five is a reject, and twice as much of materials is wasted with each rejection. That automatically means per-unit cost of the engines is increased, as the loss due to the faulty one is spread between four good ones, instead of nine.

Then there's the matter of redundancy. If you have, say, 15 engines, 2 or 3 flaming out won't mean loss of mission. If you have two and lose one, you won't be going to space today.

The aviation industry is way more mature now - in the beginning they did use many smaller engines because scaling that much up simply wasn't technologically viable - and also for reasons similar as with SpaceX currently, economy of scale, reliability, redundancy. With constant, steady stream of revenue from existing production of smaller engines, innovation, improvements in safety and reliability, and active competition, they were able to develop incrementally larger and more powerful engines without running at "infant mortality" problems of new production, where before it becomes profitable, there are a lot of problems to solve and costs to bear.

SF.
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  • The chance of a glitch ruining the engine is higher for the larger engine, 20 % instead of 10 % for the smaller one. – Uwe Aug 20 '21 at 20:29
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    @Uwe Generally speaking, if you increase your clearances linearly with size, you are not using scale to your advantage. If you'd increase the gap of an oil lubricated gliding bearing, you'd need to pump much more oil into it and do so at a higher pressure to keep the parts from touching. The oil film does not care how big the parts are, as long as there is a thin film of oil all around. As such, it would be much more prudent to increase the number of oil channels feeding the bearing, and keep the gap size the same if possible. Size changes due to temperature may force bigger gaps, though. – cmaster - reinstate monica Aug 20 '21 at 22:01
  • I think with more trustable drives the larger ones would be not only more economic, but also more effective. A drive capable to create double thrust would likely weight lesser than twice, and also producing it would cost lesser than twice. – peterh Aug 21 '21 at 11:24
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    @peterh "Better is the enemy of good". First SpaceX needs to get a working and revenue-generating Starship, then they can work on optimization. – SF. Aug 21 '21 at 12:07
  • I think two arguments here are valid: economy of scale (producing thousands of engines having 2MN each is cheaper "per N" than producing hundreds with 20MN each); and redundancy is a large bonus as demonstrated by (partly) successful missions in spite of single engine failures. The "larger production failure impact with larger engines" part seems phony though: 10% is 10%. – Peter - Reinstate Monica Aug 22 '21 at 10:10
  • @Peter-ReinstateMonica - "producing thousands of engines having 2MN each is cheaper "per N" than producing hundreds with 20MN each" Have you got a source for that? As mentioned earlier, rocket engines do not necessarily scale in a simple way. – WhatRoughBeast Aug 22 '21 at 13:36
  • @WhatRoughBeast It is an iron law of industrial production that the unit cost falls with the number of units. Wee see that with solar cells and wind turbines. Ramping up the numbers is essential for reducing production cost. That's natural and unsurprising. (Imagine you'd produce just one single engine with a GN of thrust, a one-off prototype, essentially hand-built: that'd be the worst case.) Producing a million engines à 1 kN would be best, but that's too many engines. Using as many as fit under the rocket is probably the optimum. You'd need a reason for a deviation from that basic rule. – Peter - Reinstate Monica Aug 22 '21 at 14:21
  • @WhatRoughBeast Ramping up the numbers and driving down the cost is exactly the point at which SpaceX is now, see this part of the feature Everyday Astronaut did on the SpaceX factory: "How do we make a Raptor where the cost per ton of thrust is under a thousand dollars." The answer is putting a lot of effort in the manufacturing process (which is "10 to 100 times harder than the engine design"), which has a better pay-off with high unit numbers. – Peter - Reinstate Monica Aug 22 '21 at 14:43
  • @ReinstateMonica - That does not answer my question. Sometimes a small number of larger units IS more economical than a large number of small ones. Tractor-trailers vs minvans is a good example, at least for long-distance transport. Marine crude oil carriers is another. Likewise cargo container ships. Long-range passenger/cargo aircraft. Open-pit mining equipment. Aircraft carriers. – WhatRoughBeast Aug 22 '21 at 17:30
  • @Peter-ReinstateMonica Consider the same for CPUs. Same filure rate per cm^2 of wafer, but make the CPU big enough and you practically guarantee there will be a fault somewhere, your yield dropping to maybe 20% (manufacturers salvage it by disabling faulty cores and selling the 8-core CPU as 6-core, 4-core, 2-core...) . Make it tiny, and sewwt, and most of production will be fine, faulty units well isolated. – SF. Aug 22 '21 at 21:24
  • Everyone is overusing XKCD references these days, but yours was subtle and well-placed. – JohnEye Aug 23 '21 at 09:32
  • @WhatRoughBeast "Sometimes a small number of larger units IS more economical than a large number of small ones." - Look closely at when that "sometimes" happens, as it's true for all your examples: a mature industry, where a "large number of small ones" is a period in the past when the industry was developing and growing rapidly, and the current growth led it there... and especially that in your examples it's not a "small number of larger units", it's a BIG number of larger units - when maintaining a myriad of small ones becomes untenable. – SF. Aug 23 '21 at 11:55
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    Also, "thousands of 2MN vs hundreds of 20MN" - "rocket engines do not necessarily scale in a simple way. " - once again, yes, it doesn't - in particular with problems of combustion stability cropping up exponentially, requiring extremely sophisticated injector systems, possibly multi-chamber construction, new problems related to thermal expansion, and all sorts of problems absent in smaller engines. As the size goes up, up to a to certain point $/N is dropping gradually, but then it ramps up. Raptor is around the sweet spot. – SF. Aug 23 '21 at 12:05
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It's not clear to me from your question if you're asking about Starship, or about Super Heavy.

Starship is the upper stage of the Starship/Super Heavy launcher, and has relatively moderate 6 engines on it; 3 "sea level" Raptors that can gimbal to point their thrust plus three more optimized for performance in vacuum with large fixed nozzles. One working sea level engine out of the set of three is required for landing on Earth; failure to light at least one engine guarantees destruction of the vehicle. The vacuum engines are not usable here.

Unlike an airliner, the engines are turned off during most of the descent and only start a few seconds before landing, which adds a large risk factor (as the development flights of Starship have shown painfully). Furthermore, an airliner even has a good chance to land with zero working engines. So the landing modes just aren't directly comparable, and the actual 3:1 redundancy isn't excessive.

Super Heavy, on the other hand, mounts 29 31 engines. Here, the driver is engine commonality with the upper stage and economy of scale. SpaceX only has to develop one really good methane engine, and then concentrate on how to reduce production costs over a large number of them.

Russell Borogove
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5

Others have already mentioned the advantages in redundancy and manufacturing scale. Other advantages:

  • It's structurally more efficient to place engines near the skin of the vehicle. The Superheavy booster in particular takes advantage of this, with its outer ring of engines actually protruding somewhat beyond the diameter of the vehicle.
  • Using a large number of engines makes it much easier to achieve a wide effective throttle range by shutting engines down, which is important for recovery of the vehicle.
  • Smaller, lighter engines are easier to transport and handle. Raptors are small enough to be moved around with forklifts, which makes them easier to install or swap out.
  • Smaller engines are easier to test. Engine test stands are smaller, and vehicles can test fire single engines or subsets of their engines.
  • Smaller engines are less likely to inflict severe damage on the vehicle in the event of a major failure.
  • A large number of smaller engines is actually quieter, due to how incoherent noise sources add together. In short, with the same total sound power, the multiple incoherent sources will partially cancel each other out, resulting in lower average pressure levels.
  • The same goes for vibrations internal to the vehicle. People who've flown on both Crew Dragon and the Shuttle have remarked about how smooth the portion of the flight powered by the 9-engine booster was in comparison, and how the 1-engine upper stage actually felt rougher than the Shuttle.
Christopher James Huff
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    "Raptors are small enough to be moved around with forklifts, which makes them easier to install or swap out." – Indeed. SpaceX has improved installation times tremendously. Just one year ago, it took roughly a day to install an engine, a couple of weeks ago, they installed all 29 engines on B4 in one night. (Although that was only a fit check, so it might be the case that the plumbing was not hooked.) The engine installation before the static fire test of B3 took only about an hour per engine. – Jörg W Mittag Aug 24 '21 at 12:34
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As airline safety improved, they focused more on cost. Without safety, cost didn't matter. SpaceX engineers asked "what's the largest engine we can make and still have room for vacuum engines and multi-engine-out capability for landing?" The answer was the current engine size of raptor. There were more factors, but that's the gist.

Next, since engines are so expensive, they decided to mass produce that size. Mass production leads to innovations in cost and quality, which should increase safety.

Lastly, you want max thrust on liftoff to reduce fuel costs. So you pack the booster full of them, which ends up being 29-32 engines. That plus 3 on the ship and 6 more with vacuum nozzles means 38-41 total, which is a lot.

As safety is proven and technology improves, we may see fewer, larger engines on Starship V2. Bigger has historically meant more efficient, and fewer might lead to lower cost. But by then 3D printing or some other innovation might make fewer/bigger engines a bad strategy. Time will tell.

Dexterious22
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  • I might not be very bright, but I don't see what additional facts (apart from speculations) this answer has brought, to what the OP question has already hinted at, and to what others have already answered. – Ng Ph Aug 23 '21 at 13:43
  • The last paragraph is roughly what happened with aircraft jet engines. The 747-100 needed four engines partly due to the tech limit on how powerful an engine could be, and partly due to the tech limit on how reliable an engine could be. – Codes with Hammer Aug 23 '21 at 15:55
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Let's say you want to build a big rocket (or airplane). You're going to need a lot of thrust. You can get a lot of thrust from a bunch of small engines or just a few big ones. Now, we know that more engines are more expensive than fewer engines, right?

But wait! How many rockets are we going to build? Probably not a lot. Of the heavy lift rockets, there have only been 13 Saturn Vs, 5 Shuttles, 2 Energias, and probably will only ever be 10 SLS(es?). Let's consider Starship--it's going to be reusable, so even though Musk wants a crazy launch rate, there's probably not ever going to be more than 15-30 built.

So what if we put only a few massive engines on each of our vehicles? There's probably going to ever be 50 (at most!) built. While that's not quite low enough that basically every engine will be hand-crafted, it's going to be pretty close. Building the tooling to make a rocket engine will always be very expensive, and with only 50 engines to divide that cost over, each engine is going to be wildly pricey.

Let's compare this to aircraft engines. The PW4000 and the GE90, two of the most common engines you'll find in a Boeing 777, have each had over 2,500 manufactured--and recall, this is on a century's worth of incremental work into jet engine manufacture by P&W and GE. Much of their tooling is probably shared with previous engines. These are far, far cheaper a peace than the 50 engines you're planning on building.

So when building a rocket, a market in which volume is very low and development costs are very high, it's actually more expensive to use fewer engines, because so few of them will ever be built. It makes more sense to develop a smaller engine--something which (we haven't mentioned this yet) is MUCH easier to design & validate than a very large engine--and just make a bunch of them.

Note: we haven't even mentioned the redundancy issues yet--if one of 20 engines dies, it's not that big a deal. If one of 4 engines die, you're not going to space today.

Afterthoughts: if we ever get to the point where there's 100s or 1000s of rockets being built a year, and engine design has matured to be very reliable, I expect you'll start to see only a few engines per rocket. But we're building 10s (at ABSOLUTE best!) of rockets per year.

Anton Hengst
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    "there's probably not ever going to be more than 15-30 built" – Elon Musk estimated they need about 1000 Starships. Estimating about one Booster for every 5 Ships (a totally uninformed guess on my side), that is a total of ~1200 rockets with a total of ~15000 engines. They have already built 17 vehicles, and have started construction on at least 2 more; they have also built around 70 engines. 8 of those vehicles and about 18 of those engines have already flown. – Jörg W Mittag Aug 22 '21 at 09:54
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    "But we're building 10s (at ABSOLUTE best!) of rockets per year" – SpaceX is already building 10 Ships per year by hand: they don't even have the factory yet! And they intend to have multiple factories near multiple launch sites (very likely at least one at Starbase and one at Cape Canaveral), including on Mars. – Jörg W Mittag Aug 22 '21 at 09:55
  • You should have consulted some public launch manifest, such as this – Ng Ph Aug 23 '21 at 15:15
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    clearly I should have clarified: 10s of rockets per design per year. It's not like a 777 & an a321 share engines & ought to be counted in the same mass-production tally. I'm not a moron; I know more than 10 rockets are being launched between the Altas Vs and the Long Marches & the Falcons & the Antares & the Soyuzes & H-2s (?) & the whatever the ISRO launches & the Delta IV when they manage to not scrub. Are they all sharing engine tooling? absolutely not. – Anton Hengst Aug 24 '21 at 07:47
  • AH! This makes sense. Thx. – Ng Ph Aug 24 '21 at 20:38
  • @AntonHengst - Sometimes, the best answer gets voted down. – TheMatrix Equation-balance Feb 24 '23 at 03:47