9 ms·
380s for ISP for the Raptor vacuum engine seems durn good, considering the tradeoffs between hydrogen and methane. The most efficient engines we've flown, the
by hoorayimhelping 7y ago
380s for ISP for the Raptor vacuum engine seems durn good, considering the tradeoffs between hydrogen and methane.
The most efficient engines we've flown, the RL-10 [0] (used on the Delta IV and Atlas rockets as part of the Centaur Upper Stage) and the RS-25 [1], (the Space Shuttle Main Engine) get around 450-460s for their specific impulse. These engines use liquid hydrogen and liquid oxygen. The issue with liquid hydrogen is twofold: it is not very dense, and to keep it from boiling away in liquid form, it needs to be really cold. So you need huge, insulated tanks to store it. Hydrogen is everywhere, but it's kind of a pain to use as rocket fuel.
Merlin Vacuum [2], which is currently used on the Falcon vehicles gets 311s. The RD-0110 [3], used on the Soyuz gets 326s. These use RP-1, which is highly refined kerosene. It's super easy to work with. On earth. Where we have 200 years of infrastructure in place to support using hydrocarbons. It takes that infrastructure to refine it into a form that rocket engines can use without gunking up the works with unburnt carbon and other particulate crap.
Raptor uses methane, which is kind of like a jack of all trades between density, efficiency, ease of processing, storage, and ease of use in engines. This is basically how SpaceX seems to operate: rather than optimizing a single part of the system (like trying to get the most efficient engine), they try to optimize from a systems level. I don't really know much more than that, because there hasn't really been a lot of stuff for civilians to read on methane engines. This is kind of the cutting edge of the second golden age of space exploration. This freaking rules, what a time to be alive.
[0] https://en.wikipedia.org/wiki/RL10 https://en.wikipedia.org/wiki/RL10
[1] https://en.wikipedia.org/wiki/RS-25 https://en.wikipedia.org/wiki/RS-25
[2] https://en.wikipedia.org/wiki/Merlin_(rocket_engine_family) https://en.wikipedia.org/wiki/Merlin_(rocket_engine_family)
[3] https://en.wikipedia.org/wiki/RD-0110 https://en.wikipedia.org/wiki/RD-0110
- codeulike 7y agoHere's a Wired article about the raptor engine - it's "a full-flow staged combustion engine, only the third engine in history to employ this technique ... The previous two attempts at such an engine, one in the Soviet Union in the 1960s and another in the US in the early 2000s, never made it beyond testing." https://www.wired.co.uk/article/spacex-raptor-engine-starship https://www.wired.co.uk/article/spacex-raptor-engine-starshi...
- deleted 7y ago[deleted]
- mlindner 7y agoI'm sorry but did anyone do a rudimentary spelling and grammar check on that? "Raptor is designed to power the new reusable vehicles SpaceX’s is building, the Starship spacecraft and the Super Heavy rocket." Also they call Tim Dodd an "industry expert"...
- Klathmon 7y agoAnd if you want to get a lot more detail on what makes a "full flow staged combustion engine" so special, this article from Everyday Astronaut goes into some amazing detail on this and many other engines. https://everydayastronaut.com/raptor-engine/ https://everydayastronaut.com/raptor-engine/
- aaronblohowiak 7y agoThis was AMaZING. Exactly the kind of content I want to consume on HN
- oska 7y agoThis guy is amazing. Look out for his interview with Elon Musk that should be released in the next 24 hours on his YT channel [1]. [1] https://www.youtube.com/channel/UC6uKrU_WqJ1R2HMTY3LIx5Q/videos https://www.youtube.com/channel/UC6uKrU_WqJ1R2HMTY3LIx5Q/vid...
- jakoblorz 7y agohttps://youtu.be/Sdwy9fzQzl4 https://youtu.be/Sdwy9fzQzl4 Scott Manley does a great job explaining such things!
- baybal2 7y agoI'm surprised the guy being a youtuber. He is the codecs guy at Apple Computers
- kbenson 7y agoEh, it's just another form of blog at this point. Not the greatest for searching, but a valid medium for expression.
- inopinatus 7y agoI think they might be suggesting that Scott may have specific exemption from Apple's PR policy (summary: "say nothing to anyone") to be allowed an independent public voice - particularly on a technical/engineering topic. Apple is not, so far as we know, currently developing orbital-class rocketry, but there's genuine potential for product topic overlap in Project Titan vs Tesla and perhaps more besides. I would not be at all surprised if there's a List Of Things Apple PR Hereby Politely Requests That Scott Doesn't Mention. When I was at AWS I was similarly forbidden from mentioning anything to do with video games to just about anybody, despite not having anything to do with video games myself, and many other topics besides the obvious (e.g. "don't disclose EC2/S3 capacity numbers"[1], "don't reveal how Glacier works"[2]), and in fact there was a whole programme of PR training and tiered permission to speak at conferences or to the press/analysts that boiled down, mostly, to knowing what you could and couldn't say, and how diplomatically you parried questions on the latter. Either that or Terry Pratchett's law of rewritten rules applies. [1] no-one would believe me anyway [2] it's a massively redundant array of vinyl records
- afterburner 7y agoIt's also great self-marketing.
- 7y ago
- JumpCrisscross 7y ago> The issue with liquid hydrogen is twofold: it is not very dense, and to keep it from boiling away in liquid form, it needs to be really cold It’s also a bitch to work with. When it’s not trying to immolate or suffocate you it’s busy turning every metal part it touches to garbage [1]. [1] https://en.m.wikipedia.org/wiki/Hydrogen_embrittlement https://en.m.wikipedia.org/wiki/Hydrogen_embrittlement
- avar 7y ago> This is basically how SpaceX seems to operate: rather than optimizing a single part of the system (like trying to get the most efficient engine), they try to optimize from a systems level. They're optimizing for "real" reusability, as in eventually aiming for airline-like turnarounds. None of the earlier engines you noted were reusable except the RS-25, and given how long it took to turn the Shuttle around one could argue it doesn't really count as "reusable" in sense SpaceX is aiming for. Aiming for reusability and ease of maintenance leads to different optimization paths, in particular we should expect that any one component is going to be less powerful, it'll need to account for wear & tear, not use once and crash it into the ocean. But raw performance on a single trip doesn't matter much if you can just deliver the same payload in two trips to orbit instead.
- walrus01 7y ago> and given how long it took to turn the Shuttle around one could argue it doesn't really count as "reusable" in sense SpaceX is aiming for. Not just the time but the cost, if you take the figure of how much the space shuttle program cost, and divide it by total number of flights (including the two lethal ones), it cost somewhere between $900 million to $1.1bn every time the thing launched. It was very far from the early 1970s dream of an economically reusable spacecraft.
- mobilefriendly 7y agoIndeed, the space shuttle was so expensive it burned up much of the rest of NASAs budget-- keeping that disastrous, failed program running is why NASA doesn't have manned spaceflight capability at the moment.
- jabl 7y agoWell, from an outsider perspective, the SLS/Orion program is pure pork. Nobody holding the purse strings cares whether the thing ever flies. Heck, probably they even prefer if it never flies (always just a few years and tens of billions in the future); a failed launch is potentially politically catastrophic.
- tim333 7y agoMethane was chosen to a large extent because you could make it fairly easily on Mars which is necessary if you want to fly people there and then get them back again.
- tim333 7y agoMusk talks about making it on Mars here https://www.youtube.com/watch?v=sOpMrVnjYeY&feature=youtu.be&t=3900 https://www.youtube.com/watch?v=sOpMrVnjYeY&feature=youtu.be...
- novalis78 7y agoISRU on Mars makes Methane fuel and excellent choice. Zubrin’s favorite in ‘A case for Mars’.
- avmich 7y agoRD-58MF - https://en.wikipedia.org/wiki/RD-58 https://en.wikipedia.org/wiki/RD-58 - uses kerosene (or syntin?) and has Isp of 372 s. Methane, which is cryogenic and less dense, ought to deliver more? Otherwise bigger tanks and operational issues will eat out the benefits.
- greglindahl 7y agoYou can raise the Isp of any vacuum engine by using a bigger nozzle. The RD-58MF is a small engine and there's room for a relatively huge nozzle. That's not true for big engines. For example, Merlin vacuum has a 1:165 expansion ratio. The RD-58MF has a 1:500 ratio. The Wikipedia article for the RD-58 family shows the increase in Isp from various expansion ratios. https://en.wikipedia.org/wiki/RD-58 https://en.wikipedia.org/wiki/RD-58
- avmich 7y agoRight. There is however an argument that the smaller total impulse a stage does (RD-58 is used for upper stages, where the stage mass is closer to the stage payload), the smaller the relative mass of fuel+engine of that stage is, and so the fixed mass of the bigger nozzle added to the stage costs more, in Tsiolkovsky equation, that the mass for the bigger stage. Going further, we can argue that for larger stages it's beneficial to spend some mass on nozzle, because that mass is used for larger impulse provided by the stage. Interesting that using http://rocketworkbench.sourceforge.net/equil.phtml http://rocketworkbench.sourceforge.net/equil.phtml I couldn't get declared Isp (380) for methane-LOX for reasonable nozzle expansion ratios. Maybe the model is too approximate...
- jccooper 7y agoThere won't be much literature because Raptor is essentially the first production methane engine. Everyone in the past seems to have decided to optimize strongly for either Isp, bulk density, or storability, and methane is the best at nothing.
- simonh 7y agoRight, because in the past we were building first stages which had to be optimised for operations in the lower atmosphere, and upper stages which had to be optimised for vacuum. Hence the Saturn V had an RP-1 powered first stage and Hydrolox second stage. But the raptors on the second stage have to operate efficiently in a vacuuum, but also work well and sea level for the landing. So even taking Mars out of the equation (which we really shouldn't, but just for sake of argument), it's requirements are quite different to anything we've built before.
- iblaine 7y agoMusk thinks like a software engineer. Create simple, reusable, components that can scale efficiently. I’m taking some liberties in saying this, but the gist remains the same. Musk keeps it simple and focused.
- jonstewart 7y agoAgreed! Only a software engineer would risk a product launch delay because of an uber-bikeshedded pet feature (Model X doors) or wait until the last minute to ship and then construct a makeshift “factory” in a parking lot and make staff go through a death march, QA be damned.
- starpilot 7y agoThe Model X doors are terrible. It's an SUV that you can't put a roof rack on. Does seem like a case of geek myopia over usability.
- bobsil1 7y agoOr… optimized for a different case (ease of access for small kids).
- bryanlarsen 7y agoSliding minivan doors get you both.
- FullyFunctional 7y agoI haven't used or wanted a roof rack in 30 years (extra mass and drag at the highest location? No thanks), but the Model X has a lot of storage space, including the frunk so it's a non-issue. The falcon doors also provide a surprising benefit: cover in the rain. And they are very practical with small children.
- londons_explore 7y agoOptimized for showy and unique features, when a young Tesla really needed its products to stand out.
- trappist 7y ago> This is basically how SpaceX seems to operate: rather than optimizing a single part of the system (like trying to get the most efficient engine), they try to optimize from a systems level. A system of local optimums is an inefficient system. Sort of the self-identified takeaway of the book _The Goal_, and I haven't stopped thinking about this statement since I read it.
- doomlaser 7y agoI'm no rocket scientist, so bear that in mind, but something has been bugging me about the flight paths for this vehicle [0]. It has to make some very large, specific orientation changes several times during descent, using the heat shield on one side of the vehicle, and then only rotating to upright orientation in the lower atmosphere, after quite a lot of falling sideways. The orientation maneuvers seem much more complicated than their current booster re-entry flight paths [1]. and even more complex than the space shuttle [2]. [0] https://www.youtube.com/watch?v=LQTnWEHl5qU https://www.youtube.com/watch?v=LQTnWEHl5qU [1] https://www.youtube.com/watch?v=6YyV-otP3pI https://www.youtube.com/watch?v=6YyV-otP3pI [2] https://www.youtube.com/watch?&v=TOieURpnbm0 https://www.youtube.com/watch?&v=TOieURpnbm0 This appears to me to make each flight much more complicated to pull off, introducing more points for it to fail without contingencies, and it makes me nervous. But, again, I am an just an unenlightened observer from an unrelated field. I don't even play Kerbal Space Program! I'm sure they've weighed a million different options and their risks, and have arrived at this one after a lot of thinking, but it has been bugging me. The stainless steel construction certainly looks beautiful out there in Texas.
- wurst_case 7y agoLearning how to reliably control the damn thing on descent is certainly their greatest hurdle. Refueling in orbit being the second biggest. I expect things to go boom at least a few times before they learn to land properly. Saying people Wil go to orbit within a year is most likely not to happen.
- zaroth 7y agoElon said they learned a lot of what they need for orbital refueling just from docking with the ISS.
- InTheArena 7y agoIt’s actually similar to the shuttle, In a couple of ways simpler, and a couple of ways more complex. The shuttles body was also designed to both generate lift and be a blunt body - with a transition of mode between them. The blunt body allowed it to survive reentry while the lifting body allowed it to glide. However that glide was way more complex then NASA generally talked about. The shuttle pulled some aggressive s-turns to bleed off energy during the re-entry. This approach is more or less the same as what the starship is doing here, except instead of using a glider trajectory for the controlled landing they instead use retropropulsive landing. This is needed because what SpaceX is landing here is a second stage, not a first stage. Way more heat, way more power.
- quotemstr 7y agoSpaceX has a good design philosophy. They use cheap, reliable, and sturdy components, take the hit on payload fraction, and just make really big rockets to make sure _total_ payload is adequate. I'm reminded of the old quip apocryphally attributed to Stalin: "quantity has a quality all its own"
- rjf72 7y agoYou missed one of the most important parts about methane as well: The Sabatier Reaction [1] Hydrogen + CO2 => Methane + Water This is an extremely important reaction. Mars, for instance, has effectively unlimited Hydrogen and CO2 which also means effectively unlimited Methane and Water. How just absurdly convenient is that? But it also plays a major role in things like life support. We breathe out CO2. On Earth where we have an enormous atmosphere this doesn't matter. But in closed systems, such as the ISS or what will be the habitation regions on Mars, this is a major issue that needs to be dealt with or we'd kill ourselves with our own exhaust. NASA, on the ISS, used to deal with life support by producing oxygen from imported water, and discarding the produced hydrogen. They'd then collect exhaled CO2 from the air using CO2 scrubbers and also discard that. That was a pretty inefficient system that required large amounts of imported water to sustain. Now they use the Sabatier Reaction to convert the exhaled CO2 into water and methane. Only the methane is discarded. And as a result of this, instead of needing to import large amounts of water, now all they need to import is a small amount of hydrogen to keep the reaction running. On Mars this will result in a really cool system. What will be an increasingly large scale life support system will be literally creating rocket fuel as a byproduct. Of course methane is also a controlled and efficient gas, so it can also be safely used for things like cooking dinner. [1] - https://en.wikipedia.org/wiki/Sabatier_reaction https://en.wikipedia.org/wiki/Sabatier_reaction
- adrianN 7y agoIf you invest more energy you can also turn Methane into longer chain hydrocarbons and produce things like plastics or lubricants.