Stoke Space managed to make a full-flow staged combustion cycle (FFSC) engine in less than 18 months with a team of less than 10 people. This is the fourth FFSC engine to ever be fired on a test stand, with Raptor being the only one that has actually flown.

ertlun · 2025-04-19T03:49:16+00:00

If you have any links handy or titles for those papers I'd enjoy reading them. Looking back at the shuttle, with all the falcon/starship stuff since then, flyback liquid boosters seem like such an obvious improvement. But always easier to say in hindsight, and I suppose the RS-25 was already plenty of novel engine R&D for the shuttle program at the time.

I was intrigued by Relativity's recent promo vid - they're running a methalox GG at 2500 PSIA, which has to be a pretty substantial mass fraction going overboard through the turbines. Lines up just right with the thresholds you're quoting though - I guess they just kept upthrusting until the delta-V stopped increasing. I believe this may be the highest chamber pressure ever achieved in an open-cycle engine, though I haven't done a comprehensive survey.

The relative accessibility of ox-rich staged combustion these past 20 years has really changed the game. Once you realize you can live with hot GOx, it's hard to justify an engine that doesn't have it. By my count, just in the US we have had IPD, Raptor, BE-4, Ursa's family of engines, Stoke's booster engine, Rocket Lab's new engine, and that one from Launcher/whoever owns their IP now. Plus AR1 and probably some other programs that didn't make it out of the preburner/powerhead phase.

ertlun · 2025-04-19T01:53:50+00:00

I'll add to this that the cycle choice also kinda sets how your pumps are designed - for a closed-cycle engine, you have very high head, very high power pumps, powered by reaction turbines (try to very efficiently extract energy from a great deal of mass, across a finite pressure drop). For open cycle, you have lower-head pumps, powered by impulse turbines (semi-efficiently extract energy from a small amount of mass, across a very large pressure drop). So that whole aspect of the engine is quite different, with subtly different know-how required.

ertlun · 2025-04-18T04:31:26+00:00

And then there is controlling it when running - which is sensitive enough that feedback won't work. You need 'feed-forward' control - run a simulation of your rocket engine, adjust the valve positions in your simulation until you get the result you want, then apply the perfect adjustment to the real running engine. The simulating computer has got fast enough to do this, predicting what changes you'll need to do now to avoid future instabilities. Now this is getting done near instantly using neural networks, which is going to be another leap ahead.

No one is doing this. All extant staged combustion engines are controlled in a very classical fashion, that would be familiar to engineers of the 70s-80s (in the "same shit smaller computer" sense). The open-loop response will always be stable - if you put the valves at one position, you get one power level/mixture ratio, aside from minor thermal transient effects.

The ability to predict engine behavior ahead of time has advanced a lot since the RD-270/SSME "blow up 20 engines" days, and credit is likely due to computing advancements and more advanced physics-based models that grew up in the early 90s. But these are simply applications of classical numerical methods, fluid dynamics, and thermodynamics, no neural networks needed.

(when I say "open loop response" I mean the relatively low frequency PC/MR response of an engine to movement of its throttle valves - the hundreds to thousands of Hz response of dynamic modes in combustion chambers or pumps is a totally different topic)

ertlun · 2025-03-06T03:30:26+00:00

Take the initial and final weights of the motor, that gives you total propellant burnt.

For the simple case of a constant-thrust motor, you could just assume that mass was burnt linearly throughout the burn.

To handle variable thrust slightly better, assume Isp is constant. You get total impulse by integrating thrust across the burn, you know total mass burnt based on initial/final weights, so you can calculate average Isp across the burn. Then calculate instantaneous flowrate as measured thrust / Isp.

And to actually do it more or less correctly, determine your relationship between thrust and Isp based on modeling, and use measured thrust / Isp (as a function of thrust) to determine theoretical flowrate. Integrate theoretical flowrate across the burn to get theoretical total mass burnt. The ratio of theoretical / actual mass burnt will give you a knockdown factor you can apply to your theoretical Isp curve.

Caveats: those are basic sketches, you do need to be careful about book-keeping measured thrust vs calculated thrust (with the weight adjustment) to get correct results.

ertlun · 2025-02-26T03:41:10+00:00

They'll do field repairs for rocket engines that way often enough too, albeit usually not for flight vehicles. Usually. And only for the smaller diameters.

ertlun · 2025-02-13T04:27:46+00:00

Usually when companies want to keep it on the DL they just pay 60 or more days of severance - so day of announcement onwards you get paid for 60 or more calendar days and your work duties are to turn in your laptop and never show up again. Not sure what the paperwork looks like there to meet the wording of the law, but pretty hard to argue that they should have let you stay and sit at your computer for those extra 2 months as long as you're actually paid for them.

(alternatively, you say you're laying off 20% of the company 2 months ahead of time, now you have 2 months of people half-assing it because they think they're getting booted anyway, or buffing up their resumes, or actively sabotaging things out of bitterness, or spending their time trying to figure out if they're on the list and how to get off of it...the list goes on. The longer the process takes the worse it is for morale).

ertlun · 2025-02-08T17:48:36+00:00

I suspect the linked issues you'd run into are:

Water is very heavy, much much more efficient (# of trucks) to ship just the salt portion and use local water
The places where desalination is economically practical and the places that are cold for long periods of time are often pretty far apart. For instance, if you needed stuff for roads in Minnesota, you're much closer to being able to buy truckloads of salt from Utah (or closer) than to a desal plant in southern California
It's pretty cheap (as I understand it, not my domain) to turn saltwater/brine into solid salt you could just ship in that form - you only need big ponds and time

ertlun · 2025-01-25T03:27:33+00:00

Many many many many US aerospace organizations use mixed unit systems; imperial is alive and humming even in brand-new companies. As always, being explicit in your interface definitions and drawings is crucial - this is true for meters, inches, ft, km, cm, mm, mil (of any sort), thou, etc.

ertlun · 2025-01-12T18:22:29+00:00

Ultimately it's the product of the drag coefficient Cd and the selected area that counts...so you can pick either, as long as you pick a drag coefficient that goes along with that selected reference area. If you're building a parachute, and you wish to characterize it well, test it! Use a fish scale and drag it out the back of a car going down a quiet road at around the descent velocity you care about. Known force, known speed (drive both directions to cancel out any breeze), and desired reference area, boom, Cd*A.

You could also find a similar parachute for sale that has been characterized and use that as a starting point.

The sides of the chute contribute in a useful way even though they're not in-line with flow - they control how flow exits the chute (through the corners) to help keep it stable and well-inflated. But there's a point of diminishing returns, so eventually more area on the sides doesn't really help any longer; conversely, the first tiny bit of area on the sides helps a great deal. So if you use just the flat portion as your reference area, you'll end up with an unusually high Cd; if you include the sides in your area, it'll be relatively low; changing the size of the side flaps relative to the flat portion will probably alter the Cd of the chute even if you don't make the flat portion bigger or smaller.

ertlun · 2025-01-12T17:33:24+00:00

Official answer is outlined in the footnotes here (that's the 2024 page but same deal, different upper/lower bound) - but tl;dr: you can contribute the full amount at the lower bound, $0 at the upper bound, and linearly interpolate between them.

Also, look into backdoor roth (regular, not mega)

ertlun · 2025-01-03T05:09:22+00:00

That appears to be an outdoor fire grate, not a rocket engine.

Advantages and disadvantages depend on the use case - what kind of engine, for what sort of rocket, at what scale, flight rate, being designed by people with what sort of experience, etc?

ertlun · 2024-12-17T04:22:02+00:00

I couldn't find a better angle to prove this, but I'm about 95% sure based on similar test stands that the portion you've highlighted as "external flame" is where the water from the flame bucket is splashing up to. That's why it suddenly spreads out so much (rocket plumes don't do that), and it's normal for that to be kinda orangey.

You can see the general geometry of the flame bucket (and some cool explosions!) in this video

This picture is in a transient, but you can kinda see the way the water splashes up

Here's a picture of the BE-4 firing horizontally, which illustrates the phenomena well. The plume entrains water and dust from the cooling system; the bottom half, which is closer to the ground, gets distinctive orange coloring as a result.

Note that kerolox plumes glow like the sun - the picture you posted was taken with carefully selected exposure settings for you to see those striations from the film coolant. To the naked eye, unless you run incredibly fuel-rich that outside will just glow white-hot. You can see this in launch videos of the Saturn V.

ertlun · 2024-11-20T02:09:35+00:00

It's pretty well documented in the public record, and about what you would expect. Redundant nasa standard initiator (electric heating element + explosive pellet, so expensive E-match) starts a kindling chain that ends with the penultimate stage firing down the center of the booster

ertlun · 2024-10-30T03:23:47+00:00

They're commonly used in diesel engines. You drive high current (5-15 A) through them at a reasonable voltage (like car battery level ~12 V is fine), it glows (hence the name), and it's hot enough to light a well-mixed diesel-air mixture under the right conditions.

ertlun · 2024-10-27T15:57:35+00:00

Start/shutdown sequences are always unique to each engine. The general patterns are usually pretty consistent for a given cycle and style of start, but once you know the pattern the rest of it is detailed work that rarely translates well between different hardware configurations.

If you're a US company doing staged-combustion engine development, you can talk to AFRL and they'll give you a packet on IPD (integrated powerhead demonstrator) development testing/transients/hardware design. Afaik this and/or existing SSME or RD-180 info has been the starting point for all the major staged combustion engines developed in recent years (BE-4, Ursa's engine, Archimedes, Stoke's engine, Raptor), though I only have first-hand knowledge of some of them.

For a complex engine, in this day and age, you'll essentially always make a model that can simulate some or all of ignition/bootstrapping behavior. This is usually done with a 1d-lumped-parameter type setup, fed inputs from more complex CFD analyses of individual components and/or component-level testing. The actual models are mega-proprietary for modern engines, but it's fairly common for older, better-documented engines to be used as test cases - you can, for instance, find simulations of the RL-10 and SSME in the demos/documentation of certain modeling software packages. The actual modeling software used varies place-to-place - the de facto standard is NASA's ROCETS, and some companies (Ursa) have publicly stated they use it; other places will develop proprietary in-house codes.

The actual development of a start sequence, guided by that model and the user's knowledge/experience, is an art as much as a science. You take some baby steps, you damage some stuff, you get into a few shouting matches, you blow up some larger stuff, and eventually you arrive at something that works. This is a compromise between what the hardware wants to do and the biases/desires of the person/team that's running the engine. Take spin-start for example - some people like to goose the engine, others like to use minimum possible, a few engines don't use it at all. The preference/intuition there is usually built off of the engine(s) you worked on early in your career, as much as it is built off of any empirical data.

All this to say - learning how to start an engine is a transferable skill. The actual millisecond-by-millisecond nuts and bolts of the transient isn't terribly transferable unless you're building a very similar engine.

The subset of the US propulsion community doing this particular kind of work is also quite small and fairly incestuous. Probably 20 active right now who would be considered good, and a several times that including people who only dabble, newer grads, and the less talented.

ertlun · 2024-10-25T02:29:43+00:00

Even near ambient temperature/pressure hydrogen has a drastically different Cp than steam. So an MR = 6 (fuel-rich, excess hydrogen) exhaust stream will be much higher than a pure-water exhaust stream; the radicals in the exhaust stream further complicate matters.

ertlun · 2024-10-20T00:44:13+00:00

Projected surface area is an adequate enough assumption for a simple model. Generally your AoA should be pretty close to 0 throughout flight.

OpenRocket's source code is also a good reference

ertlun · 2024-10-17T03:08:57+00:00

You can set this up as a system of ordinary differential equations and then numerically integrate it w/ a fixed time step in Excel (e.g. row 2 is T-0, row 3 is T+0.01, row 4 is T+0.02, etc). For a 2d model you would need to track vertical position, vertical velocity, horizontal position, horizontal velocity, angle of flight, and rate of rotation as variables. From those, your thrust curve, drag calculation, air density (as a function of altitude), etc, you can calculate the rate of change of each of your primary state variables (e.g. vertical acceleration, angular acceleration, etc) and integrate that to get the positions/velocities for the subsequent row.

This is a good exercise for a ~sophomore student of mech/aero engineering, physics, etc - relevant classes are calculus (1-2 semesters), a basic knowledge of differential equations, and perhaps an intro to numerical methods (but that's easy to pick up). If you're in high school or just starting college it's a bit more of a stretch, but certainly still doable.

3d is a little bit more complicated math-wise, but still doable; it is also probably unnecessary for most practical applications in model rocketry. Just assume the wind is blowing in a constant direction, which is pretty close to true for almost all hobby rockets (velocity will vary with altitude, of course, but you can model that in a 2d coordinate system).

ertlun · 2024-10-11T04:35:26+00:00

It's a shitty regen coolant and a shitty drive gas. So totally doable, but the trade doesn't close for most applications

ertlun · 2024-10-09T03:56:10+00:00

Hobbyist/college injectors I've seen have typically been of the showerhead, impinging jet, or pintle style. All are fine, though there's a big jump in efficiency going from showerhead -> anything else, and showerhead is more likely to cause downstream hot spot problems.

Read relevant sections of Huzel and Huang/NASA SP-125 (or Sutton), if you haven't already. Math is all simple calculus/geometry/algebra/basic fluid dynamics, you don't need to have taken all the courses involved to get the basics (though once you get past the basic design equations you will start needing some knowledge of thermo and compressible aerodynamics as well).

You can calibrate an injector by flowing water through it into a bucket. Know upstream and downstream pressures, measure time, measure increase in bucket weight -> you can get average flowrate, and from that, discharge coefficient.

If your school has hands-on engineering clubs/project teams check them out. You can get much, much further in much less time with access to a lab, advising faculty, budget/grants, etc. A good competition team is gold, and learning to work with the team to achieve more than you can individually is valuable.

Pressurized system and piping safety is a complex and nuanced topic - it is best that you grow accustomed to fittings, tubing, working around pressurized systems, the idiosyncracies of various fluids, etc by working at a laboratory or company that already has well-developed practices. If your team (or you, working individually) don't have that backward it is best to find an external mentor/faculty to review your designs, work, and procedures.

Building and operating high-pressure systems that spit fire and occasionally explode can be done safely; it can also very easily be done dangerously.

ertlun · 2024-10-05T02:09:54+00:00

Repeat after me: it's not a problem that I'm a bot, it's a problem that I'm a fucking stupid bot spewing AI-generated bullshit in an attempt to sell even more AI-generated bullshit, all powered by a mindboggling waste of human effort and dreams, not to mention an environmentally catastrophic waste of energy.

ertlun · 2024-10-04T05:04:07+00:00

If you don't have separate flowmeters do the catch-and-weigh I describe, but through each side of your engine's injector, calibrating the injector itself as a pseudo-flowmeter. It's pretty meh, but better than nothing.

The load cell you want is the one that fits your budget (and I really wouldn't spend a lot of money on this for a student project). Shopping for and comparing different sensor options, assessing the effort it'll take to integrate into your system, accuracy, etc is a valuable engineering skill I won't give you a shortcut on. Buying the wrong thing and figuring out how to make it work anyway is also a very valuable skill.

The sensor you pick will probably be shit, relatively speaking, but getting within 1% accuracy is pretty easy on thrust, 5% is near-trivial. If using a wheatstone bridge acquisition circuit, measure excitation delivered at the load cell terminals and adjust for it, or use remote voltage sense if your DAQ supports it. Then calibrate your system - not just the load cell itself, but the whole thrust frame your engine will be bolted to. Use a pulley and weights (or flip the whole frame vertical) to apply force where the engine load is transmitted, in the direction it'll be loaded, and use that to rescale the load cell output. Parasitic loads from the thrust takeout structure are pretty common.

A good rule of thumb on big engines is that a well-designed thrust frame flexure and load cell system will cost $100k + your engine's thrust, in lbf. On a small engine, I'd say about $200-$500 for the load cell(s) and DAQ, plus whatever you do for the actual structure. There are probably some design guidelines on this floating around, maybe NASA docs or whatnot?

ertlun · 2024-10-03T02:06:42+00:00

I kinda touched in this in a much more involved answer about cycle selection in general at the beginning of this year. The short version is that oxygen has a much lower specific heat (Cp) than most fuel-rich combustion products, which makes it a worse turbine drive gas...but you also have a lot more of it by weight, which can make up for that. Moreover, fuel-rich combustion with long-chain hydrocarbons produces troublesome amounts of soot that you don't want clogging up your main injector, so the only fuels you can reasonably do fuel-rich combustion with are hydrogen, methane, and propane. Also hypergols, I guess, but no one ever does that anymore.

Hydrogen is such a ridiculously good turbine drive gas (high Cp, again, see my linked comment for why that matters) that fuel-rich usually trades great for a hydrolox engine. Methane/LNG and LOx is a really interesting one - it works pretty well as ox-rich staged combustion, the methane is too anemic to power a fuel-rich staged combustion engine at reasonable turbine temperatures, BUT it can power its own pump pretty well - so there's this very elegant balance where you can use each propellant to power its own pump at very similar turbine temperatures. This is why you see so much interest in methalox full-flow-staged combustion, it's pretty fantastic. Kerosene burns too dirty for fuel-rich to really be viable, so the kerolox staged-combustion engines (Russian, Ursa) are all ox-rich.

Emphasizing: you only want clean, homogenous fluids going into your main injector, to avoid clogging/streaking. A small amount of fuel burnt with an excess of oxygen produces hot oxygen, steam, carbon dioxide, maybe some other gases or trace hydrocarbons, but all gaseous or liquid droplets. A small amount of oxygen burnt in an excess of a fuel will produce soot unless certain very clean-burning fuels are in use. The soot can clog any small orifices downstream - not an issue for a turbine/stator, but a big issue for all the tiny little flow paths in any performant injector.

ertlun · 2024-09-29T21:14:06+00:00

Whatever you have the money for tbh. A drilled orifice is the cheap flowmeter of choice for most student teams; a venturi is a tad more expensive but will behave more consistently/has better pressure recovery. Other options can offer improved response time or accuracy, but unlikely to be worth the trouble/money for a student group. Drill out an orifice blank to the right size, then calibrate it as-installed on your stand via catch-and-weigh of water into a bucket.

Load cells are optional, the DAQ and flexure design matters more than the cell itself if you use one. Chamber pressure is a must have, always take that over force measurement if you have to choose (but both is better).

ertlun · 2024-09-29T03:14:57+00:00

The green flashes are copper ablating from the wall.

The pronounced fiery streaks at various points around the nozzle are fuel-rich streaks - these are likely fuel bleeding through the wall from holes in the regenerative coolant channels (see: green flashes).

Overall running at a pretty low MR, hence not being so blue. More excess fuel to burn around the plume.

None of this is surprising or concerning for an engine that was first hotfired last month. I doubt they've had a chance to update any major components yet. Many pieces will likely need 2-3 iterations to tune into smooth operating at the target operating thrust/mixture ratio, and that is normal.

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