Are Aerospike Engines Better Than Traditional Rocket Engines?(everydayastronaut.com)
everydayastronaut.com
Are Aerospike Engines Better Than Traditional Rocket Engines?
http://everydayastronaut.com/aerospikes/
78 comments
Here's an interesting interview the author did with Elon Musk: https://www.youtube.com/watch?v=cIQ36Kt7UVg
Fun to see how interested Elon is with the engineering of rockets, looks like he could talk for days if he had the time.
Fun to see how interested Elon is with the engineering of rockets, looks like he could talk for days if he had the time.
This is such a good interview. Lots of engineer philosophy dropped here. Love his approach on thinking things are always wrong and to constantly question interfaces and standards.
"All models are wrong, but some are useful"
https://en.wikipedia.org/wiki/All_models_are_wrong
https://en.wikipedia.org/wiki/All_models_are_wrong
Apart from climate models of course. Science is settled.
That's why researchers should always report uncertainty bounds. On that note, I don't think I've ever heard a researcher give a definitive answer as to what climate change _will_ do. We can very precisely measure the change in ocean surface temperature and acidity and come up with reasonable guesses for the amount of CO2 added to the atmosphere every year, but the future always involves additional uncertainty.
This simply isn’t true. Climate science has an incredibly long history of failed predictions, with the less specific predictions being obviously more resilient to falsification. If you care about the existence of climate change deniers, then you should know who Competitive Enterprise Institute are, most writing on climate change denial can be traced back to their publications. The reason being that they have an essentially limitless supply of alarmist predictions that have been falsified.
https://cei.org/blog/wrong-again-50-years-failed-eco-pocalyp...
https://cei.org/blog/manufacturing-alarm-dana-nuccitellis-cr...
https://wattsupwiththat.com/2017/10/30/some-failed-climate-p...
Just look at all the other child comments, most of them immediately jumped to the defence of supposedly irrefutable climate models. The reason climate deniers have so much support is that climate scientists, their communicators and their advocates have little credibility outside the true believers. If you don’t want to take my word on it, here’s a Stanford study on this exact topic:
https://www.sciencedaily.com/releases/2019/10/191018112145.h...
https://cei.org/blog/wrong-again-50-years-failed-eco-pocalyp...
https://cei.org/blog/manufacturing-alarm-dana-nuccitellis-cr...
https://wattsupwiththat.com/2017/10/30/some-failed-climate-p...
Just look at all the other child comments, most of them immediately jumped to the defence of supposedly irrefutable climate models. The reason climate deniers have so much support is that climate scientists, their communicators and their advocates have little credibility outside the true believers. If you don’t want to take my word on it, here’s a Stanford study on this exact topic:
https://www.sciencedaily.com/releases/2019/10/191018112145.h...
Climate models are a perfect example of the above statement. They're not good enough to predict the detailed future with certainty (weather forecasts only go out for a few days), but they're useful and valid for reasoning about large scale trends.
I wholeheartedly believe in anthropogenic climate change and I disagree with this comment with every fiber of my being despite not knowing which "side" you're actually on.
I'm actually kind of impressed.
I'm actually kind of impressed.
I’m on neither side. I also wholeheartedly believe in anthropogenic climate change, but I think the commentary on climate science is about as ignorant and anti-scientific as the views espoused by young earth creationists. Climate models are flimsy as hell and have an atrocious track record of making predictions. Yet “the science is settled” (or a close approximation to it) is the only acceptable view in most circles. This gives climate change deniers a remarkably strong platform, because they can just say “climate scientists (or more often climate science communicators) are full of bs” and then point to countless examples where that is absolutely true. Science is never settled, in any case that you think it is, you’re not dealing with science, you’re dealing with an opinionated dogma. A statement which almost everybody would agree with, on any topic except climate change.
Computer models are used to try to predict the local effects of climate change, not prove whether it’s real. The basic science is settled, and no one needed computers to do it. Heck the hypothesis was first proposed over 100 years ago.
Computer climate models have trouble returning accurate predictions for exactly the same reasons it’s hard to make accurate weather forecasts. They’re attempting to do things on the edge what we know is possible. Personally, I would not call that “flimsy as hell.”
Computer climate models have trouble returning accurate predictions for exactly the same reasons it’s hard to make accurate weather forecasts. They’re attempting to do things on the edge what we know is possible. Personally, I would not call that “flimsy as hell.”
Relativity is a model. Yet it is settled.
Still 'wrong' though.
Still 'wrong' though.
Seems like a pretty creative use of the word ‘settled’.
F = M * A is never correct. Yet, it’s simple and useful enough in most situations to be ‘settled’ and people will probably still use it in 10,000 years. Thus, demonstrating these are different qualities when it comes to models.
The key feature of science is that all theories are open to further scrutiny and falsification. To say “the science is settled” is probably the least scientific statement you could possibly make. A deeply ironic position to take for people who’s key complaint about their detractors is that they don’t appreciate science.
When comparing two alternatives, we can most definitely conclude that the science is settled, at least until actual evidence to the contrary shows up. Being open to accepting new evidence in the future in no way means we can't act confidently on what we know now, taking into account our state of knowledge/uncertainty.
The science on a heliocentric solar system is settled. The science establishing a link between anthropogenic carbon emissions and average global temperatures is also settled - built on foundational chemistry and physics, supplemented with observational evidence here on Earth, and validated by the planetary science done to understand Venus's temperature.
Sure, there's a lot of remaining uncertainty about short term effects, but mostly in the category of "just how bad is it likely to get?"
The science on a heliocentric solar system is settled. The science establishing a link between anthropogenic carbon emissions and average global temperatures is also settled - built on foundational chemistry and physics, supplemented with observational evidence here on Earth, and validated by the planetary science done to understand Venus's temperature.
Sure, there's a lot of remaining uncertainty about short term effects, but mostly in the category of "just how bad is it likely to get?"
Science as a search for truth cares deeply about accuracy, engineering doesn’t. Discovering say F=MA was wrong was a huge deal, but existing results still apply. New models fill in gaps, but the improved models needed to account for everything the old one correctly predicted.
So while science moves on, engineering does not care about the 15th decimal place. Which is why we keep getting value from approximations.
So while science moves on, engineering does not care about the 15th decimal place. Which is why we keep getting value from approximations.
> This is such a good interview. Lots of engineer philosophy dropped here.
The interview is great, and it really highlights the difference in the way that founders and engineers think compared to those in the finance and commercial fields. If this was an investor interview, they'd be asking about deadlines and revenue projections. If it was a Joe Rogan interview, the questions would be vague questions about social, political, or philosophical repercussions. Those are all important questions, but they don't actually address what any business or organization is really trying to do. You can learn a lot more about a person or organization by speaking their language and letting them talk the way they are comfortable than by stuffing them into your own rubric of what they should be expected to say.
The interview is great, and it really highlights the difference in the way that founders and engineers think compared to those in the finance and commercial fields. If this was an investor interview, they'd be asking about deadlines and revenue projections. If it was a Joe Rogan interview, the questions would be vague questions about social, political, or philosophical repercussions. Those are all important questions, but they don't actually address what any business or organization is really trying to do. You can learn a lot more about a person or organization by speaking their language and letting them talk the way they are comfortable than by stuffing them into your own rubric of what they should be expected to say.
He is the lead designer! So he must be interested quite a bit to do that on day-to-day basis.
Yeah exactly. I think a lot of people don’t realize that. Elon is so busy and no other person with his level of power is really doing that kind of ground work. But I think about it this way: Elon can do whatever he wants, and he loves rockets. Being the lead engineer on the first Mars colony rockets (and first reusable first stages) is his life’s work. It’s what he wants to be doing.
Everyday astronaut has some of the best space content on YouTube, which is even more impressive given that he has doesn't have any kind of formal training in it. If you are into space and especially spacex I would highly recommend his channel.
This video has been in the works for many months and knowing his work it is probably one of the most comprehensive works on aerospike engines outside of academia.
This video has been in the works for many months and knowing his work it is probably one of the most comprehensive works on aerospike engines outside of academia.
I love Tim Dodd! I'm actually really surprised to see him show up on hacker news. I've been watching his youtube videos for quite a while now.
Anybody else interested in space stuff should also check out Scott Manley. I also like Marcus House also has some really great stuff.
These guys focus a lot on SpaceX but they also do a lot of other videos on general space stuff.
Marcus House: https://www.youtube.com/channel/UCBNHHEoiSF8pcLgqLKVugOw
Scott Manley: https://www.youtube.com/user/szyzyg
Anybody else interested in space stuff should also check out Scott Manley. I also like Marcus House also has some really great stuff.
These guys focus a lot on SpaceX but they also do a lot of other videos on general space stuff.
Marcus House: https://www.youtube.com/channel/UCBNHHEoiSF8pcLgqLKVugOw
Scott Manley: https://www.youtube.com/user/szyzyg
Tim Dodd is great! An alternative great podcast (that Todd backs) is Main Engine Cutoff (MECO). Highly recommend you give it a listen if you are interested in Space News, and thoughtful discussion on recent topics.
https://mainenginecutoff.com/
I’ll have to check our Marcus House. Scott Manley is so good! Super talented.
> Everyday astronaut has some of the best space content on YouTube.
Agreed, Everyday Astronaut and Scott Manley are my go to for space and rocket news and videos.
Agreed, Everyday Astronaut and Scott Manley are my go to for space and rocket news and videos.
In a previous article, he renamed the gas generator on the Merlin and F-1 rocket engines a "preburner" and now everyone is copying him.
> ... given that he has doesn't have any kind of formal training in it.
As someone who does have some training in it, I can say that "best space content on YouTube" is akin to "best brain surgeon in a room full of circus clowns."
He gets a lot of stuff right, don't get me wrong. But the appearance of credibility is down to enthusiasm, specialty of focus, and editing and delivery. It's fine as far as it goes, but it's infotainment. It's more important that you be entertained and feel smart than sit down and struggle with the actual application of engineering principles and achieve any kind of reliable knowledge.
As someone who does have some training in it, I can say that "best space content on YouTube" is akin to "best brain surgeon in a room full of circus clowns."
He gets a lot of stuff right, don't get me wrong. But the appearance of credibility is down to enthusiasm, specialty of focus, and editing and delivery. It's fine as far as it goes, but it's infotainment. It's more important that you be entertained and feel smart than sit down and struggle with the actual application of engineering principles and achieve any kind of reliable knowledge.
You're conflating "doing brain surgery" and "learning about how the brain works". No one is claiming to be an aerospace engineer after watching one of his videos.
Taking an esoteric concept and making it understandable for the masses takes far more talent and dedication than you might think, and it's something that experts often fail at (or don't attempt). This guy makes no false claims about his credentials or the intended purpose of his videos, and your condescending gatekeeping shows a real lack of understanding for what this guy is actually doing and why it's so valuable.
Taking an esoteric concept and making it understandable for the masses takes far more talent and dedication than you might think, and it's something that experts often fail at (or don't attempt). This guy makes no false claims about his credentials or the intended purpose of his videos, and your condescending gatekeeping shows a real lack of understanding for what this guy is actually doing and why it's so valuable.
What I'm saying is that the video contains numerous misconceptions, errors, and dubious bits of hand-waving. I'm not saying that people think they're engineers after watching. I'm saying that he is not trained as one, and that it shows in his work. I'm also saying that "what this guy is doing" and "why it's so valuable" are both undermined by his lack of training.
Long ago, some cranky academic wrote on his very well-sourced and specialized website, back when there was still that sort of thing, something like, "Everybody said that the internet was going to let anybody say anything at all. It turns out they were right."
I'm fine with the backlash from what you call my "gatekeeping." (I imagine that some gates need to be kept, but I suppose that's another story.) My point is that "the best on YouTube" is not as grand a qualification as one might suspect, and is certainly not a guarantee of accuracy.
Long ago, some cranky academic wrote on his very well-sourced and specialized website, back when there was still that sort of thing, something like, "Everybody said that the internet was going to let anybody say anything at all. It turns out they were right."
I'm fine with the backlash from what you call my "gatekeeping." (I imagine that some gates need to be kept, but I suppose that's another story.) My point is that "the best on YouTube" is not as grand a qualification as one might suspect, and is certainly not a guarantee of accuracy.
I’d be curious about the errors and inaccuracies - I’m not trained in the field, and to me it didn’t seem like he got into any real detail about how the aerospike actually worked from a physics perspective other than that it did indeed work. Seemed magical.
Expound on "some gates need to be kept." This forum comments on medical topics, finance, economics, and software. I'd argue that for the majority of people only the last one is something they are experts in.
You could've compared this to a brain surgeon among medical students, or even software engineers, something technical/noncreative.
You could've compared this to a brain surgeon among medical students, or even software engineers, something technical/noncreative.
> the video contains numerous misconceptions, errors, and dubious bits of hand-waving.
Would you care to give a few examples so that I can appreciate their significance? Are they major errors or are you nitpicking?
Would you care to give a few examples so that I can appreciate their significance? Are they major errors or are you nitpicking?
Well, just like every other edutainment channel out there like Veritasium or SmarterEveryDay. Nothing wrong with that, most of us are not trying to become aerospace engineers or physicists.
I like my entertainment to also be reasonably accurate so I'll take it. He doesn't make me feel smart, but his content does fill a curiosity itch. Especially since I will not be considering a career in rocket engineering!
If you like his content, Go with Scott Manley. He has at least some formal training as he is an astrophysicist by trade. Massive knowledge and in-depth bits of info.
Scott Manley is a Software Engineer, currently at Apple.
Formal education != employment != what you actually know.
(Scott Manley wins at all three, though.)
(Scott Manley wins at all three, though.)
And did a few years at Napster.
By education at least then, with a bs/ms in the area.
> specialty of focus, and editing
These two are perfectly legitimate ways of using the medium of video to avoid making mistakes.
These two are perfectly legitimate ways of using the medium of video to avoid making mistakes.
[deleted]
This has been a labor of love for Tim - and most of the space community has been watching for this to come out. He recently had a interview with Bridensteine at NASA (who despite expectations seems to be doing a amazing job at NASA) and a geek out moment with Elon Musk. He's shifting the reporting of space news.
It is a pet peeve of mine that people call all spike engines "aerospikes." An aerospike engine has a truncated physical spike that is replaced with expanding gasses. In other words, it has an aerodynamic spike, or aerospike. Spike engines that taper to a line (for linear engines) or a point (for cylindrical engines), are just "spike engines."
I know I've lost this one among popular and lay audiences. It's just way too cool to say 'aerospike.'
I know I've lost this one among popular and lay audiences. It's just way too cool to say 'aerospike.'
Pet peeve of mine is that you could have a missile propelled by an aerospike that also uses an aerospike to decrease drag.
I was under the impression that the linear spike engines also used the mechanism of the air pressure keeping the gases in the right shape?
The issue is that an aerospike engine is a spike engine that has been truncated, with the truncated portion replaced with an aerodynamic "spike," which also eliminates base drag.
Is it even practically possible to have a non-truncated spike engine? If you don't truncate the spike, won't it get so thin that it'd just burn/melt off? It seems like all spike engines are truncated to at least a minimal degree.
Or is the matter whether or not the truncation has aerodynamic effects? (Do any truncations not?)
Or is the matter whether or not the truncation has aerodynamic effects? (Do any truncations not?)
> Throughout history, there have been a number of aerospikes that have made it really far in development, but to date, none have ever really flown
http://www.astronautix.com/g/garveyaerospike.html
During 2005, GSC and California State University, Long Beach (CSULB) conducted several notable small launch vehicle R&D activities through their partnership in the California Launch Vehicle Education Initiative. Using a single-chamber, liquid-propellant, annular aerospike engine concept developed by CSULB, the GSC/CSULB team validated the basic design and ignition sequence with a successful static fire test at the Reaction Research Society's Mojave Test Area (MTA) in June 2003. The team then mounted one of these 4,444-newton thrust LOX/ ethanol ablative engines onto their Prospector 2 vehicle and proceeded to conduct the first-ever powered liquid-propellant aerospike flight test at the MTA in September 2003. In response to several issues observed during that flight, modifications were made to the engine fabrication process. Another flight test with the Prospector 4 vehicle followed in December 2003. Performance was entirely nominal, resulting in complete recovery of the vehicle and key trajectory data.
http://www.astronautix.com/g/garveyaerospike.html
During 2005, GSC and California State University, Long Beach (CSULB) conducted several notable small launch vehicle R&D activities through their partnership in the California Launch Vehicle Education Initiative. Using a single-chamber, liquid-propellant, annular aerospike engine concept developed by CSULB, the GSC/CSULB team validated the basic design and ignition sequence with a successful static fire test at the Reaction Research Society's Mojave Test Area (MTA) in June 2003. The team then mounted one of these 4,444-newton thrust LOX/ ethanol ablative engines onto their Prospector 2 vehicle and proceeded to conduct the first-ever powered liquid-propellant aerospike flight test at the MTA in September 2003. In response to several issues observed during that flight, modifications were made to the engine fabrication process. Another flight test with the Prospector 4 vehicle followed in December 2003. Performance was entirely nominal, resulting in complete recovery of the vehicle and key trajectory data.
What altitude did it reach?
1370 meters (4,500 feet), apparently.
http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.544...
http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.544...
Thanks. The air pressure is about 85% of sea level pressure at that altitude. Maybe that's enough to get some data about how the engine responds, but it's nothing like a real launch, and I think it's borderline fair for Tim Dodd to say these that "none have ever really flown" in the context of orbital rocketry.
I think Tim didn't know about that launch, that's why decided to add to the discussion. Not having data is not the same as not having flown; not only the flight has symbolic value but also, I'm sure, the flight serves as an incentive to all participants, especially students. 1.3 km altitude for a liquid fuel rocket is a very real launch, believe me, for amateur participants.
In the context of the video, which focused on technical details, I personally don't think the symbolic or incentive value is relevant. I'm glad you raised the point though.
The abrupt jump in the altitude plot looks like that sensor was malfunctioning.
Tim Dodd is super charming. He's such an antidote to common cynicism. He's approachable and passionate and humble. And cool that he interviewed literally the top 3 rocket CEOs in the US, all accomplished professionals with each having a different perspective (but having sort of a consensus on this topic).
I have a bit of a polarizing view about the fascination for things such as aerospike engines, wenkel car engine, etc. While I think it’s cool, what’s cooler are technologies that are optimized across many dimensions from manufacturability to cost, performance to durability, across the entire spectrum. While I understand and appreciate the author’s passion, hard work and his ability to explain things so clearly; I find the fascination, almost fetishization of outlier technologies that have been proven suboptimal for many reasons, to be unexplainable. As an engineer, I am personally fascinated by practical technologies that are simple, cost effective, manufacturable, durable, etc etc, technologies that are optimized across a wide domain to be useful... such as Traditional rocket engines and Piston IC engines to be equally “cool” if not cooler.
But it’s good to learn why certain technologies don’t work and why they haven’t taken off. There is definitely something to learn from failures as much as to learn from optimal designs.
Overall, I applaud the author for this amazing video.
But it’s good to learn why certain technologies don’t work and why they haven’t taken off. There is definitely something to learn from failures as much as to learn from optimal designs.
Overall, I applaud the author for this amazing video.
As supporting technologies improve and materials science marches on, fringe technologies can sometimes leap quite suddenly into the mainstream. For example: the Stirling-cycle engine. It was a suboptimal technology compared to steam and IC engines for nearly 200 years, until Sweden found a use for it in their Gotland-class submarines. As it turns out, subs built with free-piston Stirling engines can run even more silently than their nuclear-powered competitors:
https://en.wikipedia.org/wiki/Gotland-class_submarine
https://en.wikipedia.org/wiki/Gotland-class_submarine
Nuclear powered submarines can be as quiet. They aren't typically as quiet as they could be, because they have pumps to circulate coolant around the reactor spaces, and they typically have the prop being directly driven by a turbine (through a gear train).
Both of those have solutions: natural circulation, and turbo-electric drive. Natural circulation is seen on the Ohio class submarines[0], in which the heat gradient inside the reactor is enough to cause it to circulate without powering a pump to move it around. It only works well at low power, but if you're trying to make a good imitation of a hole in the water, that's enough. And it's not like the Gotland-class is any better at going fast.
Turbo-electric propulsion was seen on the USS Tullibee[1]. It definitely was an advantage, in terms of making the submarine quieter. But it had a disadvantage in being heavier than the machinery required for geared drive.
And the USN wants nuclear submarines. A Gotland had a pair of 75kW generators onboard for power/propulsion. A S9G reactor drives a 30MW pumpjet on a Virginia class. SSNs can go faster when necessary, and have more power available to run all their systems.
[0] https://en.wikipedia.org/wiki/S8G_reactor [1] https://en.wikipedia.org/wiki/USS_Tullibee_(SSN-597)
Both of those have solutions: natural circulation, and turbo-electric drive. Natural circulation is seen on the Ohio class submarines[0], in which the heat gradient inside the reactor is enough to cause it to circulate without powering a pump to move it around. It only works well at low power, but if you're trying to make a good imitation of a hole in the water, that's enough. And it's not like the Gotland-class is any better at going fast.
Turbo-electric propulsion was seen on the USS Tullibee[1]. It definitely was an advantage, in terms of making the submarine quieter. But it had a disadvantage in being heavier than the machinery required for geared drive.
And the USN wants nuclear submarines. A Gotland had a pair of 75kW generators onboard for power/propulsion. A S9G reactor drives a 30MW pumpjet on a Virginia class. SSNs can go faster when necessary, and have more power available to run all their systems.
[0] https://en.wikipedia.org/wiki/S8G_reactor [1] https://en.wikipedia.org/wiki/USS_Tullibee_(SSN-597)
> what’s cooler are technologies that are optimized across many dimensions
That sounds boring to me - there's no room for improvement. The aerospike and wenkel engines and ramjets are exciting exactly because they have not been explored so well and there may be a lot of improvements yet to be discovered.
That sounds boring to me - there's no room for improvement. The aerospike and wenkel engines and ramjets are exciting exactly because they have not been explored so well and there may be a lot of improvements yet to be discovered.
The Pan-Am spaceplane in Kubrick's 2001: A Space Odyssey featured an aerospike.
Rest in peace Lockheed VentureStar :(
Amen.
That thing was so close to being ready; just let down by the composite tanks. The state-of-the-art in composites has advanced so much since the 80s, I'm sure if they tried again now they'd have no problem.
A let down by the NASA administration who testified to Congress that there was no point to it without the composite tanks. Even though the alternative Al-Li tanks already being designed were better by every relevant engineering metric. The X-33 could have launched but the funding was pulled.
SSTO would probably more relevant to return from Mars rather than to go there in the first place - building a solid booster or building a mars recoverable one might be crazy.
However, the atmospheric pressure in Mars is a lot lower than on earth, which means the same diameter scale issues don't apply.
However, the atmospheric pressure in Mars is a lot lower than on earth, which means the same diameter scale issues don't apply.
And the orbital velocity is also much lower, which makes talking about Mars SSTOs kind of weird in the same sense that talking about Lunar SSTOs is rather weird. (If you need a multi-stage rocket to get off of the Moon, I really want to see your mission architecture)
I think Starship is intended to go SSTE (Single Stage to Earth) from Mars, for instance.
I think Starship is intended to go SSTE (Single Stage to Earth) from Mars, for instance.
Yep, Starship should be definitely SSTO from Mars, Moon and likely about any non-Earth survivable (hello Venus!) landing spot in the Solar System.
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This is an excellent article.
This summer I read H&H and RPE as well as reading some papers and talking to rocket engineers, and this is the best article on aerospikes at a high level that I've read.
This summer I read H&H and RPE as well as reading some papers and talking to rocket engineers, and this is the best article on aerospikes at a high level that I've read.
What an extraordinarily comprehensive, comprehensible and informative article. I don't think I've read anything quite like it before.
Wow. One of the best articles I have read in a while. So many great analogies, made the material extremely easy to consume.
That's a rare talent.
That's a rare talent.
For a TL;DR this is a good video: https://www.youtube.com/watch?v=-SGIiO1APig
I remember EverydayAstronaut had a chat with Elon Musk about aerospikes recently, don't know if it was featured in the video.
I remember EverydayAstronaut had a chat with Elon Musk about aerospikes recently, don't know if it was featured in the video.
Another one https://www.youtube.com/watch?v=K4zFefh5T-8
Incredible Video!
Oh my god. I live aerospikes, and i love expander cycles, even though neither of them are actually practical, so the idea that each one might be able to solve the main problem with the other one has given me an enormous space boner.
What about expander cycle engines is not practical? The RL-10 which uses an expander cycle has been used for a very long time(1962) and it's future looks bright as well[1].
[1]: https://en.wikipedia.org/wiki/RL10
[1]: https://en.wikipedia.org/wiki/RL10
Ah, i was inaccurate, sorry. They are very practical - but there's been a limit to how big you can make them, based on the need to get enough surface area in contact with the exhaust to vapourise the fuel. AIUI, that's why they're great for upper stages, but haven't been used for first stages. Combining them with an aerospike might let them scale up to first-stage size.
Sorry for the late reply. I'm curious why you think an aerospike nozzle would help expander cycle engines scale up to bigger sizes?
Expander cycle engines are limited because you need heat to run the turbo pump. And the heat is provided by running propellant around engine to cool it off. The advantages Tim Dodd mentions for larger engines (propellant flow increases faster than surface area of the engine), are exactly the things that make expander cycle engines hit their limit: not enough area to heat up the propellant to run the pumps enough for the amount of propellant you need for the thrust you want.
Aerospikes have more surface area, which means more area to cool... which is more area for the propellant to flow through to get heated up to run the engine, which means more power available to give more fuel.
Aerospikes have more surface area, which means more area to cool... which is more area for the propellant to flow through to get heated up to run the engine, which means more power available to give more fuel.
> But all in all, I think the best way I can summarize the Aerospike is to compare it to like the rotary engine seen on cars like the Mazda RX-7 and RX-8 and their predecessors.
This is a great analogy.