Expander Cycle Rocket Engines – Using Waste Heat To Drive Your Rocket

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Another installment of ‘Things Kerbal Space Program Doesn’t Teach’ – explaining the expander cycle rocket engines in more detail. Expander cycles use the waste heat from the combustion chamber and nozzles to boil liquid hydrogen and power the turbines. The main advantages are cooler, less chemically active turbine environments, but if used in a closed cycle design the total thrust is limited.

Most of this material is at a pretty high level, I’m not a rocket scientist, I only play one on the internet.

Most of the flow schematics are taken from this excellent article by William Green

I also took some o

Also, Emre Kelly’s story on the Dragon capsule RUD

John Kraus’s Delta IV photo is available from his site:

  1. Yenadar

    Ha! Humorous to hear a different way of saying the conclusion I came to a few years back:
    – The essence of any engine is control and management of pressure differentials.

  2. timothy vince

    I wonder if it would be also possible to use smaller internal rocket powered venturi pump jets as propellant pumps,.. ..at least say for fuels and oxidizers that are thermally stable and/or won’t polymerize.

  3. Вася Сиськин

    Next time tell about Combustion tap off cycle

  4. highvoltagefeathers

    Don’t forget about the Relativity Space Aeon, it’s going to be a fully 3D printed open expander cycle engine.

  5. nmnmnm


  6. SoLongSidekick

    2:23 feels pretty prophetic today.

  7. ReptarGG

    Any details on that shirt? Looks really cool.

  8. thoriginalSpaceDiver

    Hi I was wandering if there has ever been a idea to use magnetic constriction to adjust the nozzle geometry?

  9. Billy Doyle


  10. TheAtom

    Where did you get that Starship & Starhopper model?

  11. KnightsWithoutATable

    Some things just work so well they keep getting used for decades.

  12. omdev shastri

    Thanks for knowledge sir…………..XD

  13. slikrx

    How do they keep the boiling liquid from creating massive back pressure, or simply locking a check valve closed?

  14. Maarku

    Scott, if you ever make it out to Utah (my home state) you need to go out to ATK and experience the booster rocket tests!

  15. penroc3

    hafnium alloys are definitely worth looking into, i think a ICBM used the expanding nozzle if i remember correctly

  16. Markle2k

    11:23 Metricphiles getting triggered by all these USC/”Imperial” units. Not just feet, inches, and pounds-mass, but BTUs and Rankine!

  17. Geof Cornelsen

    Scott, you’re a computer programmer. I know that takes smarts, but how in the world did you teach yourself the complexities of all this rocket science? I’m amazed at your level of knowledge! Very impressive!

  18. Bouzsi

    To be fair, an exploding Dragon engine *is* kind of the most expander an engine could be…

  19. PsychoLucario

    so they’re like the rocket equivalent of air cooled engines, limited by their size due to the surface area requirements to manage heat

  20. FalcoFury

    I love learning from you, Scott. Your voice is like my light in the dark.

  21. Dimitri Edgar Metz

    Wow, all this data busted into my head. I need a 30 minute YouTube break now.

  22. Markus Daxamouli

    Thank you for even attempting to explain this..

  23. david johanson

    I not Scott Manley and I can’t fly safe. 🤤

  24. cdrbmw

    Great video as always. Thank you very much.

  25. Marcelo Miranda

    its much easier than staged combustion cycle and very efficient to “small engines”.
    although not lose it elegancy

  26. Uncle Ned

    It is truly amazing how something so impossibly complicated is made to seem trivial.

  27. M Pidg

    I thought I was pretty sharp with propulsion with a career in business and some military aviation. Space technology…how hard can it be.? Well you just crushed me with this one. Thank You. I needed a come-uppance. 😉

  28. making in China

    You wanna know why the delta’s exhaust it red?
    …something something …soviet something something joke…

  29. Владимир Кузнецов Vovacat17

    Rocket Lab be like “screw you, li-ions are the best”

  30. Synchronizor

    I love expander-cycle engines. It’s just such an elegant concept.

  31. rune12358

    Hey, I was in that twitter conversation! How weird it is to be replied to by elon. My twitter exploded, and I went from zero to ten followers in an hour (I don’t tweet much! ^^’). Still, the conversation with Tory was pretty awesome in its details. :)

  32. Jan Negrey

    Thank you for explaining that. I did know that limitations in the Closed system were due to cube-square law, but it is nice to refresh memory.

  33. artemkras

    I appreciate the new Starship models behind you )

  34. BrokenLifeCycle

    So the dual expander cycle is kinda like the full-flow version of expander cycle engines.
    I do wonder, though. Do rocket engines overall gain any significant performance when it preheats its fuel and/or oxidizer as a coolant before burning it?

  35. RocKiteman _ 2001

    I’m Scott Manley. TURBO-pump SAFE…😊

  36. Nazamroth

    I have an idea! We should pre-mix the oxidizer and fuel in one bigger fuel tank to reduce the complexity of the rocket designs! Less tanks, fewer moving parts, more fuel!
    What could possibly go wrong?

  37. Thomas Chow

    compare closed dual split expander cycle with full-flow staged combustion? man that’d be so technical but I’ll be so happy

  38. Željko Radojković

    I still prefere the *Full-flow staged combustion cycle*

  39. Phos9

    I’m kinda curious if powering the turbopumps by tapping exhaust from the engine bell has ever been considered.
    Haha my question was answered later in the video. I wasn’t expecting it to come up, it seems like a pretty different principle from the expander cycle.

  40. Against NAZO!

    Make the rockethull to a giant capacitor and then drive electric pumps.
    My proposal. Greetings from Germany.

  41. Rozni Yusof

    I wish he would also talk about the specific impulse or exhaust velocity of the engines as well as thrust.

  42. 44R0Ndin

    I do think that a RL-25 with a different nozzle optimized for pure vacuum use might get better specific impulse than the RL-10.
    I KNOW that it would get much better thrust than the RL-10.

  43. Jeff Vader

    The more you learn about rocket science, the more complicated it gets!

  44. zapfanzapfan

    Those were some cool (or maybe hot…) new expander cycles I had not seen before. Thanks for some rocket design with my morning coffee!

  45. Scott FW

    With its liquid fuels boiling in to gasses it _almost_ sounds like an expander cycle engine is a type of steam engine.

  46. Asger Vestbjerg

    NIce explanation
    Thanks for sharing :-)

  47. Duck Goes Quack

    5:32 I dont know about the “will”part. Bfr may kill the program.
    Dont get me wrong, the more spaceships the better. But if there’s a cheaper option, I dont see why they wont pick that.

  48. Strati Mitchell

    That blog post, and the follow up NTR post, are PURE GOLD Internet manna!

  49. harbl99

    “Yeah, we get a good power/weight ration out of this thing, but you know what we _really_ need to do?”
    “Slap a turbocharger on it.”


    I believe the combustion chamber geometry must be changed.In this way we can use back pressure to operate the fuel and oxygen pumps. But we need extreme cooling solutions for components.

  51. Major Dakka

    Scott: …neutral hydrogen gas…
    Hydrogen embrittlement: what am I, a joke to you?

  52. Jean-Louis Beaufils

    IIRC the RL-10 was used on DC-X and was also planned for DC-Y. So it WAS used as a liftoff engine.

  53. RichTheWolfGaming

    Absolutely no one:
    Some batshit crazy rocket scientist: Gas Generator Augmented Expander Cycle
    Me: What in the name of Jebediah Kerman’s Mother

  54. Dr Vidya

    All these possible engine design variants.
    Is there a game where you can build all this plumbing?

  55. Renato Digitalo

    hey scott
    can you link your rocket models somewhere?
    im interested, maybe you said it sometime somewhere but i cant find info on it…
    thanks and of course fly safe

  56. Elon Musk Fan

    Scott Manly video is the highlight of my day
    I’m not a Scott Manly bot btw… But that’s exactly what Scott Manly supporting bot would say….

  57. Sean McDonough

    5:32 – _Powered_ the space shuttle, unless they pulled them back out of the museums while I wasn’t looking.

  58. Colby Baker

    Hey, a couple of us were wondering about a hybrid between the expander cycle and the expander bleed cycle where you can switch back and forth to suit your needs. Would that work? Idk with the turbo pumps probably being so different for each.
    Also, could you drive only one pump with the expansion cycle and the other with another cycle to maximize efficiency and thrust?

  59. Wilford Lewis McGee

    in my office at work i have that F1 @0:52 engine turbopump assembly cutaway hanging on my wall 36×24.
    great conversation starter

  60. AverageJoe8686

    I now have a 6″ Newtonian on an Equatorial with a clock drive. I’m super exited! :D

  61. shadfurman

    I knew Bob Ross was cool and was doing Bob Ross impersonations before the world wide web, in the days of gopher, Veronica, and Archie.

  62. Samovar maker

    Would the expander cycle work well with Liquid Methane?

  63. Ross h23

    The rl 10 is a workhorse of an engine logging so many flight hours awesome look into how it works

  64. diGritz1

    Well these all seem a bit better then some of my concepts but I remain confident.
    Although my current prototype was quite promising. Due to refinements in the design and upgrading the rotational power distribution packages I have dramatically improved efficiency. This has allowed me to eliminate 3 of the 7 guinea pi…….. engines. This in turn gives me better balance, allows a reduction in lettuce…… fuel and reduces the overall waste produced by the 4 remaining engines. I will continue to be inspired by my hero Ralph Kramden. I too just want to send my loved ones to the Moon.

  65. Gerry Larsson

    I wish I understood 3% of what you’re saying, but it’s absolutely fascinating. Okay maybe I understand 3% but I love how much detail you get into. Thanks again, Scott.

  66. David Messer

    I’m amazed that after so many years that rocket design hasn’t settled on a few basic designs that everyone uses. It shows that rockets are still in their infancy and have a lot of improvements yet to go.

  67. Dylan Rutledge

    could you do a video about the MLS (maritime launch service) in Canada. The will be launching the ukrainian cyclone rocket

  68. Jaran Noiseux-Mackay

    Can you do a vijeo on modern solid rockets? All this edumaction on liquid rockets makes me realize that they must me more complex than I’ve thought about…

  69. tihzho

    Cryogenic H2 & He … In another life I use to service vacuum pumps and systems. The most unusual pump I had to service (for the first time) was a cyro-pump using liquid helium in a “cold head” that’s purpose was the condense and freeze all the residual gas left in the vacuum chamber. The liquid helium was produced by the system and there was a “temperature gauge” to indicate when the helium was cold enough to liquefy. How this was done was with SS sealed system pipe system with pressure gauge and a small bulb in the cold head and was filled H2 at 7 bar.
    The pressure gauge scale wasn’t in pressure, rather it read the lower end of the Kelvin scale. The idea was that as the temperature dropped to cryogenic levels so did the pressure. The gauge needle would sit there doing nothing and then would slowly drop to 14K at which point the H2 would freeze in the bulb. The pressure would still drop past this point as more H2 condensed and froze. From memory the low end of the scale was 4K which is the temp of LH2.
    The reason for the service was they opened the connection to the gauge and let all the H2 out so it didn’t work as it should. All I had to do was evacuate the system, fill with H2, evacuate again to and then fill to 7bar. The client was very surprised that it was so simple to fix and they just paid a lot of money for me to do something they could have done.
    Its the old story – “you’re not paying me for turning the screw; rather you’re paying me for knowing which screw to turn.” haha

  70. Rik Schaaf

    Would it be possible to have an open/closed hybrid expander cycle engine? That way you could run the engine in the open cycle configuration when more thrust is needed, and in closed cycle config when the thrust is not required, so that you can take advantage of the higher efficiency.

  71. Antoine

    6:18 You forgot about the RL10A-5 wich flew on the DC-X wich was used in atmospheric conditions

  72. Bevin Smeith

    I like how the Rutherford engineer upper stage has two batteries one ejects when drained reducing the total weight to SECO down just a wee bit.
    Your going to need all the efficiency you can get with a 17m tall rocket.

  73. Ilya

    One of the best channels on YouTube. Keep up the good work.

  74. nesa1126

    This made me wonder how tf merlins survive all that and actually can be reused…

  75. James Monahan

    Is the RL-10 the engine that whistles at 80,000hz? And damaged a payload back around 2009?

  76. Sean McDonough

    7:01 – Couldn’t you (theoretically, at least) get around this by using a fuel-oxidiser combination with a higher flame temperature?

  77. Jerry Rupprecht

    Thanks for expanding my knowledge on this subject!

  78. Khitish Kumar Bhol

    And the Japanese are going with the (Open) Expander Bleed Cycle for their rocket engine LE-9, which solves the limiting thrust problem of Closer Expander Cycle, so it can give high thrust while also reducing complexity and cost albeit at a less efficiency.
    But the low cost n simplicity of it is unmatched by others I think.

  79. John Campbell

    Hast thou forgotten the DC/X (and DC/XA)??

  80. Joe Holland

    Just in case you don’t know what 472 Rankine is, its -10C or 12F or 262K.

  81. YZFoFittie

    Expander Bleed Cycle is just another word for, “turbo waste gate”… CHANGE MY MIND!

  82. Jake S

    Ah, the smile in my face when I saw 14m43s of video!

  83. Nuran On

    tl;dr: Expander Cycles are the rocket engine equivalent of Pulling Yourself Up By Your Bootstraps

  84. Spike The Lizard

    The RL-10 is my Realism Overhaul vacuum engine of choice!

  85. Kevin Street

    Not going to lie, I had to back up and replay parts of this video a bunch of times – but it was really fascinating! Thanks for the introduction to Expander Cycle rocket engines!

  86. Daniel Hems

    Nice one Scott !!!! super informative !! extremely high KTT ratio (Knowledge To Time). loved it …

  87. Norman Mattson

    I hope the day will come when we look upon this technology as being antiquated and quaint kind of like steam locomotives.

  88. Magnus Skaug

    Kerbal space program doesnt teach. docking ports

  89. Major Dick

    Idunno what Bob Ross was doing in that web page but now all I can think about is adding a ‘happy little rocket’ to the background of a painting.
    ..Maybe some ejecta from stage-sep
    .. Little sun glistening off the main body
    .. and a slightly broken con-trail

  90. himssendol

    4:10 that cut out gives you a view of how intricate and technologically advanced these engines are. And apart from spacex all these are thrown away after one use. 🚀🗑😱

  91. injustice fellow

    This is low-key a things Kerbal space program doesn’t teach episode

  92. galfisk

    This was great, please do more of these more in-depth videos on other cycles, other engines, and other space tech stuff.

  93. Ivo Drinković

    Just w8 for the day this chanel hits long deserved 1 milion subs

  94. Festivejelly

    Maybe you could expand on this? :P

  95. phaseed

    Thank you Scott for talking about a great Engine like the RL-10, but Sorry Scott RL-10A5 used on the DCX and DCXA launched many times. I worked on the engine test program from 1991 to 1995 for P&W. You also didn’t note that the RL-10A5 was very throttleable from down to 5% to 100% in less than 3 seconds, This is why it was selected for the CC program to land on the Moon. Most likely will be again just a modified Aces engine RL-10C-5. Best regards Ed.

  96. chris_0611

    DC-X was a ‘first stage’ that was lifted against Earth Gravity, using 4 RL-10’s!
    Another cool idea is to use an expander cycle with an Aerospike nozzle. This would supposedly give some more nozzle area so the total expander cycle engine could have more thrust than with a de laval nozzle (it would ‘scale’ better).

  97. MrVipitis

    I love how Elon, Tim, Tory, Destin and Scott just have a casual discussion in rocket engines on in some tweet reply chain of something unrelated.

  98. Mikicat

    5:38 when you see this after Simple RL-10 schemat

  99. SmarterEveryDay

    I feel like you made this video to answer my questions. Thank you very much, it’s great.

  100. Recoil Rob

    I worked building RL-10’s back in ’89-90 when they were developing the improved thrust version that has the moveable skirt and they really ran into problems. When you increase the fuel flow needed to increase the thrust the skirt cools down which then reduces the expansion which slows the pumps. It’s a really delicate balancing act on the early engines and the first attempts to increase the thrust did achieve more but they lost impulse which is no good for a second stage engine. The nozzles supplying fuel and oxygen in the ‘shower head’ were flowed then carefully adjusted by hand to get exactly the flow needed which was a bit tedious and often took many trips to the flow test to get it right. Happily things were worked out and the engineers at P&W were and still are the finest in the world.

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