View source for Expander cycle

{{short description|Rocket engine operation method}}
[[Image:Expander rocket cycle.svg|thumb|right|275px|Expander rocket cycle. Expander rocket engine (closed cycle). Heat from the nozzle and combustion chamber powers the fuel and oxidizer pumps.]]

The '''expander cycle''' is a power cycle of a [[bipropellant rocket]] [[engine]]. In this cycle, the fuel is used to cool the engine's combustion chamber, picking up heat and changing phase. The now heated and gaseous fuel then powers the turbine that drives the engine's fuel and oxidizer pumps before being injected into the combustion chamber and burned.

Because of the necessary phase change, the expander cycle is thrust limited by the [[square–cube law]]. When a bell-shaped nozzle is scaled, the nozzle surface area with which to heat the fuel increases as the square of the radius, but the volume of fuel to be heated increases as the cube of the radius.  Thus beyond approximately 3000 [[newton (units)|kN]] (700,000 lbf) of thrust, there is no longer enough nozzle area to heat enough fuel to drive the turbines and hence the fuel pumps.<ref name="y655"/> Higher thrust levels can be achieved using a bypass expander cycle where a portion of the fuel bypasses the turbine and or thrust chamber cooling passages and goes directly to the main chamber injector. Non-toroidal [[Aerospike engine|aerospike]] engines are not subject to the limitations from the square-cube law because the engine's linear shape does not scale isometrically: the fuel flow and nozzle area scale linearly with the engine's width.  All expander cycle engines need to use a [[cryogenic fuel]] such as [[liquid hydrogen]], liquid [[methane]], or liquid [[propane]] that easily reaches its [[boiling point]].

Some expander cycle engines may use a [[gas generator]] of some kind to start the turbine and run the engine until the heat input from the thrust chamber and nozzle skirt increases as the chamber pressure builds up.

Some examples of an expander cycle engine are the [[Aerojet Rocketdyne]] [[RL10]] and the [[Vinci (rocket engine)|Vinci engine]] for [[Ariane 6]].<ref>{{cite web |url=http://www.esa.int/Our_Activities/Launchers/Launch_vehicles/Ariane_6 |title=Ariane 6 |website=www.esa.int |access-date=21 February 2017}}</ref>

== Expander bleed cycle ==
[[Image:Expander bleed cycle (coolant tap-off).svg|thumb|right|250px|Expander bleed cycle. Expander open cycle (Also named coolant tap-off).]]
This operational cycle is a modification of the traditional expander cycle. In the bleed (or open) cycle, instead of routing all of the heated propellant through the turbine and sending it back to be combusted, only a small portion of the heated propellant is used to drive the turbine and is then bled off, being vented overboard without going through the combustion chamber. The other portion is injected into the combustion chamber. Bleeding off the turbine exhaust allows for a higher turbopump efficiency by decreasing [[backpressure]] and maximizing the pressure drop through the turbine. Compared with a standard expander cycle, this allows higher engine thrust at the cost of efficiency by dumping the turbine exhaust.<ref name=aiaa-paper2003-4597 /><ref name=mhitr-vol48n4p36t43 />

The [[LE-5A|Mitsubishi LE-5A]] was the world's first expander bleed cycle engine to be put into operational service.<ref name="tsj1993">{{cite web|url=https://www.jstage.jst.go.jp/article/tsj1973/21/3/21_3_138/_pdf|archive-url=https://web.archive.org/web/20210528150902/https://www.jstage.jst.go.jp/article/tsj1973/21/3/21_3_138/_pdf|script-title=ja:わが国の液体ロケットエンジンの現状と今後の展望|page=10|language=ja|author=Akira Konno|publisher=Turbomachinery Society of Japan/[[J-STAGE]]|date=October 1993|archive-date=May 28, 2021|access-date=January 24, 2022}}</ref> The [[LE-9|Mitsubishi LE-9]] is the world's first first stage expander bleed cycle engine.<ref name="nikkei20210202">{{cite web|url=https://xtech.nikkei.com/atcl/nxt/column/18/01540/00008/|archive-url=https://web.archive.org/web/20220124045358/https://xtech.nikkei.com/atcl/nxt/column/18/01540/00008/ |script-title=ja:H3ロケットの主エンジン「LE-9」熱効率向上で世界初に挑戦|language=ja|author=Shinya Matsuura|publisher=Nikkei Business|date=February 2, 2021|archive-date=January 24, 2022|access-date=January 23, 2022}}</ref>

[[Blue Origin]] chose the expander bleed cycle for the [[BE-3#BE-3U |BE-3U]] engine used on the upper stage of its [[New Glenn]] launch vehicle.<ref>{{cite web |url=https://www.blueorigin.com/news/new-glenn-completes-second-stage-hotfire |title=New Glenn Completes Second Stage Hotfire |publisher=Blue Origin |date=September 23, 2024}}</ref>

== Dual expander ==
In a similar way that the [[Staged combustion cycle (rocket)|staged combustion]] can be implemented separately on the oxidizer and fuel on the [[Full flow staged combustion cycle|full flow cycle]], the expander cycle can be implemented on two separate paths as the '''dual expander cycle'''. The use of hot gases of the same chemistry as the liquid for the turbine and pump side of the turbopumps eliminates the need for purges and some failure modes. Additionally, when the density of the fuel and oxidizer is significantly different, as it is in the [[Hydrogen|H<sub>2</sub>]]/[[LOX]] case, the optimal turbopump speeds differ so much that they need a gearbox between the fuel and oxidizer pumps.<ref name=sutton-rpe7edp221p227 /><ref name=ustpo-patent7418814 /> The use of dual expander cycle, with separate turbines, eliminates this failure-prone piece of equipment.<ref name=ustpo-patent7418814 />

Dual expander cycle can be implemented by either using separated sections on the [[Regenerative cooling (rocket)|regenerative cooling system]] for the fuel and the oxidizer, or by using a single fluid for cooling and a [[heat exchanger]] to boil the second fluid. In the first case, for example, you could use the fuel to cool the [[Rocket engine#Combustion chamber|combustion chamber]], and the oxidizer to cool the [[Rocket engine nozzle|nozzle]]. In the second case, you could use the fuel to cool the whole engine and a heat exchanger to boil the oxidizer.<ref name=ustpo-patent7418814 />

== Advantages ==
The expander cycle has a number of advantages over other designs:{{citation needed|date=March 2011}}
;Low temperature:After they have turned gaseous, the propellants are usually near room temperature, and do very little or no damage to the turbine, allowing the engine to be reusable. In contrast [[gas-generator cycle (rocket)|gas-generator]] or [[staged combustion cycle (rocket)|staged combustion]] engines operate their turbines at high temperature.
;Tolerance: During the development of the [[RL10]] engineers were worried that insulation foam mounted on the inside of the tank might break off and damage the engine. They tested this by putting loose foam in a fuel tank and running it through the engine. The RL10 chewed it up without problems or noticeable degradation in performance. Conventional gas-generators are in practice miniature rocket engines, with all the complexity that implies. Blocking even a small part of a gas generator can lead to a hot spot, which can cause violent loss of the engine. Using the engine bell as a 'gas generator' also makes it very tolerant of fuel contamination because of the wider fuel flow channels used.
;Inherent safety: Because a bell-type expander-cycle engine is thrust limited, it can easily be designed to withstand its maximum thrust conditions. In other engine types, a stuck fuel valve or similar problem can lead to engine thrust spiraling out of control due to unintended feedback systems. Other engine types require complex mechanical or electronic controllers to ensure this does not happen. Expander cycles are by design incapable of malfunctioning that way.
;Higher vacuum performance: Compared to a [[pressure-fed engine]], pump-fed engines and hence, expander cycle engines have higher combustion chamber pressures. Increased combustion chamber pressures allow for a reduced throat area A<sub>th</sub>, and therefore, leads to a larger expansion ratio, e = A<sub>e</sub>/A<sub>th</sub> for an identical nozzle exit area A<sub>e</sub>, which ultimately leads to higher vacuum performance.

== Usage ==
Expander cycle engines include the following:
*[[Aerojet Rocketdyne]] [[RL10]]<ref name="p&w2003"/>
*[[Pratt & Whitney]] [[RL60]]
*[[ArianeGroup]] [[Vinci (rocket engine)|Vinci]]
*[[Chemical Automatics Design Bureau|CADB]] & [[Pratt & Whitney]] [[RD-0146]]
*Chinese [[YF-75D]]
*[[Mitsubishi Heavy Industries]] [[LE-5|LE-5A / 5B]]
*[[Mitsubishi Heavy Industries]] [[LE-9]]
*[[Aerojet Rocketdyne]] & [[Mitsubishi Heavy Industries|MHI]] [[MARC-60]] (MB-60)
*[[Blue Origin]] [[BE-3U]] and [[Blue Origin#BE-7|BE-7]]
* [[Avio]] [[M10 (rocket engine)|M10]]
* [[Demonstration Rocket for Agile Cislunar Operations]] (DRACO) nuclear thermal engine

== Comparison of upper-stage expander-cycle engines ==

{| class="wikitable" style="text-align:center;"
|+ Specifications
! 　
! [[RL10]]B-2
! [[BE-3U]]
! [[Vinci (rocket engine)|Vinci]]
! [[YF-75D]]
! [[YF-79]]
! [[RD-0146]]D
! [[LE-5B]]
! [[LE-9]]
|-
! Country of origin
| {{USA}}
| {{USA}}
| {{FRA}}
| {{PRC}}
| {{PRC}}
| {{RUS}}
| {{JPN}}
| {{JPN}}
|-
! Cycle
| Expander
| Expander bleed cycle
| Expander
| Expander
| Expander
| Expander
| Expander bleed cycle, <br />chamber expander
| Expander bleed cycle
|-
! Thrust, vacuum
| 110&nbsp;kN (25,000&nbsp;lbf)
| 769&nbsp;kN (173,000&nbsp;lbf)<ref name="rsuqSn7ifpU">{{cite web |title=New Glenn Completes Second Stage Hotfire! |url=https://www.blueorigin.com/news/new-glenn-completes-second-stage-hotfire|website=BlueOrigin |access-date=1 November 2024}}</ref>
| 180&nbsp;kN (40,000&nbsp;lbf)
| 88.36&nbsp;kN (19,860&nbsp;lbf)
| 250&nbsp;kN (56,200&nbsp;lbf)
| 68.6&nbsp;kN (15,400&nbsp;lbf)
| 137.2&nbsp;kN (30,840&nbsp;lbf)
| 1471&nbsp;kN (330,000&nbsp;lbf)<ref>{{Cite web |last1=Watanabe |first1=Daiki |last2=Manako |first2=Hiroyasu |last3=Onga |first3=Tadaoki |last4=Tamura |first4=Takashi |last5=Ikeda |first5=Kazufumi |last6=Isono |first6=Mitsunori |date=December 2016 |title=Combustion Stability Improvement of LE-9 Engine for Booster Stage of H3 Launch Vehicle |url=https://www.mhi.co.jp/technology/review/pdf/e534/e534028.pdf |access-date=2024-03-13 |website=Mitsubishi Heavy Industries Technical Review}}</ref>
|-
! Mixture ratio
| 5.88
|
| 5.8
| 6.0
| 6.0
|
| 5
| 5.9
|-
! Nozzle ratio
| 280
|
| 240
| 80
| 160
|
| 110
| 37
|-
! [[Specific impulse|''I''<sub>sp</sub>]], vacuum (s)
| 462<ref>{{Cite web |url=http://www.rocket.com/rl10-engine |title=RL10 Engine &#124; Aerojet Rocketdyne |access-date=2017-06-06 |archive-url=https://web.archive.org/web/20170430183058/http://www.rocket.com/rl10-engine |archive-date=2017-04-30 |url-status=dead }}</ref>
| 455<ref>{{Cite web |last=Limp |first=David |date=2024-11-12 |title=The BE-3U's nozzles release hydrogen-rich steam at speeds of roughly 10,000 mph |url=https://x.com/davill/status/1856436665788854463 |access-date=2024-11-29 |website=Twitter}}</ref>
| 457
| 442.6
| 455.2
| 470
| 447
| 426
|-
! Chamber pressure (MPa)
| 4.412
|
| 6.1
| 4.1
| 7.0
| 5.9
| 3.58
| 10.0
|-
! LH<sub>2</sub> TP (rpm)
|
|
| 65,000
| 98,180
|
|
| 52,000
|
|-
! LOX TP (rpm)
|
|
|
|
|
|
| 18,000
|
|-
! Length (m)
| 4.14
|
| 4.2
|
|
| 3.358
| 2.79
| 3.8
|-
! Dry mass (kg)
| 277
|
| 280
| 265
| 
|
| 285
| 2400
|}

== See also ==
*[[Gas-generator cycle]]
*[[Combustion tap-off cycle]]
*[[Staged combustion cycle]]
*[[Pressure-fed engine]]
*[[Electric-pump-fed engine]]

== References ==
{{reflist|35 em|refs=
<ref name="y655">{{cite book | title=Liquid Rocket Thrust Chambers | publisher=American Institute of Aeronautics and Astronautics | publication-place=Reston ,VA | date=2004-01-01 | isbn=978-1-56347-223-7 | doi=10.2514/4.866760 | page=636 | editor-last1=Popp | editor-last2=Hulka | editor-last3=Yang | editor-last4=Habiballah | editor-first1=Michael | editor-first2=James | editor-first3=Vigor | editor-first4=Mohammed }}</ref>
<ref name=aiaa-paper2003-4597>{{cite conference |url=http://www.la.dlr.de/ra/sart/publications/pdf/2003-4597.pdf |title=Studies on Expander Bleed Cycle Engines for Launchers |first1=Martin |last1=Sippel |first2=Takayuki |last2=Imoto |first3=Dietrich |last3=Haeseler |date=July 23, 2003  |conference=39th AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit |publisher=[[AIAA]] |archive-url=https://web.archive.org/web/20160303181821/http://www.la.dlr.de/ra/sart/publications/pdf/2003-4597.pdf |archive-date=2016-03-03 |access-date=2016-09-25}}</ref>
<ref name=mhitr-vol48n4p36t43>{{cite journal |url=https://www.mhi-global.com/company/technology/review/pdf/e484/e484036.pdf |title=Development of the LE-X Engine |first1=Masahiro |last1=Atsumi |first2=Kimito |last2=Yoshikawa |first3=Akira |last3=Ogawara |first4=Tadaoki |last4=Onga |date=December 2011  |journal=Mitsubishi Heavy Industries Technical Review |publisher=[[Mitsubishi Heavy Industries]] |volume=48 |issue=4 |pages=36–43 |archive-url=https://web.archive.org/web/20151224013743/https://www.mhi-global.com/company/technology/review/pdf/e484/e484036.pdf |archive-date=2015-12-24 |access-date=2016-09-25}}</ref>
<ref name="p&w2003">{{cite web |title =Pratt & Whitney Space Propulsion – RL60 fact sheet |url =http://www.pw.utc.com/StaticFiles/Pratt%20&%20Whitney/Assets/Documents/space_2003_status_rl60.pdf |access-date =2008-12-28 |archive-url =https://web.archive.org/web/20120328031450/http://www.pw.utc.com/404.asp |archive-date =2012-03-28 |url-status =dead }}</ref>
<ref name=sutton-rpe7edp221p227>{{cite book|last1=Sutton|first1=George P.|last2=Biblarz|first2=Oscar|title=Rocket Propulsion Elements: an introduction to the engineering of rockets|date=2000|publisher=John Wiley & Sons, Inc|isbn=0-471-32642-9|pages=221–227|edition=Seventh|url=http://www.ewp.rpi.edu/hartford/%7Eernesto/S2013/EP/MaterialsforStudents/Lee/Sutton-Biblarz-Rocket_Propulsion_Elements.pdf|access-date=26 September 2016|archive-url=https://web.archive.org/web/20160119113628/http://www.ewp.rpi.edu/hartford/~ernesto/S2013/EP/MaterialsforStudents/Lee/Sutton-Biblarz-Rocket_Propulsion_Elements.pdf|archive-date=2016-01-19|chapter=Section 6.6|url-status=dead}}</ref>
<ref name=ustpo-patent7418814>{{cite patent |country=US |number=7,418,814 B1 |status=patent |title=Dual expander cycle rocket engine with an intermediate, closed-cycle heat exchanger |gdate=2008-09-02 |fdate=2005-06-30 |invent1=Greene, William D. |assign1=The United States of America as represented by the Administrator of the National Aeronautics and Space Administration |class= |url=https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20090006594.pdf }}</ref>
}}

== External links ==
*[https://web.archive.org/web/20120414212704/http://www.aero.org/publications/crosslink/winter2004/03_sidebar3.html Rocket power cycles]

{{spacecraft propulsion}}
{{Thermodynamic cycles|state=collapsed}}

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