Copyright 2024 Robert Clark
Staged combustion engines such as the Russian RD-180 and American SSME are regarded as the utmost in efficiency because they achieve high vacuum Isp while being able to achieve high thrust at sea level. They achieve this by operating at high chamber combustion pressure.
SpaceX is developing the Raptor engine also as a staged combustion engine. However, a surprising fact is a medium performance, mid-level pressure and cheaper engine such as the Ariane 5’s Vulcain engine or Delta IV’s RS-68 can get higher performance than a staged combustion engine by using altitude compensation.
See the graphic of the Isp of the Vulcain engine with an altitude compensating nozzle:
CONTRACT NO. NAS8-32996
The nozzle extension is just a well-known, simple way to accomplish it but there may be simpler or more lightweight methods of accomplishing it.
Aerospike in 3D exhaust injection. UPDATED, 1/10/2023: Extension to single nozzles.
https://exoscientist.blogspot.com/2023/01/aerospike-in-3d-exhaust-injection.html
SSME based SSTO’s. UPDATED, 6/28/2021 - Extension to the Delta IV Heavy.
https://exoscientist.blogspot.com/2021/06/ssme-based-sstos.html
ESA's Callisto reusability testbed as an *operational* TSTO and SSTO. UPDATE, 7/1/2019.
https://exoscientist.blogspot.com/2019/05/esas-callisto-reusability-testbed-as.html
Altitude compensation attachments for standard rocket engines, and applications, Page 6: space shuttle tiles and other ceramics for nozzles. UPDATED: 3/6/2018
https://exoscientist.blogspot.com/2017/12/altitude-compensation-attachments-for.html
Altitude compensation attachments for standard rocket engines, and applications, Page 5: metal foil expandable nozzles.
https://exoscientist.blogspot.com/2017/08/altitude-compensation-attachments-for.html
Altitude compensation attachments for standard rocket engines, and applications, Page 4: the double aerospike.
https://exoscientist.blogspot.com/2016/10/altitude-compensation-attachments-for.html
https://exoscientist.blogspot.com/2016/06/altitude-compensation-attachments-for.html
Altitude compensation attachments for standard rocket engines, and applications, Page 2: impulse pressurization methods.
https://exoscientist.blogspot.com/2016/01/altitude-compensation-attachments-for.html
Altitude compensation attachments for standard rocket engines, and applications.
https://exoscientist.blogspot.com/2014/10/altitude-compensation-attachments-for.html
The Coming SSTO's.
https://exoscientist.blogspot.com/2012/05/coming-sstos.html
https://exoscientist.blogspot.com/2016/01/altitude-compensation-improves-payload.html
Robert Clark
9 comments:
Higher chamber pressure allows higher expansion ratio at sea level. No amount of altitude compensation will increase Vulcains efficiency to where RS-25 is.
SpaceX even uses a shorter vacuum nozzle on some missions.
It is really hard to beat a fixed-geometry bell nozzle because the list of potential failure modes is far smaller. -- GW
GW, I’d like to see a numerical simulation of the suggestion positioning multiple nozzles such as on the Falcon 9 or the SuperHeavy in a 3-dimensional configuration emulating the aerospike instead of a single plate. That way you don’t having moving parts, but it emulates the altitude compensation of the aerospike:
https://exoscientist.blogspot.com/2023/01/aerospike-in-3d-exhaust-injection.html
Bob Clark
Rok, remember every fixed nozzle sea level engine operates at an intermediate efficiency level: it is always overexpanded for sea level operation. According to that graphic showing Vulcain ideal expansion if it is given ideal expansion at sea level, I.e., not overexpanded, Its sea level Isp would exceed that of the SSME. Then if also given ideal expansion at high altitude it would also exceed that of the SSME at high altitude and vacuum. Then a variable nozzle extension could get the Vulcain to exceed the performance of the SSME all the way from sea level to vacuum.
Bob Clark
Bob and ROK: I have just posted on "exrocketman" an article exploring how best to compromise-design a fixed-bell rocket engine for ascent to LEO, compared to standard sea level and vacuum designs. I have another article in work that addresses this compromise-design option vs the options of extendible bells and free-expansion nozzle designs. -- GW
Sea level optimised Vulcain would be like RS68.
They have same chamber pressure. One has nozzle ratio of 45, the other 25.
One has sea level isp of 320s the other 360s.
If im not mistaken SSME has 366s.
Sorry, got lost and confused. The posting I had put up was about 1-stage to LEO vs 2-stage to LEO. Both the sizing of fixed bells and a brief discussion of extendible bells and free-expansion nozzles were together in the posting that was "in work". I just finished it and posted it on "exrocketman" this morning (3-4-24). -- GW
Rok, even the RS-68 is overexpanded for sea level, as is every ground-launched engine. I’ve seen a couple of numbers for the sea level Isp of the RS-68, either 362s or 365s. For either value, it is quite close to the sea level Isp of the SSME, an expensive staged combustion engine. So if the RS-68 was given a variable nozzle that was even shorter still at sea level, it would have a sea level Isp exceeding that of the SSME.
Then the variable nozzle at high altitude and vacuum would be made to expand out to ultra high levels such as the 625 to 1 discussed in that “ORBITAL TRANSFER VEHICLE (OTV) ENGINE STUDY” report to get 482+ s Isp, exceeding that of the SSME also at high altitude and vacuum.
Robert Clark
A traditional fixed-geometry sea level nozzle design sizes the expansion ratio from max chamber pressure to an exit pressure that is atmospheric at sea level. That maximizes sea level Isp, but thrust and Isp do not grow very much (only about 6%) while flying out into vacuum.
A typical "vacuum" design has nothing to do with "optimization" and everything to do with design constraints, usually space constraints within an interstage. In effect, the nozzle gets designed to some exit/throat area ratio that meets those constraints! These designs are usually not operable at sea level, at any throttle setting, because of backpressure-induced flow separation in the nozzle, which causes catastrophic failures.
However, if you operate your ascent engine nozzle overexpanded at sea level, but not quite flow-separated, at some moderately-high throttle setting, you will decrease sea level Isp, but the larger expansion ratio increases the vacuum Isp, and also the ascent-averaged Isp, if you choose a high-enough nozzle-design throttle setting. I usually use about 80-85% of max chamber pressure for that, driven more by ignition and throttle-up needs, because there is no need for using a min throttle setting deep in the atmosphere with an ascent engine. I get around 17% thrust and Isp gain flying out into vacuum.
Designing for sea level-incipient separation at 85% max chamber pressure has produced for me designs within about 5% of the vacuum Isp of a modest vacuum design. The ascent-averaged Isp exceeded that of the traditional sea level design, by around 3%, despite the drop in sea level Isp! And I got this with only modest modern-technology characteristics: chamber pressures well under 3000 psia, nonzero dumped bleed for the cycle, and any propellant combination you might want. There is no need to push the state-of-the-art hard to do this!
Such is coming really close to an "ideal-expansion notion", especially in terms of ascent-averaged Isp, and this is a totally fixed-geometry design approach, which maintains high engine thrust/weight, and avoids any leak risks associated with bell extensions and such-like! It also has a vacuum Isp fairly close to the traditional vacuum designs, quite unlike the free-expansion approaches, which all suffer catastrophic streamline divergence effects as they go above the lower stratosphere. Those are all inherently-lousy vacuum engine designs. Prandtl-Meyer expansion physics tells you so, and in no uncertain terms.
Fixed-geometry, conventional-bell, ascent engines which exceed the ascent-averaged Isp of traditional sea level engines, and yet approach fairly closely the vacuum performance of modest vacuum designs. I think that's quite the remarkable engine design result! I put some of this on "exrocketman". -- GW
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