On the successful firing of its Merlin 1D engine, Space Exploration Technologies, with more than a note of self congratulation, states that the "enhanced design makes the Merlin 1D the most efficient booster engine ever built" further noting that it has "a vacuum thrust-to-weight ratio exceeding 150",
True - it has the best ever thrust-to-weight ratio, taking this record from the Russian NK-33 (now made under the US designation AJ-26). However, for most rocket engineering cognoscenti, it is specific Impulse, the momentum change per kg of propellants used, that is the true measure of rocket efficiency.
As such, with its "open" gas generator cycle and using the propellants of LOx (liquId oxygen) and kerosene, the Merlin 1D will never match the efficiencies of the LOX/Liquid hydrogen powered Vinci or RL-10 engines. Under this measure, at circa 310 seconds, the Merlin 1D's vaccuum specific impulse is about a third less than the 465 seconds or so that these expander cycle engines can manage.
And if you really want to be a trouble maker you could mention electric ion thrusters whose vaccuum specific impulses are measured in the thousands of seconds - albeit with very low thrust.
on June 26, 2012 7:11 PM | Reply
The merlin 1D is not even the most efficient in terms of Isp for LO2/Hydrocarbon engines, (which is a more reasonable comparison as opposed to considering other propellant combinations).
As pointed out, the AJ-26 has a considerably higher Isp across all operating altitudes. This is because the AJ-26 uses a staged combustion cycle, as opposed to the gas generator cycle merlin.
on June 26, 2012 7:45 PM | Reply
Comparing a hydrocarbon engin with a hydrogen burning one is confusing things. Better to say efficiency is the part of available energy is "produced". If you state output as Isp, then the Isp compared to ehat is possible from the energy content of the propellant combination. Usually it is assumed the energy from condensing the H2O of the exhaust is not recovered--that that remains water vapor.
I agree that the closed cycle engines made famous by USSR are more efficient because there is not the secondary flow for a turbine. That all goes through the combustion chamber and inefficiency of the turbine, pumps appears as additional enthalpy of the propellant. Like eating the watermellon and the rind.
With Microlaunchers development there is a plan to eventually build such. A simple diagram of that appears at the last slide of a presentation given 2005: http://www.flightglobal.com/blogs/hyperbola/2012/06/comment-merlin-1d-is-not-quite.html .
on June 27, 2012 1:23 AM | Reply
The press release specifies "booster engine" - all the engines you mention are 2nd stage or in-space engines. Semantics, but..
on June 27, 2012 5:39 PM | Reply
Note that SpaceX's claim is rigorously true: they said the most efficient booster (ie, first-stage) engine. You can't use electric propulsion to get a vehicle off the ground, and you wouldn't want to use RL-10s. When launching a staged vehicle from the ground, T/W is honestly a much more important parameter than ISP, due to the need to overcome gravity losses. This is why the high-ISP hydrogen engines in the shuttle and Ariane 5 have to be augmented by relatively low-ISP (but high-thrust) solid boosters.
So, SpaceX is correct: in the realm of things that you'd want to get a spacecraft airborne, they come out tops.
on June 27, 2012 6:55 PM | Reply
You are correct in your analysis that thrust-to-weight (T/W) ratio is, to some extent, the more important parameter over specific impulse (Isp) for a "booster engine". Nevertheless, we still consider Isp as the true measure of rocket efficiency. This is the reason that NASA is examining using liquid propellant engine types for the strap on boosters of the Space Launch System (SLS) over Shuttle-style solid rocket boosters.
It should be noted that the LOx/Kerosene RD-170 of the Soviet era Energia heavy lift rocket had a vacuum Isp of 338 sec - which remains somewhat better than the Merlin 1D. As it is, at 137:1, the NK-33 (AJ-26) has nearly as good a T/W as the Merlin 1D (150:1) but has a significantly better vacuum Isp (331 seconds versus 310 seconds). In other words, on pure performance, it would probably make a better booster engine. One thing though. Space X and its low cost production techniques could make the Merlin 1D the better booster engine if cost effectiveness/value is taken as the key measure.
Of course, the Merlin 1D, and, by the way, the NK-33, are both probably undersized in terms of overall thrust. This is one of the key reasons why the heavy lift Falcon 9 will need so many of them (27!) for lift off, and is the reason the much larger Merlin 2 has been proposed for development.
on July 5, 2012 2:34 PM | Reply
As previously mentioned, the NK-33(AJ-26) is still better in Isp at 331 s, though the Merlin 1D supersedes it in thrust/weight ratio.
The NK-33 is legendary among SSTO supporters because it has both a high Isp and high T/W ratio. Contrary to common belief, SSTO's are actually [i]easy[/i] to produce with the current high efficiency engines and lightweight stages available now. People who say they are unfeasible are basing that on the engines and stages from the 50's and 60's.
Knowledgeable sci/fi writers such as Robert Heinlein and Arthur C. Clarke writing during that period supposed rockets to the Moon had to be nuclear powered because of the low efficiency engines available then. See for example the film [u]Destination Moon[/u]:
Destination Moon.
http://www.youtube.com/watch?v=DsisGSBlQqo
Actually in the key measure of Isp the Merlin 1D is still no better than the engines we had then. However, we've had the necessary engines and stages for SSTO's since the 1970's. See the discussion here:
The Coming SSTO's.
http://exoscientist.blogspot.com/2012/05/coming-sstos.html
Bob Clark