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I'm primarily looking for comparison of currently operational orbital launch vehicles by system-specific impulse, or Issp, and to get a fair impression of achievable rocket propellant performance and not merely theoretical maximum sea-level and vacuum specific impulse of propellants, or Isp, per weight. For the sake of having some direct quotes also in answers, please provide the following information:

  • Highest Issp achieved by a complete launch vehicle configuration, and
  • Highest Issp achieved by a single rocket stage.

For completeness, please also quote most relevant technical information about them, e.g. total impulse delivered by the mass of contained propellant, total (wet) mass of the propulsion system, propellants used, etc. To keep the length of answers manageable, we shall limit ourselves to only the highest performing launch vehicles and stages, but if you can also provide links to any such external launch vehicle and rocket stage comparisons by Issp that is more complete, then even better.

Edit to add: Some related lists that might help you answer my question:

Kudos to @PearsonArtPhoto for the first link that also enables sorting of rocket engines by their thrust-to-weight ratio, which is essentially what this question is about and that does nearly all the work to answer the second part of my question. As same rocket engines might be used on different launch systems and their stage configurations, some additional work would have to be done, but it is an excellent start.

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    $\begingroup$ The answer would require data that I have not generally seen publicly available. In particular in order to integrate the total impulse, you would need to know the thrust profile as a function of time. Or if not the thrust profile, the throttling profile as a function of time, the Isp as a function of altitude, and the altitude as a function of time. $\endgroup$
    – Mark Adler
    Dec 1, 2013 at 16:52
  • $\begingroup$ TWR has very little to do with the question, because engine mass is usually very small compared to propellant mass. $\endgroup$
    – Russell Borogove
    Jan 17, 2016 at 1:15

2 Answers 2

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The current launcher with the highest overall system specific impulse is almost certainly the Delta IV Heavy.

As far as I can tell from the list of current orbital launchers on Spaceflight 101, it is the only one that uses hydrogen-fueled engines on all stages. All the others use hypergolics or kerosene or solids somewhere in the stack, which all have far lower specific impulse than hydrogen.

As Mark Adler notes, exact figures for system specific impulse are hard to come by, because they require knowledge of the Isp versus altitude curve of the engines and the throttle schedule of the launcher. The complexity of throttle scheduling in particular means that the calculation is best done as a discrete-time iterative simulation rather than analytically.

At each time step of simulation:

  1. The engine specific impulse is determined based on the current altitude
  2. The current thrust is computed from the specific impulse and throttled propellant flow rate
  3. Momentary impulse is computed by integrating thrust over the duration of the time step and summed to total impulse.

The final total impulse divided by non-payload launch mass gets you system specific impulse.

I have been developing such a simulation. I've made educated guesses about throttling and done my best to research the other issues. The simulation isn't completely accurate, but the results it gives are reasonably aligned with reality.

In the end, the system specific impulse contest isn't even close, with Delta 4 Heavy (RS-68A Upgrade) 20% better than anything else I've looked at, because the Isp of hydrogen is so much better than other common propellants.

My simulations yield the following for max-payload launches to 200km LEO:

Launcher         Issp      Comments 
Falcon 9 FT      259 s     all kerosene
Ariane 5 ECA     262 s     solids + hydrogen
Saturn V/INT21   293 s     kerosene + hydrogen
Delta IV Heavy   352 s     all hydrogen

Calculating stage specific impulse for a single upper stage is simpler; the burn is assumed to take place in vacuum, so it's just the propellant fraction of the stage times the specific impulse of the engine. The winner appears to be the Delta IV's 5-meter DCSS upper stage, with a stage specific impulse of 409.5 s.

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  • $\begingroup$ Good answer with the Issp of some launchers so to have better idea how much differences do this launchers have with each other. Very helpful info. But can you please show in a more detailed answer or mabye comment calculations that you have done, at least calculations of one of the launchers. $\endgroup$
    – Mark777
    Apr 18, 2016 at 11:08
  • $\begingroup$ Is that better? $\endgroup$
    – Russell Borogove
    Apr 18, 2016 at 13:52
  • $\begingroup$ Yes definietly yes. I thought that you mabye have found somewhere those data or detailed info materials that are neccesary to make calculation because i have searched even before to find info needed for Issp calculation but without result. This work that you have done is a great job. $\endgroup$
    – Mark777
    Apr 18, 2016 at 20:54
  • $\begingroup$ In Delta Heavy result i guess it is about for the RS-68A upgraded version, right? And the simulation that you have done it's kind of program that you should buy or that you can find and get from internet ,or something else. I am sorry because it sound like a disccusion that has not to do with question but i was interested to know about Issp and also i have saw even in other questions in your comments for simulations done to find payload mass in orbit, so i would like to know how this simulations can be done for Issp in this case or even for other things of rocketry. $\endgroup$
    – Mark777
    Apr 18, 2016 at 20:55
  • $\begingroup$ The D4H is with RS-68As, yes. Unfortunately, the simulation is not off-the-shelf software, it's something I'm working on myself, inspired by Braeunig's work. braeunig.us/apollo/saturnV.htm I may open source it in the future, but I'm not ready to yet. I have a certain amount of trust in the simulator because the results for e.g the Apollo/Saturn V stack match the well-documented actual flight well enough. $\endgroup$
    – Russell Borogove
    Apr 18, 2016 at 21:23
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Using data from Wikipedia rocket launch's, the highest ISP for a first stage engine that has publicly available data is the Vulcain engine, used by Ariane 5. The value would be somewhere between 318 to 446, depending on exactly how you count it.

The highest stage is GSLV Mk I or the Delta III/IV upper stage, which both achieve 462 ISP.

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  • $\begingroup$ That page is missing a lot of data; RS-68 on the Delta IV has a sea level ISP of 365s, so could well give Ariane a run for its money for system specific impulse. $\endgroup$
    – Russell Borogove
    Dec 17, 2015 at 23:10
  • $\begingroup$ If you have more information, feel free to add your own answer;-) $\endgroup$
    – PearsonArtPhoto ♦
    Dec 17, 2015 at 23:11
  • $\begingroup$ and I thought SSME Isp were around 500 but they are no longer operational. $\endgroup$
    – Jim2B
    Dec 18, 2015 at 6:06
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    $\begingroup$ I just checked their stats, SSMEs were 450+ in space and 360+ at sea level. $\endgroup$
    – Jim2B
    Dec 18, 2015 at 6:10
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    $\begingroup$ @SF.: efficiency in a vacuum is higher than when there's an atmosphere pushing back on the exhaust. $\endgroup$
    – Hobbes
    Jan 17, 2016 at 8:59

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