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Browsing a comparison of orbital launch systems on wikipedia, I notice that launcher capable of launching payload in GTO can launch in GTO approximately all the mass of the payload they can launch in LEO (seems reasonable), and that launcher capable of launching heavier payload in LEO are able the one able to launch heavier payload in GTO (seems logical). 2 lines stroke me:

  • Ariane V ECA:

    • LEO payload: 21000kg
    • GTO payload: 10865kg
  • Falcon 9 Full Thrust (expended):

    • LEO: 22800kg (more than Ariane V)
    • GTO: 8300kg (less than Ariane V)

If a launcher is more powerful, it should launch heavier payload both in LEO and in GTO. Why this is not the case for this 2 launchers?

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That Wikipedia article may be incorrect. There are several versions of the Ariane 5 with different performance figures, and the article seems to mix 2 of them.

  • the Ariane 5 ES ATV, which is optimized for heavy LEO missions has a 21 t payload to LEO. It uses a second stage (EPS) which has lower performance than the ESC-A but has the advantage that it can be restarted. This version was developed for the ATV missions, which were heavier than the then-current Ariane 5 versions could support. This version can launch 7.5t to GTO.

  • the Ariane 5 ECA has a second stage (ESC-A) optimized for GEO. Arianespace don't specify a maximum payload for the ECA version to LEO. You can see this in the user manual. As far as I know, they don't use the ECA version for LEO missions. If I remember correctly, there's a structural limit on the ESC-A which limits performance for LEO missions. I did find a LEO payload of 20.6 t on Bernd Leitenberger's website (which has pretty good information usually), but I can't verify that from primary sources. You can see a fair amount of bickering about these figures on the Talk pages attached to the Wikipedia article.

This is the big difference with the Falcon 9, I think: the F9 second stage uses kerosene, while the ESC-A stage uses hydrogen and has a higher Isp.

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  • $\begingroup$ If the wikipedia article is incorrect, what are the real figures? Is the comparison still valid (the one launching the heavier payload in LEO being the one launching the least in GTO)? $\endgroup$ – Manu H Jun 17 at 6:21
  • $\begingroup$ as I said, the real figures are incomplete. I've found a GTO payload for the ES version, but no LEO payload for the ECA. $\endgroup$ – Hobbes Jun 17 at 12:17
  • $\begingroup$ +1 for taking the time to sort this all out. $\endgroup$ – uhoh Jun 17 at 12:25
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    $\begingroup$ If I understand correctly, the comparison is unfair as we compare the falcon 9 to 2 versions of Ariane V: one optimized for LEO, the other optimized for GTO (not fitted to put heavy payload into LEO) $\endgroup$ – Manu H Jun 17 at 13:16
  • $\begingroup$ yes, that's it. $\endgroup$ – Hobbes Jun 17 at 14:00
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To expand a little more on the last sentence of the answer and pick up on @GittingGud's comment. Rocket performance is not as simple as

If a launcher is more powerful, it should launch heavier payload both in LEO and in GTO.

For any particular fuel (and to a lesser extent, engine design) the rocket needs to use up a certain proportion of its initial mass as propellant (fuel and oxidiser) in order to accelerate the rest of that mass through a given velocity change. This proportion is determined by a number called the $I_{sp}$ which is higher for more efficient systems where you need to use up less mass.

The upper stage of Ariane has a significantly higher $I_{sp}$ than the upper stage of Falcon 9, so although the Falcon 9 delivers more mass to LEO, a higher proportion of that mass has to be used to make the velocity change to GTO, leaving less for the payload.

As a very loose analogy -- a truck can get a larger payload to 60mph than a Formula 1 car, but the racing car can get a larger payload to 150 mph.

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In general I think its clear this sort of thing should be expected. Rockets don't just "scale up" to be twice as powerful. That's not how the rocket equation works.

Take for example 2 rockets launched side by side. Together they will have twice the payload to any given orbit than just one. However no number of rockets will ever get any any of them past where an empty payload rocket could get to.

However even a tiny bit better rocket could. It might also take a really tiny payload with it. So these rockets are always better than any number of the slightly worse rockets launched side by side. However, just less than this and some number of the worse rockets will always manage to lift as much.

Hence there are 2 things that make up how "powerful" rocket its. One how much dV is has free of payload, and 2 how big is it (how heavy). There are a lot of things that effect the "empty" dV, but the main things are mass ratio and Isp. Staging make this more complex too, but the idea remains.

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