Watching the Amazonas5 launch, and at 1st stage separation, the commentator noted that the 2nd stage would ignite before the separation in order to "maintain constant acceleration."

I'm wondering how this works - surely the 1st stage is still in the way, so the rocket will be pointed directly at the top of the 1st stage.

Alternatively, if this is beneficial to the launch in some way, why don't all launches do this?

Why do the Russians use these fence-like interstage fairings? is related, but not really a duplicate.

  • $\begingroup$ Its called hot-staging and is to povide ullage. The russians like it, see N1 and Soyuz. $\endgroup$
    – Polygnome
    Sep 11 '17 at 21:00

The initial exhaust from the upper stage fires at the first stage and is deflected and exits through the openings in the interstitial fairings, as shown in the markup of a Proton rocket below:

enter image description here

The reason for this is to maintain the ullage of the upper stage tanks (if the fuel/oxidizer slosh forward at separation they won't flow into the pumps/engine evenly and things tend to end badly). Starting the upper motor while still under thrust is one way to ensure proper ullage, another method is to use small solid rockets to separate the stages and push the fluids back in the tanks.

A related question: Why do the Russians use these fence-like interstage fairings?

  • $\begingroup$ How do western designs address the ullage problem? No air-gap, perhaps, with some kind of sealed expansion chamber so it won't burst? Never mind, it's in the referenced other question. $\endgroup$
    – Peter Wone
    Sep 12 '17 at 2:56

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