What types of propellant are used in the second stage of multi-stage rockets? For example, the Falcon 9. Is it the same combination as the first stage? I've found plenty of information about the first stage but not the second stage.
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1$\begingroup$ Depends on the rocket. This information's usually available on Wikipedia. $\endgroup$– DylanSpCommented Dec 15, 2017 at 17:35
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$\begingroup$ Spaceflight101 is another good source for information, usually diving a little deeper into technical details than Wikipedia. spaceflight101.com/spacerockets/falcon-9-ft $\endgroup$– Russell BorogoveCommented Dec 15, 2017 at 19:02
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3 Answers
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Falcon 9: second stage uses the same propellant as the first stage (RP-1). This is somewhat unusual, many launchers have a second stage optimized for performance, using liquid hydrogen.
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3$\begingroup$ As unusual as the R-7 rocket family used from 1957 until today in many sucessful flights, using kerosene and LOX for all stages. From first manned orbital flight to the transfer to ISS today. $\endgroup$– UweCommented Dec 15, 2017 at 23:48
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1$\begingroup$ Also some orbital launch vehicles (notably a lot of Orbital's) are "upside down" in terms of Isp from what one might naively expect. E.g. liquid on the first stage, solid on the second stage. $\endgroup$ Commented Dec 17, 2017 at 3:00
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Propellants of second stage varies from UDMH to RP1. Usually propellant is selected on the basis of required characteristics of launch vehicle, its efficiency, and its ease of handling. For example ARCA is using RP1/HP due to its higher ISP. Falcon uses RP1/LOX. Whereas Indian PSLV uses N2O4/UDMH in its second stage. In comparison to this Japan's SS520 uses solid propellant.
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Second stage fuel usage is often a historical artifact. That is, while some boosters are developed in toto (Falcon 9) others are built out of parts of other systems.
Some are liquid fueled with anything from LH2 to solid fuels.
Even a new booster, like Antares, starts with a Kerosene first stage (Started with NK-33 engines, moved onto RD-191) but then pairs it with a solid upper stage. In that case, the developer Orbital ATK was known as a solid booster house, and they effectively outsourced the first stage components.
For the first stage you want more efficient thrust to get off the ground, and for upper stages you care much more about efficiency to get you to orbit.
