I am a bit confused - If it is correct that RL 10 and LR 87-LH2 both were developed in 1950 by Aerojet, and both used LOX and LH2 as the propellants, if so, then why was there a huge difference in their thrust at SL? was there a large difference in their size? or anything else?

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    $\begingroup$ Why is it confusing that two hydrolox engines made by the same manufacturer at about the same time would have different amounts of thrust? They'd be somewhat redundant otherwise. $\endgroup$ Commented May 31, 2021 at 18:25
  • $\begingroup$ I was wondering what made the same manufacturer make two different engines, using same propellants, in the same year (well, almost same..) with different thrusts. As I understand, in those days, rockets engines were designed against some purchase order (supply or feasibility study or otherwise) from some of the defense agencies - FOR CREATING MISSILES, having more payload capacity and larger range.. So if an engine with higher thrust is available, who would like to use a smaller engine!!! $\endgroup$
    – Niranjan
    Commented Jun 1, 2021 at 13:52
  • $\begingroup$ Rocket design isn't about maximizing thrust. Sticking an oversized engine on a rocket will at best reduce payload capacity and range, especially on an upper stage where added engine mass comes directly out of payload mass. At worst it could cause it to break up due to acceleration, or by reaching excessive flight speeds while still within the atmosphere. Rocket engines have always been sized to match their intended application, just like every other form of propulsion. $\endgroup$ Commented Jun 1, 2021 at 16:02
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    $\begingroup$ "if an engine with higher thrust is available, who would like to use a smaller engine" -- If an engine 1/5 the weight is available, who would like to use a heavier one? $\endgroup$ Commented Jun 1, 2021 at 23:14

1 Answer 1


There was a large difference in their size, and at the time, they were developed by two entirely different companies.

In general, the propellant combination of a rocket engine does not determine its thrust, just as an internal combustion engine running on diesel fuel may power anything from a lawnmower to a tractor-trailer truck.

The LR87 was developed by Aerojet. It was a large, twin-combustion chamber, turbopump engine originally developed as a kerosene-LOX engine for the first stage of the Titan I ICBM; it was converted to use hypergolic propellants for the Titan II, and then evaluated as a candidate for use with hydrogen as a fuel as an upper stage for the Saturn rocket family. The LR87-LH2 never went into production, as Rocketdyne's J-2 proposal was selected for the Saturn IB and V.

The RL10 was a much smaller expander-cycle engine -- about 135kg in its 1960s incarnation -- designed by Pratt and Whitney for the Centaur upper stage, which was very small by comparison with the Saturn upper stages. The LR87 was over 5 times the mass of the RL10.

Pratt & Whitney merged with Boeing Rocketdyne to form Pratt & Whitney Rocketdyne in 2005; in 2013, P & W Rocketdyne was acquired by GenCorp and merged with Aerojet to form Aerojet Rocketdyne, by which time the LR87 was well out of production.

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    $\begingroup$ The RL-10 was designed by Pratt & Whitney Aircraft at what was then called Florida Research and Development Center for the Centaur (upper stage for the Atlas used as a space launch vehecle). See en.wikipedia.org/wiki/RL10. $\endgroup$
    – W H G
    Commented Jun 1, 2021 at 20:34
  • $\begingroup$ Hah, really good point. $\endgroup$ Commented Jun 1, 2021 at 22:53

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