How many hydrolox engines had a methane variant?

Question

Russia, has developed many rocket engine concepts over the years and its noticeable that they already tried liquid methane as a propellant back in the 90s. How many of their cryogenic engines had a liquid methane variant? Have any other countries also developed similar capabilities and tried liquid methane in the past?

Answer 1

According to some old usenet postings by Henry Spencer now on yarchive, the RL10 was experimentally used with methane:

"CH4/O2 offers the second highest Isp (385 s)... A significant disadvantage of CH4/O2 is that no flight-rated engine is currently available; development of one based on Pratt and Whitney RL-10 engine technology would probably take three years and cost on the order of $30 million."

One reason to be reasonably confident in this low number is that the RL10 has already been run on LOX/CH4, with only minor modifications. That was done only experimentally, and you'd need a bit more work to certify that configuration for production use, but it shouldn't be hard.

According to Clark's Ignition! (1972), German rocket pioneer Johannes Winkler experimented with a methane/LOX engine in 1930. Clark also mentions with some horror a 1957 proposal to use pre-mixed methane/LOX as a monopropellant. ("JPL later demonstrated that you could make the mixture detonate merely by shining a bright light on it.") Clark also mentions using methane with other oxidizers:

Rocketdyne, and TRW, with NASA contracts, as well as NASA itself, have concentrated their efforts on OF2 and the light hydrocarbons: methane, ethane, propane, 1-butene, and assorted mixtures of these. (In most of their motor work, they used a mixture of oxygen and fluorine as a reasonably inexpensive surrogate for OF2.)

Spencer also mentions the RL10 having been fired with FLOX (fluorine-LOX mixture), so it seems probable that Clark is talking about RL10 here as well.