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The LV-T15 "Faithful" is a liquid engine that is unlocked in the start in career mode, however the engine has a big fuel consumption and a poor specific impulse.

The Kerbodyne KR-1L- "Hippo" Liquid Fuel Engine is an upper stage engine for 3.75m parts.

The FL-T50 is a tank the size of the FL-T100, but has a smaller fuel quantity.

The Mk 1-1 cockpit is a smaller version of the Mk 1-2 cockpit.

The Mk 1-3 cockpit is a larger version of the Mk 1-2 cockpit

The LV-A "Atomic" Engine is a Nuclear Engine which consumes only oxidizer and has higher thrust and a higher specific impulse than the LV-N.

The "Dusk" is an ion engine with higher thrust and lower specific impulse.

The "Neon" is an ion engine with lower thrust but higher specific impulse.

The HECS3 is a probe core with monopropellant inside but contains less electric charge.

The "Probe" monopropellant engine is built for the HECS3. 

The HECS4 is a probe core with liquid fuel and oxidizer built in.

The LV-101 is an upper stage engine with higher specific impulse than the LV-909.

The L-808 is a nuclear mixed mode turbojet which has a higher thrust than the RAPIER engine in space.

The N-1 is a nuclear reactor which requires radiators to function properly.

The X-88 is a Turbo Scramjet engine which uses a small amount of oxidizer.