RAPIER as alternative to atomic rocket motor

[ZodiaK]

Whilst deciding on which engine to use for my duna interplanetary ship, I found that the RAPIER engine rocket mode has both higher thrust and ISP than the LV-N rocket motor. Does this mean that for efficient burns, the RAPIER is better?

[Skyler4856]

No. The ISP of the RAPIER in space isn't the number given in mechjeb or KE. Mechjeb doesn't assume that the RAPIER switches to closed cycling.

[WillHop]

RAPIER in atmo has an ISP of 800, in vacuum it has an ISP of 360 - LV-N is definitely the way to go

[MalfunctionM1Ke]

I am sorry to say, that the Rapier Engine is quite thirsty in Rocket mode. So it is not quite near the Impulse of the LV-N :/

[EndOfTheEarth]

RAPIER has a much higher fuel consumption rate than the LV-N. I would still stick with Nuclear for your interplanetary burns. If you need a lot of in-atmosphere oomph for a lander, the aerospike might be a better choice, as it has the same thrust as Rapier, but better ISP vac&atmo.

Personally, my duna lander used four LV T45s and carried the three-man capsule to and from the surface with fuel to spare.

[Armchair Rocket Scientist]

Whilst deciding on which engine to use for my duna interplanetary ship, I found that the RAPIER engine rocket mode has both higher thrust and ISP than the LV-N rocket motor. Does this mean that for efficient burns, the RAPIER is better?

The RAPIER has higher ISP than the LV-N in jet mode. So does every other jet engine. In rocket mode it's heavier than an LV-T45, with less thrust and lower ISP.

Basically, the only application of the RAPIER is SSTOs, and even then its usefulness compared to a combination of turbojets and rocket engines is questionable. SSTOs designed for interplanetary flight without refueling tend to use LV-Ns for their transfer burns.

A RAPIER might be useful for a Laythe lander, though.