Why RAPIERs are Awesome

[Mobjack]

Most test with the RAPIER engines involve small SSTO planes with the MK1 cockpit. These planes don't push the engines to their limits which paints an incomplete picture of their performance. It is when building bigger space planes that the RAPIER engine advantages becomes apparent.

I've used a medium sized space plane with a MK3 cockpit, 4 Ramjet Intakes and 4 Radial Intakes as my test plane.

Here are the three configurations used:

2 RAPIER/1 Turbojet (2R/1T)

2 Turbojets/1 Aerospike (2T/1A)

3 Turbojets/2 Aerospikes (3T/2A)

Ship Specifications

[table=width: 700, class: grid]

[tr]

[td]Configuration[/td]

[td]Mass[/td]

[td]Jet Thrust[/td]

[td]Jet TWR[/td]

[td]Rocket Thrust[/td]

[td]Rocket TWR[/td]

[/tr]

[tr]

[td]2 RAPIER/1Turbojet[/td]

[td]23,389kg[/td]

[td]605kN[/td]

[td]2.64[/td]

[td]350kN[/td]

[td]1.53[/td]

[/tr]

[tr]

[td]2 Turbojets/1 Aerospike[/td]

[td]22,589kg[/td]

[td]450kN[/td]

[td]2.03[/td]

[td]175kN[/td]

[td]0.79[/td]

[/tr]

[tr]

[td]3 Turbojets/2 Aerospikes[/td]

[td]25,449kg[/td]

[td]675kN[/td]

[td]2.70[/td]

[td]350kN[/td]

[td]1.4[/td]

[/tr]

[/table]

The 2R/1T configuration weighed 800 kg more than the 2T/1A ship, but had about 33% more thrust from the jet engines and 100% more thrust from the rockets.

The 3T/2A weighed more than 2000kg compared to the RAPIER version, but only had about 11% percent more thrust from the jet engines and the exact same thrust from the rockets.

The Aerospike does have a higher ISP than the RAPIER engine, but the 3T/2A ship has 2 tons in extra dry mass compared to the RAPIER ship. I haven't done exact calculations, but my guestimations tell me that the RAPIER configuration has more dV.

Flight Characteristics

I was able to get the 2R/1T plane into LKO orbit with ease and had plenty of dV left. I went to about 1600m/s at 25km, the center Turbojet flamed out, I switched the RAPIERS to the closed cycle, then the Turbojet fired back up again.

The 2T/1A configuration didn't have enough thrust to do the job efficiently and it took twice as long to reach 20,000m. At around 22,000 - 24,000 meter mark, I've hit a wall where I didn't have enough speed and atmosphere to feed my Turbojets, and didn't have enough thrust from the Aerospike to push my ship above the atmosphere. I had to use all three engines together much earlier which wasted a lot of fuel compared to the other designs, but managed to get into orbit. This is clearly the worst design.

The 3T/2A configuration was able to get up into orbit fairly easy, but it required more management with the action groups and it used more fuel than the RAPIER configuration. I would characterize it as a more clunky and less efficient version of the 2R/1T. One thing I noticed was the RAPIER engines flame out less violently compared to Turbojets.

Conclusion

From what I see, it is almost always better to have a RAPIER engine than a Turbojet/Aerospike pair. The RAPIER has about 15% less thrust than the Turbojet and 8% less ISP than an Aerospike, but you end up saving 1 ton in dry mass which will more than offset the difference. The difference is however slight, which means Squad did a good job balancing the RAPIER engine.

I believe the 2 RAPIER to 1 Turbojet is a good ratio to making SSTO planes since you need more thrust in the atmosphere to overcome the drag. I should have tested a 3 RAPIER version too, but I imagine its performance would be the same or slightly worse than the 2R/1T configuration.