Anyone made any RAPIER based heavy lifters?

[Soda Popinski]

Obviously, the RAPIER is tailor made for space planes / SSTO. Has anybody made any heavy lifters using this system? I'm currently using an asparagus based lifter to get 70t payloads into LKO (using FAR + KIDS). Thinking of trying to make a more fuel efficient lifter.

Edited December 24, 2013 by Soda Popinski

[Murph]

I wouldn't even consider them for that role, frankly. They are too small, underpowered, and too low impulse. There are much better alternatives.

[Soda Popinski]

I wouldn't even consider them for that role, frankly. They are too small, underpowered, and too low impulse. There are much better alternatives.

Low Impulse for travel past LKO? Or Low Impulse compared to turbojet based lifters? Compared to rockets, the atmospheric fuel efficiency wins with the RAPIER. I was thinking of quad clusters of X200-32 tanks + one jumbo tank. 3 of them would give a thrust of 2100 kN. With 1 jumbo tanks + 3x X200-32 + 70t payload, that's under 1600 kN of weight, for a TWR of ~1.3.

I was thinking about Scott Manley's reusable lifter with it's Turbo Jet cluster & single Poodle engine (think it was a Poodle).

[Murph]

360s is not that great for the second half of the ascent, and the lack of power combined with lower max air speed makes them less efficient in the first half, than turbojets. I've built a cargo delivery plane which can do 70t+ to LKO without a problem, and it really needs much bigger and better than the stock plane parts, so that's B9 for me. Once B9 is in the picture, quad SABRE Ms are a much more appealing alternative to a large number of smaller, mediocre engines.

[tavert]

By the time you're out of the atmosphere, you don't really need as much thrust as the vacuum-mode Rapier provides, considering how many of them you would need to get an acceptable vertical-lift TWR off the pad. Turbojets have better TWR and top speed in air-breathing mode, and you can add a smaller number of rocket engines to finish the orbital insertion with the mass you save.

[Murph]

By the time you're out of the atmosphere, you don't really need as much thrust as the vacuum-mode Rapier provides, considering how many of them you would need to get an acceptable vertical-lift TWR off the pad. Turbojets have better TWR and top speed in air-breathing mode, and you can add a smaller number of rocket engines to finish the orbital insertion with the mass you save.

Yes, that's not a bad summary. They provide less thrust when you need it most, and too much (which isn't really a problem in that phase) when you don't need it.

[Soda Popinski]

By the time you're out of the atmosphere, you don't really need as much thrust as the vacuum-mode Rapier provides, considering how many of them you would need to get an acceptable vertical-lift TWR off the pad. Turbojets have better TWR and top speed in air-breathing mode, and you can add a smaller number of rocket engines to finish the orbital insertion with the mass you save.

I've never built a turbojet powered heavy lifter. I might try turbojet boosters with a RAPIER cluster in the center (sort of want to get that in there somewhere).

Thanks for the thoughts.

[antbin]

I was excited to update my 40 ton heavy lifter SSTO for the RAPIER engines, but after trying them out I lost interest.

The RAPIER has exactly the same altitude/speed performance curve as the turbojet, so SSTOs built with it fly the exact same flight profile, just with lower thrust/weight. With the RAPIER's reduced thrust it would have even less power to overcome the 10km "stall zone".

[thereaverofdarkness]

The RAPIER has exactly the same altitude/speed performance curve as the turbojet, so SSTOs built with it fly the exact same flight profile

Have you actually tried it? The RAPIER engine works better than the turbojet in the upper atmosphere once you get past 20km, and can carry a lot more mass without switching to rocket mode.

[Murph]

Have you actually tried it? The RAPIER engine works better than the turbojet in the upper atmosphere once you get past 20km, and can carry a lot more mass without switching to rocket mode.

With 1:1 or 2:1 intake:engine, the air-breathing RAPIER is close to the turbojet, including past 20km, but more than 15% less powerful. I've no idea (and don't care) how they compare with excessive intakes. I can't see how it can possibly "carry a lot more mass" with its power output. I certainly wouldn't say that it works significantly better, more that it's at best close to the turbojet, and at worst significantly inferior. The one exception where it does seem to win is for micro space planes, with very low total mass and just a single engine.

The performance curves in the part.cfg are close to identical between the turbojet and RAPIER, just the RAPIER is significantly less powerful and has a slower max air speed.

[Soda Popinski]

I went ahead and played around with a mix of RAPIER engines and Turbojets, and made a Medium Lifter. This relatively tiny thing (compared to my Mainsail powered 70t asparagus heavy lifter), got 33t to orbit...barely (about 2 seconds of fuel left for a 100km x 70.1 km orbit).

It's got a quad cluster of RAPIERS in the center, and 4 boosters with quad turbojets. It takes off vertically (obviously), but spends a lot of time horizontal for not a space-plane. Maybe I should slap some wings on it. I take off with 2/3 of the intakes closed, and open them up incrementally as FAR tells me my air usage is about to go under 100%, then as air starts running out, I shut off the outer ring of turbo fans, inner ring, then start throttling back the RAPIERs (maintaining acceleration the whole time), jetison the boosters when the fuel runs dry (happens soon after shutting down the inner turbojets), and when I'm just out of air, toggle to closed cycle, close intakes, go full burn to 100km apoapsis. Not as easy as my Mainsail asparagus system, but definitely more fuel efficient (7 jumbo tanks worth of fuel compared to less than 1 jumbo tank worth of it with this).

[Moar Boosters]

I'm currently working on a heavy 334 ton VTOL SSTO that uses 40 Rapier engines, and 4 nukes.

It still requires further optimisation regarding the fuel/oxidizer ratios, moar intakes.

It takes off vertically, but due to the aerodynamic surfaces, at around 10km it turns over onto it's side and then starts picking up horizontal speed like a regular spaceplane. It's taking a while to figure out the best ascent profile.

So far I've gotten it into orbit with 4000DV remaining on the nuke engines, although that's going to be reduced quite a bit if you turn on the Rapiers to perform a landing. It should currently be good for the Mun and Minmus, but I'm aiming to optimize it for return missions to Duna, Ike and Gilly.

The Rapiers are weird when using them like this. Having such a short fat craft, and the extreme gimbal range they provide, means it can survive multiple flame-outs without becoming uncontrollable. It actually flies perfectly.... more stable than any regular plane I've built.

Edited December 25, 2013 by Moar Boosters