Rapier Engines for vertical launches

[GwynJHawke]

I've been messing around with a low part count, low weight laythe lander and wad wondering how practical it is to use a rapier engie to launch a payload from a vertical launch position?  Anyone have any experience doing that?

[bewing]

As I understand it, panthers work better for that sort of thing. Are you locked into using a rapier?

[Warzouz]

6 hours ago, GwynJHawke said:

I've been messing around with a low part count, low weight laythe lander and wad wondering how practical it is to use a rapier engie to launch a payload from a vertical launch position?  Anyone have any experience doing that?

I did it back in 0.9. It was a Swivel engine 15T ship pushed at low altitude by 2 Whiplash. The lander had to climb up to 500km, where the space station was waiting... Since 1.0 has change aerodynamics this kind of lander may probably have flipped.

But the vertical air breather was working perfectly. The power is not as responsive as rocket engines, but that's OK for ascending.

I don't remember why I didn't used the rapier, maybe the total dV was lower ; I don't know. I think the rapier may have a low thrust on low speed/altitude. That may be an issue. Testing your ship on Kerbin is useful...

As an additional advice. If I would redo a Laythe mission, I would do a space plane. It's not easy to target those island with a regular lander due to atmospheric drag which alter projected landing site. Being able to fly toward an island is definitely a good idea. In any case, having a ship able to splash and take off is definitely a good idea on Laythe...

[Red Iron Crown]

It's certainly possible, the only real trouble is that the rapier's thrust is quite poor at very low speeds (i.e. during liftoff). One trick I've seen to workaround that is to run the rapier in closed cycle mode until a couple of hundred m/s of speed are accumulated, then switch to airbreathing mode and continue the ascent. 

[Streetwind]

But don't wait too long, or you're going to lose thrust to altitude faster than you can gain thrust from increased speed... or your air intakes stop collecting enough oxygen.

It's a tricky problem, and not all payloads benefit from an airbreathing launch stage. Some certainly do, however.

[GwynJHawke]

I've done a half dozen test tuns with a number of different setups. Once the ship hits 10 m/s vertical, I turn the ship onto a 45° angle and eventually just above horizontal until I hit 400 m/s and point back up to 45°. I can typically hit around 1200 m/s before open cycle burns out. I then detach that stage and switch to the command pod's engines. I'm still playing with it because I want it to be able to fit into a Mk. 3 cargo bay for the sole reason that I have a weakness for esthetics. 

[KerikBalm]

yes, you can, but the rapier static TWR is the worst of all the airbreathers. I prefer whiplashes for my VTOL airbreathers... although thats' only for multiengine designs as you need an engine that works over 30km... For smaller single engine designs, a rapier works fine.

I have a VTOL laythe ISRU lander/tanker that runs on rapiers and afterburning panthers. THe panthers obviously being for extra twr for the vertical launch.

[GwynJHawke]

As it currently stands, my Laythe VTOL has two sections.  The comand pod with a shielded docking port, 1.25m to 2.5m fuel adapter, 2 Mk.55 Thud engines on the adapter, a Mk. 2 lander can, a 2.5m service bay filled with scientific instruments and extra monoprop, an X200-8 fuel tank, and a 2.5m docking port. The ascent stage is a 2.5m docking port with 2.5m tri-stack adapter, 3 Mk.1 Lf fuel tanks, 3 engine pre-coolers, 3 whiplash engines, and 6 landing struts. I just did a test run and managed to have enough deltaV to even leave Kerbin's SOI. My biggest issue with my current design is that I just can't get it pointed up past 30-35° after I heel over horizontally to build up speed. It makes me wonder how it will perform in Laythe's thinner air and lighter gravity. I've only ever sent one way probes to Laythe.  Two landed on the polar ice caps and the other landed in the middle of the ocean. I "could" just land on the caps, but thats boring and not very exciting for me, not to mention the fact that a polar orbit isn't what I planned on entering into.

[Van Disaster]

Yeah, I have messed about & use Panthers ( or B9 equivalent ) plus a real rocket for boost stages. The advantage of the jets is their thrust starts tailing off when the fuel is running down, so your TWR doesn't climb crazily like a pure rocket booster does. Most of the thrust from the Rapier is from around Mach 3 when it'd be working as a ramjet ( this is not *quite* what a real SABRE is expected to work like... ) which is almost entirely useless for a rocket, you don't generally spend much time in usable atmosphere at that sort of speed... unless you put wings on it. At that point you might as well go all out & make it HOTOL.