How to get the most out of R.A.P.I.E.R. Engines?

[bewing]

57 minutes ago, bertibott said:

but what does one actually do against instability?

 

 

Well, my answer is to forget about tails and stabilizers and just go with canards. There are two kinds of stability, passive and dynamic.

For passive stability, you want lots of drag at the back, and very little at the front, You want the CoL vertically above the CoM. You want dihedral on the wings. And you must have the CoL a little behind the CoM horizontally.

Dynamic stability is about taking a slightly unstable plane, and having a computer (SAS system) constantly jiggle the control surfaces to keep it balanced and pointing in the right direction. This still requires more drag in the back than in the front, because there are flight regions where you don't have enough aerodynamic or gimbal authority to control orientation, so you are limited to the reaction wheels. But you can have the CoL and the CoM.at basically the same spot. The key to stability if you go this route (which seems like the way you want to go) is control authority, You need more of it. If your plane is still dynamically unstable, add more canard area. Add a second set. Add bigger rear stabilizers. And bigger reaction wheels.

The canards you were using in your Agathon don't have enough area in the moveable surface to enforce dynamic stability.

 

[GoSlash27]

On 2/12/2017 at 0:11 PM, AeroGav said:

Which version of the file did you download?   I took this one, which could be cleaner but nevertheless easily has the power to hit mach 1.  The problem is it's barely controllable , I can't get it to pitch within 10 or 15 degrees of my intended flight path.

AeroGav,

 I didn't download any of them. My install is 100% vanilla, so I can't open them. I can see by the original pic that the tail would be extremely draggy because it's not actually occluded by anything.

If he's no longer using that design, please disregard.

*EDIT* Nevermind. I just saw the Agathon II. The tail on that design is fine.
 

On 2/12/2017 at 0:11 PM, AeroGav said:

re: NERV usage - 2k dV  - even allowing for the fact you're going to lose 800dv or so  getting from the speedrun to LKO, that's a lot - enough to go to Minmus and back.   Is that really more easily done on a RAPIER only ship?     On my 30 ton designs, burning 2 tons of LF/O in the upper atmosphere when the RAPIERs switch mode, only adds about 300 m/s velocity, though we are in a climb so I suppose there's losses.

In his case, he said that he was using the RAPIERs to get to orbit and then the LV-Ns for the rest of the trip. Doing it his way, the LV-Ns aren't worth bringing along. Doing it your way, it's a different story.

If the LV-Ns are used exclusively for the trip to Minmus and back (call it 1,800 m/sec total) and have a minimum t/w of 0.5, the math works like this:

The LV-N has 60 kN of thrust, so total ship mass per engine is 12.2t. Fuel to make 1,800 m/sec at an Isp of 800s would comprise 20.5% of this mass. Plus tanks to hold it, 23.1%, or 2.81 tonnes. The Nerv is another 3 tonnes and the unused RAPIER another 2 tonnes. So you're looking at 4.39 tonnes of payload (which is airframe, empty tanks from the trip to orbit, and whatever payload) per Nerv.

Doing the same math for the RAPIER at t/w= 1 (they tend to be a bit overpowered in orbit)

The RAPIER has 180 kN of thrust, so total ship mass per engine is 18.3t. Fuel to make 1,800 m/sec at an Isp of 305s would comprise 45.2% of this mass. Plus tanks to hold it, 50.9%, or 9.31t. The RAPIER adds 2 tonnes. So you're looking at 6.99 tonnes of payload per engine.

In this case, the fuel mass savings of the LV-N simply aren't enough to overcome the mass of the engine and the penalty of hauling around an unused RAPIER. Plus, the plane itself must be larger and draggier because it has additional parallel nodes and the additional mass of the LV-Ns, which are cargo at that point.

 Now... if he does it your way, using the NERVs to achieve orbit as well as make the trip to Minmus and back, then the math flips.

Best,
-Slashy

 

 

Edited February 15, 2017 by GoSlash27

[GoSlash27]

IRT the instability problem...

I can't see the plane you're talking about (I don't do mods), but if it looks like these 2 previous designs, the problem is that the center of drag is ahead of the center of mass. The Mk2 fuselage makes crazy drag when not aligned with the airflow and there's a lot of it ahead of the wings and mass. It would create a serious pitch instability problem.

Best,
-Slashy

[AeroGav]

14 hours ago, GoSlash27 said:

IRT the instability problem...

I can't see the plane you're talking about (I don't do mods), but if it looks like these 2 previous designs, the problem is that the center of drag is ahead of the center of mass. The Mk2 fuselage makes crazy drag when not aligned with the airflow and there's a lot of it ahead of the wings and mass. It would create a serious pitch instability problem.

Best,
-Slashy

It was my guess, or something about the procedural wing.  I got errors warning the mod is not compatible with current ksp, and was unable to edit the wing shape.  I'm concerned the way it looked in the SPH wasn't the same as the way it's reporting to the physics engine.

My XKOS only has stock wings, also the nervs are mounted further forward, with less fuselage ahead of the CoM.  Don't think he'd be having this problem in a mk1 or mk3 design.

I test flew my XKOS again last night and whilst it remains very well behaved in pitch, I was annoyed to discover it now has the dreaded "slow rolling to the left" problem.  It definitely did not do that in 1.2.1  when it was built,  I suspect I've attached the wings to the NERV nacelles and in 1.2.2. that's a no-no.  

Having to keep updating your designs every time the laws of physics change is a reason why i'll never have much more than 30 craft on KerbalX . Older stuff that requires too many changes just get deleted.

I'll attach the wings direct to he fuselage then offset them outward with EEX, but that means setting up the incidence & dihedral angles again, tuning CoL  and longitudinal washout.