Rapier engine, trust?

[Sereneti]

The rapier engine, looks like, has a speed bonus, like the rapier engine,
but neither on the ingame nor the wiki are the speed bonus explained...

so did anyone have a information about the high of the speed bonus?

[Mastikator]

You mean like this thing? It's last year so I don't know if it's 100% accurate but it should still be in the ballpark.

[Sereneti]

thank you

and:
are the flame-out hardcodet, or are that a value of the intake air?

(aka dos intake-spam help, to increase the max heigh?)

[KerikBalm]

intake spam does not help

[Sereneti]

Thanks
 

[Rath]

You see, each jet can use a certain amount of intake air.  any intake air above this is wasted, therefore intake spam will not help.  If you want to know why this is, ask about air-hogging.

[KerikBalm]

Well, it has always been that way. The old airhogging allowed a jet engine to have sufficient air to operate even at 50km...

Now, the jet will flame out even when there is sufficient intake air, because thrust decreases with altitude, and at a certain altitude, it just produces no thrust, regardless of the supply of intake air.

[Sereneti]

6 minutes ago, KerikBalm said:

Well, it has always been that way. The old airhogging allowed a jet engine to have sufficient air to operate even at 50km...

Now, the jet will flame out even when there is sufficient intake air, because thrust decreases with altitude, and at a certain altitude, it just produces no thrust, regardless of the supply of intake air.

Ok
so the upper-limmit is a hard cap.
This hard-cap are interresting, too...

 

[cantab]

Yeah. The RAPIER has a strong "ram effect". If you get going fast enough the thrust starts climbing sharply and you can really pile on the speed. But if you climb too high too slow, or just have too heavy a plane for the number of engines on it, you can end up unable to get into that zone without descending. Same goes for the Whiplash, though I've less experience flying it.

[KerikBalm]

In the rapier.cfg file, this is the relevant part:

 

        velCurve
        {
            key = 0 1 0 0.08333334
            key = 0.2 0.98 0.42074 0.42074
            key = 0.7 1.8 2.290406 2.290406
            key = 1.4 4.00 3.887193 3.887193
            key = 3.75 8.5 0 0
            key = 4.5 7.3 -2.831749 -2.831749
            key = 5.5 3 -5.260566 -5.260566
            key = 6 0 -0.02420209 0
        }

* In this set of numbers, the first number is the mach number, and the 2nd number is the thrust multiplier. The next two numbers are the tangents for the curve on each side of the point.

In bold, you see that at mach 3.75, it has a thrust multiplier of 8.5

By the time you reach mach 4.5, its thrust multiplier is 7.3

At mach 6, it has a thrust multiplier of 0... the engine can't push you past mach 6, but it can push to mach 5.5 as you see the thrust multiplier at 5.5 is 3. Note that thrust decreases slightly from mach 0 to mach 0.2, but then increases to 1.8x the stationary thrust by mach 0.7. At mach 1.4, its really starting to kick to higher thrust levels... with a thrust multiplier of 4.00

        atmCurve
        {
            key = 0 0 0 0
            key = 0.018 0.09 7.914787 7.914787
            key = 0.08 0.3 1.051923 1.051923
            key = 0.35 0.5 0.3927226 0.3927226
            key = 1 1 1.055097 0
        }

This curve is like the curve above, except the first number is the atmospheric pressure, not mach number. When you reach 0.018 atmospheres (basically 2% of kerbin sea level), its only producing 9% thrust... if you're going mach 3.75 at 0.018 atmospheres... the thrust would be 8.5*0.09 = 0.765... ie 76.5% of stationary sea level thrust. If you're going mach 5.5 at 0.018 atmospheres, its 3*0.09 = 27% of stationary thrust.

There is an "ignition threshhold":

        ignitionThreshold = 0.33

I *think* if the speed multiplier * the atmosphere multiplier is below 0.33, then the engine flames out.

This means that even at mach 3.75 (where you get a 8.5x thrust multiplier), if the atmospheric density multiplier is 0.0388 or below, then the engine flames out. The altitude at which the engine flames out depends slightly on your velocity, but it will always flame out before 31km. Sometimes it may flame out at 28km, if you are already going very fast. As mentioned, mach 5.5 at 0.018 atmospheres, the multiplier is 0.27, which is below the 0.33 limit... the engine would be flamed out

 

Also, I think this was added recently, there is a flow multiplier cap:

        flowMultCap = 3.0

So that the thrust multiplier * the atmosphere multiplier means that you can't have more than 3x stationary thrust... so going mach 3.75 at sea level will only give you 3x thrust, not 8.5x thrust.

Edited May 18, 2016 by KerikBalm

[Sereneti]

hm..
it looks like that is a "step by step" calculation, not a "formula calculation"...

"ignition thereshold" -  a rapier engine has (from the wiki) a stationary trust of 105kN...
- so it should flame out at ~30kn -> but the engine dosnt flame-out even if it will reach the ~5KN trust...


so , betwen the 0.08 and the 0.018 atm the trust from the atmospheric pressure dosnt decrease...

 

[KerikBalm]

"so , betwen the 0.08 and the 0.018 atm the trust from the atmospheric pressure dosnt decrease..."

Why do you say that? btw, the word is "thrust" not "trust"

"- so it should flame out at ~30kn -> but the engine dosnt flame-out even if it will reach the ~5KN trust..."

Yes... I do seem to recall that they reach thrusts below 10 kN... I'm not quite sure how the flameout works... but I know that no amount of intake spamming will help you get higher

[Sereneti]

oh, thank you
Because there are "steps" , not a formula...

I ask that, because i have no information if i have enough air for the engines, are they trottle down because they lack of air, or because "hardcoded"...

 

 

[KerikBalm]

I think if you right click on the engines, it will display "prop requirement met" and list a percent. If its at 100%, that means they have all the intake air and liquid fuel they need.

Its "hardcoded"...

[Fearless Son]

5 hours ago, cantab said:

Yeah. The RAPIER has a strong "ram effect". If you get going fast enough the thrust starts climbing sharply and you can really pile on the speed. But if you climb too high too slow, or just have too heavy a plane for the number of engines on it, you can end up unable to get into that zone without descending. Same goes for the Whiplash, though I've less experience flying it.

From my experience with the Whiplash, it tends to favor a more aggressive ascent than the RAPIER.  With a RAPIER you want to build as much horizontal velocity as you can while in its relatively high optimal envelope.  A Whiplash can also generate tremendous thrust with a ram effect but at a much lower envelope than the RAPIER, so it tends to require a steeper profile to make orbit.  A Whiplash-driven spaceplane pointed up nearly forty-five degrees from a surface tangent will easily launch you out of atmosphere, but with only modest horizontal velocity.  It requires more substantial vacuum thrust to circularize than the RAPIER does, which means more engines and mass.  

[Sereneti]

13 hours ago, Fearless Son said:

From my experience with the Whiplash, it tends to favor a more aggressive ascent than the RAPIER.  With a RAPIER you want to build as much horizontal velocity as you can while in its relatively high optimal envelope.  A Whiplash can also generate tremendous thrust with a ram effect but at a much lower envelope than the RAPIER, so it tends to require a steeper profile to make orbit.  A Whiplash-driven spaceplane pointed up nearly forty-five degrees from a surface tangent will easily launch you out of atmosphere, but with only modest horizontal velocity.  It requires more substantial vacuum thrust to circularize than the RAPIER does, which means more engines and mass.  

i have some small orbiter (spaceplanes) that use 2 whiplash and one Rapier ...
 

[Snark]

Moving to Gameplay Questions.

[KerikBalm]

I Like the whiplash for my Vertical launch SSTOs (often a rocket-airbreathing hybrid, with the rocket engine firing the whole time, instead of only after a climb)... its static TWR is significantly higher... nearly 50%

It also continues to have a higher TWR until about mach 3.3 (although it also depends on the altitude at which you reach mach 3)

I find for my vertical launch SSTOs that the time I spend in the conditions where the rapier is better than the whiplash is not enough to justify using the rapier.

I concur, for steep ascents, use the whiplash. For slower more horizontal climbs, use the rapier

[Fearless Son]

10 hours ago, Sereneti said:

i have some small orbiter (spaceplanes) that use 2 whiplash and one Rapier ...

Aye, and that is a pretty strong combo.  The Whiplashes ought to get the RAPIER up to an altitude and speed where its thrust can be maximized, pushing ever more air into the engines by raw speed.  The only caution I would have about that approach is that you can get going too fast at too low an altitude and risk damage to the plane due to atmospheric compression overheating it.  Literally an air-friction burn.  But so long as you have a good management of the throttle and a good grasp of the flight envelope you should be good.  

 

9 hours ago, KerikBalm said:

I Like the whiplash for my Vertical launch SSTOs (often a rocket-airbreathing hybrid, with the rocket engine firing the whole time, instead of only after a climb)... its static TWR is significantly higher... nearly 50%

Sounds like you use the old tradition of air-breathing boosters.

[cantab]

For my vertical launch (and landing) SSTO I'm using two RAPIERs and two Panthers. The Panther in wet mode has the highest TWR of any jet, while still having vastly more efficiency than a rocket, so it's the natural choice for a VTOL lift engine I feel. So far I've been switching them to dry shortly after takeoff for fuel efficiency, and then once they cease producing thrust it's just the Rapiers by themselves to finish the job, but seeing the above graphs I might try leaving them in afterburning mode all the way. Might be too much thrust though, the thing already seems to have plenty of power.

[KerikBalm]

yes, the panthers have the best static TWR... but they have the same thrust as a whiplash, but with lower mass. Most of my VTOL sstos  have a rocket like the KR-2L, and the mass of the airbreathers isn't so significant... I tend to care about thrust per node more than thrust per unit mass for the airbreathers. Then they give a lot better performance later in the flight... as I said, I do these for steep ascents... and the even more massive rapier doesn't make sense to me. I use turbos when I plan on climbing at like 45 degrees down to 30 degrees when airbreathers cut out...

With rapiers, I climb at like 10-25 degrees initially, down to 5-10 when I switch over to closed cycle. It depends how fast I want to reach orbit.

I have made a laythe VTOL ssto that uses the rapier-panther combination like you mentioned. Rapiers are even more attractive on laythe because they get you even closer to orbital altitude and velocity (in terms of %).

2 rapiers and 2 panthers has a slightly better TWR than 4x turbos... and then you've also got 2x closed cycle rapiers to finish... which is great for small vtol SSTOs... but when I'm just strapping airbreathers on to my KR-2L powered SSTO rocket to supplement the KR-2Ls lackluster low altitude performance, I take the turbos. (8 of those producing 300 km or so thrust at their peak produce 20% more thrust than the KR-2L does).

As for leaving the panthers in wet mode... I'd do it... why not get more thrust from an engine with 4000 Isp, than make the rapiers do most of the work at 3200 Isp?