Rapiers seem too weak

[eddiew]

Hi there,

Basically, problem in the title. After some testing with spaceplane builds, I'm pretty sure that rapier engines only reach their power band at around 360-400m/s, after which they'll often run away with themselves and get faster and more powerful until you exceed their altitude limit. Below this speed, at any altitude, it's hard to get more than 100-150kN out of them without diving, while above this speed they'll easily head for 200-250, and beyond.

However, it can be impossible to reach that speed with what seem to be very sensible ships. This plane, with only two rapiers on it will max out at 310m/s, will not be able to reach the upper atmosphere, and will never go fast enough to engage rocket mode. If given a pair of turbojets, it will reach 750m/s at 21km.

Adding third rapier makes the difference, because it gets you past the 400m/s barrier and suddenly the power spikes up and past 5-600m/s you can shut the third engine down because the primary pair are pushing out 275kN each. But, like pretty much everyone who flies SSTOs, what I instinctively want to do is put a nerva on the back end for a bit of range.

My question is thus this:

Is it intentional that rapiers require either an excessive number of them, or additional turbojets, in order to make them reach their operating speed?

If it's intended... well, ok... but I can put two turbojets and one LV-T45 on that same plane and it will do the same job, for less money, and much lower science investment. Is that intended?

I don't believe there's a problem with the maximum thrust output - as said, once you can get a rapier past ~400m/s it'll basically run away from you - but the power curve may need some love in the low-mid range.

Right now there seems to be no compelling reason to research or use rapiers for small-medium size ships. It's possible that as craft scale up, rapiers become more appealing, but at the size illustrated here, they are not useful and do not feel like they reward players for researching them.

Appreciate thoughts

*edit*

Here's my napkin maths on where I think the problem that I (and others) are facing is coming from. (Note I'm not a modder, I may have the meaning of these configs completely wrong, and if so I apologise and just let me know. I have also used simple linear interpolation rather than drawing a proper curve. I suspect the real values are actually a little lower than I have reached.)

I'm not sure what altitude results in a pressure of 0.16atm, but taking both engines side by side, approaching mach 1, we can see that the rapier is clearly very far behind when it's trying to cross the supersonic threshold. These are also about the numbers I observe in the part menu when a rapier plane has hit its maximum velocity, vs it's turbojet equivalent which will be zooming off ahead. And remember that there are two engines on my plane! Two turbojets are worth almost exactly three rapiers - which interestingly is what I already concluded based on the illustrated configurations above.

In short, the rapier's lifting potential from ground to orbit is significantly less because on its own it can't get through the transitional phase to where it is better than the TJ at higher altitude. You have to support it with a TJ, or simply have more of them, which rather makes the point of a hybrid engine a bit redundant.

Perhaps this is by design, but if so it is a curious decision, since rapiers are an end-of-tree and expensive (in science) part. My belief was that they're there to free up an engine slot on the back from being a high TWR rocket, to being a high efficiency rocket (or simply save weight). In their current state, I'm not convinced they are capable of doing this at this scale of plane. I completely accept that larger planes scale well, and four rapiers resolves this problem; but it also means a bigger, heavier plane that needs a bigger set of interplanetary engines... etc.

Edited May 2, 2015 by eddiew

[allmhuran]

I've been making aircraft of about that size and using 4 rapiers, but if your issue is just breaking past 310-320 m/s, keep in mind that you get massively increased drag as you cross mach 1, but it will then drop again. So what you might want to do is switch over to rocket mode for a few seconds to push through the barrier, then switch back to air breathing.

[eddiew]

I've been making aircraft of about that size and using 4 rapiers, but if your issue is just breaking past 310-320 m/s, keep in mind that you get massively increased drag as you cross mach 1, but it will then drop again. So what you might want to do is switch over to rocket mode for a few seconds to push through the barrier, then switch back to air breathing.

Good suggestion - I hadn't accounted for mach effects. Unfortunately, there's three reasons that doesn't work so well...

a) The air mode will generally be delivering 130-150kN at this mach 0.95 between 10 and 15km. Rocket mode is 170; the difference is very small.

When you go back to air mode, your thrust is reset and the engine spools up, so you lose about 80m/s. Getting that while in rocket mode in thick atmosphere will burn up half your fuel.

c) You can't go higher before a and b because you lose lift, and you need a crazy AoA, which ramps up your drag and just makes the whole situation worse

Edited May 2, 2015 by eddiew

[allmhuran]

Try it, I think you'll find it works. The slightly extra thrust is all you probably need, and once you push up to, say, 400 m/s your drag will be low enough that your rapiers should light up fine before you drop back below mach 1. Works for me, although it's certainly easier to just add another engine. If you think your engines won't spool up fast enough, try setting just one or just two to switch over.

[eddiew]

I did try it... twice. It really doesn't work with twin rapier planes, and I've seen people with quad-rapiers that are also struggling in this speed range so I know it's not just me. For this particular plane, if you try rockets at 10km you're lucky to gain 100m/s for all your fuel due to thick air; by 18km the AoA is 20 degrees to stay level, and you can't overcome the increased drag.

The power curve on rapiers simply isn't competitive with turbojets when in the mach 0-1.5 range. Swap the rapiers for turbos and you can ascend rapidly all the way to 21km at 750m/s, which is what I would have expected rapiers to also deliver, being the more advanced part.

Agreed, I could add a third engine; which I indeed tried, and it was a great ride to orbit - but what I want to add is an LV-N to give some range past LKO, not another inefficient LFO rocket or air-only jet. Scaling it up to 4 rapiers and a nerva is of course an option, but it doesn't get around the original observation that two turbojets and a starter rocket engine are as good as three rapiers.

This does not seem right for an end-of-tree part.

[eddiew]

Added some napkin math to the initial post.

[allmhuran]

Maybe your ascent profile is too aggressive or you need more lift. Here's a 2-rapier ascent with a craft that looks like it weighs about the same as yours. I don't even have to switch to rockets to break mach 1 here, I just level off at about 12,000m and push up to mach 2 before climbing up to 27,000m and switching to LOX. I have significantly more lift than you do, so I can aim close to the horizon in order to pick up speed at mach 1 instead of having to waste thrust fighting gravity:

]

[eddiew]

That's very interesting, thank you!

...I have absolutely no idea how that works for you :S Your ascent barely even seems to have a hiccup in it, and I've been trying at angles ranging from 30 AoA down to barely 10m/s vertical... I'll have a go at replicating your build and see what happens. There may be something in the specific wing parts I'm using that's causing issues.

But I stand by my napkin math about low thrust vs turbojets at lower altitudes - that much at least is borne out by the numbers and it would be nice if rapiers were at least nearly as good as turbos rather than 30% weaker

[allmhuran]

The thing is, rapiers are *way* better than turbojets as you get to higher speeds and higher altitudes. So keep using regular jets for... well... regular jets. Switch to rapiers (or a combination of the two) for your spaceplanes. I have one that has a single turbojet to tool around at lower altitude or to fly back to KSC after de-orbiting, but that engine gets switched off at about 18000m because it's just not useful anymore.

[eddiew]

Well, that was fascinating... short version, your design works, mine doesn't.

Despite the increased drag of 1.0.2 it seems that wing area is not a bad thing if it means you can fly level at very low AoAs. Not entirely intuitive, since supersonic planes in the real world aren't exactly generous on the wing area... but ok, at least I know.

Much obliged for the information Allmhuran, that's a very critical detail that I shall, er, take under my wing...

(That or those big wings have way to little drag vs the small ones. Might investigate the part configs and find out for sure...)

Edited May 2, 2015 by eddiew

[allmhuran]

Being able to put some fuel in the wings certainly helps, saves you an extra part or two worth of drag (although a Mk2 liquid fuel only fuselage is probably decent as well since it provides a little lift). Good luck with your designing, let us know what you come up with!

[eddiew]

The thing is, rapiers are *way* better than turbojets as you get to higher speeds and higher altitudes. So keep using regular jets for... well... regular jets. Switch to rapiers (or a combination of the two) for your spaceplanes. I have one that has a single turbojet to tool around at lower altitude or to fly back to KSC after de-orbiting, but that engine gets switched off at about 18000m because it's just not useful anymore.

Oh that much I could see My problem has been getting to the crossover point, since rapiers lag behind significantly at subsonic speeds. It never occurred to me that the answer would be more wing, since that's generally synonymous with drag, and thus makes matters worse.

Or at least, it did under FAR I have much to learn about nuStock.

(That said, dear devs; rapiers do still suck in subsonic and trans-sonic. Rly they do. Any chance of picking that up a little and removing a common source of frustration?)

- - - Updated - - -

Being able to put some fuel in the wings certainly helps, saves you an extra part or two worth of drag (although a Mk2 liquid fuel only fuselage is probably decent as well since it provides a little lift). Good luck with your designing, let us know what you come up with!

I forgot to put fuel in the wings... it went to space anyway xD I'm sure my next thing will end up in the SSTO thread - hopefully I can spread the knowledge around a bit, cos there's still a lot of frustrated players out there struggling with SSTOs. Tbh, I'd have given up by now if I hadn't started at turbojets and succeeded... the correct method of rapier ascent is not intuitive!

[Alias72]

Oh that much I could see My problem has been getting to the crossover point, since rapiers lag behind significantly at subsonic speeds. It never occurred to me that the answer would be more wing, since that's generally synonymous with drag, and thus makes matters worse.

Or at least, it did under FAR I have much to learn about nuStock.

(That said, dear devs; rapiers do still suck in subsonic and trans-sonic. Rly they do. Any chance of picking that up a little and removing a common source of frustration?)

- - - Updated - - -

I forgot to put fuel in the wings... it went to space anyway xD I'm sure my next thing will end up in the SSTO thread - hopefully I can spread the knowledge around a bit, cos there's still a lot of frustrated players out there struggling with SSTOs. Tbh, I'd have given up by now if I hadn't started at turbojets and succeeded... the correct method of rapier ascent is not intuitive!

RAPIER is based on SABRE. Both are high altitude and speed engines. Neither will work well in airbreathing mode at low speeds.

[eddiew]

RAPIER is based on SABRE. Both are high altitude and speed engines. Neither will work well in airbreathing mode at low speeds.

In reality, sure, that's something you can say "that's how the physics is"... but in-game, does it make it more fun that your best engine in that part of the tree isn't so good at getting off the ground as a much more accessible predecessor?

I'm glad to find that you can build around the lack of low speed power (presumably within limits), but it still feels a bit of an odd choice.

[Aanker]

Modular designs might be one solution to this problem. Basically, attach one or several of the rapiers to the main structure by way of docking ports. Then have an 'engine depot' in orbit where you can switch out the undesired engine for one more suitable to the mission. I used to do this a lot. Launching and building up this kind of infrastructure is actually kind of interesting as you will need tugs, more spaceplanes to put the stuff in orbit, etc.

Though it may technically no more be an SST-Duna.

[StainX]

yeah, those RAPIERS are hard on the limit, at about 13km up you really need the TWR and get the plane level so you get the best drag/lift ratio out of there.

Its a really fine line to hit.

Maybe i just change some things in the config

[blu3wolf]

In reality, sure, that's something you can say "that's how the physics is"... but in-game, does it make it more fun that your best engine in that part of the tree isn't so good at getting off the ground as a much more accessible predecessor?

Well, yes.

The other option has no challenge.

[eddiew]

yeah, those RAPIERS are hard on the limit, at about 13km up you really need the TWR and get the plane level so you get the best drag/lift ratio out of there.

Its a really fine line to hit.

Maybe i just change some things in the config

It's insanely fine; I added half a ton to a working design and it stopped working... and yeah, I'm on the brink of increasing the power curve in the config file, because I'm just bored of it now. The design process ceased to be fun about 5 hours ago...

[StainX]

It's insanely fine; I added half a ton to a working design and it stopped working... and yeah, I'm on the brink of increasing the power curve in the config file, because I'm just bored of it now. The design process ceased to be fun about 5 hours ago...

So i guess i just play with the 1.0 aerodynamics config, the drag in 1.0.2 seems too high in my opinion

[eddiew]

So i guess i just play with the 1.0 aerodynamics config, the drag in 1.0.2 seems too high in my opinion

It was done for capsule re-entry, but it was also done in a hurry due to a lot of players having problems with it, and the result was a lack of time to really assess what it did to other parts of the game... in our case, broke SSTOs that worked in 1.0 :/

[StainX]

Also a good idea would be: a new slider in the settings menu for aerodynamics from "Fun" to "Realism"... but that would also mess up the word "stock"

[Mipe]

For some among us, "fun" is synonymous with "realism". So that setting would be hella confusing.

[StainX]

For some among us, "fun" is synonymous with "realism". So that setting would be hella confusing.

Then lets call it "Rookie" and "Scott Manley", anyway thats exactly the problem with the 1.0.0 patch that people are complaining... there are players that like a tough&realistic gameplay, some that that just like to have fun and the rookies of course!

[Kuu Lightwing]

Maybe your ascent profile is too aggressive or you need more lift. Here's a 2-rapier ascent with a craft that looks like it weighs about the same as yours. I don't even have to switch to rockets to break mach 1 here, I just level off at about 12,000m and push up to mach 2 before climbing up to 27,000m and switching to LOX. I have significantly more lift than you do, so I can aim close to the horizon in order to pick up speed at mach 1 instead of having to waste thrust fighting gravity

Thanks for the help! I actually managed to put my SSTO into orbit!

[eddiew]

Just for the interested, this is the best I've been able to come up with so far. Triple rapiers, one nerv; about 550m/s by orbit, or 900+ if you carry spare fuel in the cargo bay. But obviously you're not carrying a cargo if you do that :/

Suddenly I really want an MK2-3x1.25 adapter