Rapier closed cycle, OP or in need of a Buff?

[FleshJeb]

22 minutes ago, KerikBalm said:

or go back to stock... SSTOs for 3x kerbin are quite punishing... snipped for space reasons. Responding to all of it.

I think you might have a valid point there. If  that change were to occur, I’d like to see the rocket thrust reduced to about 80kN, (and a mass increase to at least 2.2 tons). Those nozzles are about the size of 4 Sparks.

If you’re going to be playing 3x, you might want to look into OPT. The SchramJet engines are about the most entertaining kick in the pants you can get, without being ridiculously OP.

[Guest]

I forget that you're playing with a mod system. Of course you'll want mod parts for a mod system, 3x Kerbin will radically change the balance and a lot of things will be out of whack. I'm discussing the stock game.

I'm generally opposed to rebalances of long-established parts because they will screw up lots of craft designs, and one of the terrific things about KSP is that craft that worked in 1.2 will generally speaking still work the same way in 1.7. Rebalancing new parts released in a DLC a couple of patches in is a different matter, but once they're out there and in use, they should be left alone unless they really drastically break something.

But I can definitely see that a rebalanced system would require rebalanced parts. So why not bake that rebalance into the mod, or release a companion mod with it?

Edited June 19, 2019 by Guest

[KerikBalm]

Well, I find that stock size is a bit too easy, and I find stock parts are generally balanced quite well for a 3x rescale... so much so that its almost become my standard. If its balanced so that it works well in 3x then its balanced for stock size which is more forgiving, with much greater margins, and is much more newbie friendly.

You can still SSTO on 3x kerbin with over 15% payload fraction... but not by relying on rapier closed cycle... although you can use it for some of the initial acceleration. Rapier closed cycle can get you to orbit without use of a higher Isp engine, but with basically no payload fraction.

[sturmhauke]

17 hours ago, KerikBalm said:

These numbers are quite off in my experience.

First... No way are you getting 1800 on rapiers... Unless you conflate Orbital and Surface velocity. 1650 m/s max if its barebones just for airbreathing speed. My cargo sstos get 1450-1550 on rapiers.

Meanwhile whiplashes, barebones, will get about 1450, and as a cargo laden ssto... 1250-1350

You might be able to hit 1800 m/s with Rapiers, but probably not in a space plane. I used a fighter jet design for a long distance race and hit a max of 1700 m/s at 21 km altitude. And although it was not able to maneuver in space (no oxidizer, open cycle only), it could reach low suborbital on certain ascent paths.

[Geschosskopf]

9 hours ago, Brikoleur said:

Reason I like spaceplanes is precisely because I find them easier to fly to orbit. When I build a rocket, I quite often have to do a trial or two to get the gravity turn right, as it depends on TWR with each stage and is hard to correct if you don't. On the other hand I will get a spaceplane to orbit the first time almost every time. 

Strange :).  I find building and flying your standard "payload on top" lifter rocket the simplest thing to do in KSP.   I always use the same simple general lifter configuration regardless of size so they all use the same ascent profile and handle the same in the air and in space.  Such lifters take only a few minutes to build and only a few more to get to LKO.

 

7 hours ago, KerikBalm said:

Well... it depends how marginal... you're still getting 10x the Isp you can get with LFO engines. and going flat at 30 km isn't such a problem if you keep 0 AoA and have a very aerodynamic plane. Of course there is such a thing as too low, but from what I remember, the >50% payload fraction craft start with TWR on the low end of the spectrum.

That's my understanding as well.  @GoSlash27 made a number of examples that had TWR just barely over 1.0 and a ridiculously high payload fraction, but which therefore took 30 minutes or so to reach orbit.   Impressive but not my cup of tea

 

7 hours ago, KerikBalm said:

All they'd need is a buff to their terrible vacuum Isp. I'm thinking 320 Isp

 

7 hours ago, KerikBalm said:

I'd be fine with nerfing their atmospheric closed cycle Isp... which is quite good (better than a Rhino/swivel/reliant/spark/Skiff, for comparison). I don't get the reason it has good atmospheric Isp at all... it makes no sense from a realism, design, or balance standpoint.

 

I'd go along with this myself.

[Nich]

11 hours ago, KerikBalm said:

or go back to stock... SSTOs for 3x kerbin are quite punishing... for now I switched to 2 stage designs with the rapiers just burning long enough to get the Ap high enough so that the 1st stage can be recovered.

Still, I don't see why their vacuum Isp has to suck so much. Its not realistic (the real engine its based on should have close to optimum vacuum performance), it doesn't make sense from the perspective of where the closed cycle mode will be used (high altitude/vacuum, never at 1 atm). It just reeks of artificial balance to stop them from being too good for their 1 and only 1 role.

I'd be fine with nerfing their atmospheric closed cycle Isp... which is quite good (better than a Rhino/swivel/reliant/spark/Skiff, for comparison). I don't get the reason it has good atmospheric Isp at all... it makes no sense from a realism, design, or balance standpoint.

To  be fair they are one of the most expensive nodes in the tech tree to unlock.  So I cant argue against making them a disgustingly OP engine.  Could always counter balance by adding a ton or 2

[Nich]

@Geschosskopf 1 TWR seems really high for a plane based SSTO.  Most people recommend 1 rapier per 15 tons but I think >50% requires more like 40-50 tons per rapier.

[KerikBalm]

20 minutes ago, Nich said:

@Geschosskopf 1 TWR seems really high for a plane based SSTO.  Most people recommend 1 rapier per 15 tons but I think >50% requires more like 40-50 tons per rapier.

Indeed, mine (for 3x KSP) have about 0.3 to 0.35 TWR on the runway. As fuel burns off and they hit their power curve, they do reach 1.0 at some point during the ascent though

[Nich]

In my experience you want just enough engine to get 100-120 m/s by the end of the runway and just enough wing that you don't crash into the sea with 3 degrees pitch and you will probably make it to space. Although I should try a 150 m/s version with less wing some day.

Edited June 19, 2019 by Nich

[GameTourist]

 

On 6/18/2019 at 12:45 PM, Fearless Son said:

Admittedly, I don't use R.A.P.I.E.R.s much for many of the reasons you just described.  I find most spaceplanes I build tend to be built with high-thrust Whiplashes and a low-thrust, high ISP vacuum engine.  Instead of the long, slow speed build up of the R.A.P.I.E.R., I go for an aggressive ascent at a forty-five degree (give or take) angle where the Whiplashes can build a bunch of speed and send the plane into a suborbital trajectory on air-breathing thrust alone, then uses a long, slow burn from it's vacuum engines to circularize.

Such a design is not, strictly, as efficient as more "elegant" spaceplane designs, but it gets the job done in a shorter amount of time and doesn't require quite a fine-tuned an ascent pattern.  It deals with more drag, but it also gets out of the draggy part of the atmosphere quicker and before spending too much fuel fighting it. 

Your approach makes me interested in building SSTOs again! 

Edited July 3, 2019 by GameTourist

[ajburges]

What people tend to forget is there are three vertices of efficiency: cost, time, and complexity. Gravity turn accents optimize time while most SSTO spaceplanes optimize cost. Other than challenges, you typically want considerations to all three aspects. There are plenty of trade-offs to make between them.

[Xavven]

To answer OP's original question, no I don't think the RAPIER needs a buff. It's already the best engine in the stock game for SSTO launches. It outperforms the whiplash in airbreathing mode, as it has more thrust and operates at higher altitudes, and on top of that it has great thrust in closed cycle. In my most recent spaceplane design, whiplashes + swivels could not get me to orbit in a single stage, but all-RAPIERs did so extremely easily.

[Aeroboi]

3 hours ago, Xavven said:

To answer OP's original question, no I don't think the RAPIER needs a buff. It's already the best engine in the stock game for SSTO launches. It outperforms the whiplash in airbreathing mode, as it has more thrust and operates at higher altitudes, and on top of that it has great thrust in closed cycle. In my most recent spaceplane design, whiplashes + swivels could not get me to orbit in a single stage, but all-RAPIERs did so extremely easily.

Yes, I don't see it's "mandatory" or logical for "stock" KSP and the OP mentions 3x rescale but that isn't stock KSP.

Engine variations should specialize themselves in different ways and a 330-340ISP rapier would be a turboramjet with high isp rocket capabilities doing multiple roles at once.

The most economical way is for a SSTO space plane is to be a SSTO. That means it hauls stuff to and from orbit, going elsewhere is less efficient. That means Rapiers by function should should only be used as the rocket part of a space plane to and from orbit. The only other option is sending one to Laythe, but that is the only case whereby you'd use the LV-N anyway, at least, that is the most sensible option.

In most circumstances the rapier space plane has 1500-1600m/s velocity until closed cycle mode with burned off fuel it nearly always have enough TWR not to bother coupled with the lift effect. Also, since your close to orbital velocity you have little delta-v needed and at 30-39Km use much of the oberth effect.
MECO is suspected to be within 35-45Km depending on space plane flight path. Giving the Rapier a ISP buff would do little to have any effect and it's better to bring a LV-N anyway or use another dedicated vessel with Rhino, poodle or skipper.

Adding more thrust also means larger rocket nozzles on the Rapier, while having extended nozzles for extra ISP, so that's 2 Buff's to make it a engine on par and better then many others.
Rapier shouldn't be used for VTOL and whiplashes work quite decently, and, gets you to orbit.
For VTOL SSTO I typically use as little wings since you have so much more thrust anyway you don't need as much and couple it with some vectors or aerospike's depending on weight.
 

Edited July 4, 2019 by Aeroboi

[Incarnation of Chaos]

Personally i think the main issue with the rapier is not it's performance; with a pre-cooler and inlet cones 2X rapier can push me to >Mach 4 and 25K before flaming out (This is with FAR being used though). The main issue i find is that it's the only engine of it's kind, on a endgame node and ends up being more of a curiosity than much else. Rather than buffing the existing RAPIER i would like to see dedicated spaceplane engines expanded, with 1.875m, 2.5m and 3.75m parts. And before everyone says it; i'm more than aware mods for this exist (They're awesome and i have them downloaded).

Me personally; since i already use mods i focus on reusable launchers rather than SSTO's.

[Empiro]

I also think the RAPIER is an overall well-balanced engine. It's a pretty specialized engine, and you don't always want to use it, but when you use it for what it's designed for, it works great. It's clearly balanced for Stock KSP though, which is why the ISP is so poor compared to say, the specs of the real-life in-development SABRE engine.

I wouldn't mind a bit more Vac ISP, but at the same time, nerfing the Atmospheric ISP wouldn't do much. You're only going to use it around 25 km, at which point you're at 1% sea-level pressure.

[Fearless Son]

On 7/3/2019 at 8:17 AM, GameTourist said:

Your approach makes me interested in building SSTOs again! 

Here's an example of a small spaceplane as a proof-of-concept piece I called the "Flechette":

Two Whiplash engines give this thing a great acceleration force as it rises up through the atmosphere.  The aerospike engine is activated before the Whiplashes flame out, and the additional thrust from it forces more of the increasingly thin air into the Whiplashes, allowing them to continue to contribute thrust longer than they would alone.  The LF/O engine continues to burn throughout the parabola of the launch, and cuts out at about the time it reaches the apoapsis or shortly thereafter.  It doesn't even need a separate circularization burn, the initial one is enough.  It does get a little nail-bitey though since it's not clear if you have enough delta-v to make orbit right up until the point you actually make orbit.

On 7/4/2019 at 8:44 PM, Incarnation of Chaos said:

The main issue i find is that it's the only engine of it's kind, on a endgame node and ends up being more of a curiosity than much else. Rather than buffing the existing RAPIER i would like to see dedicated spaceplane engines expanded, with 1.875m, 2.5m and 3.75m parts. And before everyone says it; i'm more than aware mods for this exist (They're awesome and i have them downloaded).

If I had a gripe with the R.A.P.I.E.R., that would probably be it.  It occurs a bit too late in the tech tree for my taste.  It's advanced, sure, but not available at a time you would find it a practical upgrade compared to everything you had been doing prior to that point.  If it were up to me, I would move the R.A.P.I.E.R. one node earlier and in it's place at the far end of the aerospace chain I would put a larger scale version of the same hybrid air-breathing/rocket engine.  Same relative balancing of mass and performance, just everything bigger.  It feels a little odd to see all these heavy spaceplaces with nearly a dozen R.A.P.I.E.R.s each to get to orbit, when a few larger versions of the same thing would probably be better suited to their mission profiles.

[Incarnation of Chaos]

6 hours ago, Fearless Son said:

If I had a gripe with the R.A.P.I.E.R., that would probably be it.  It occurs a bit too late in the tech tree for my taste.  It's advanced, sure, but not available at a time you would find it a practical upgrade compared to everything you had been doing prior to that point.  If it were up to me, I would move the R.A.P.I.E.R. one node earlier and in it's place at the far end of the aerospace chain I would put a larger scale version of the same hybrid air-breathing/rocket engine.  Same relative balancing of mass and performance, just everything bigger.  It feels a little odd to see all these heavy spaceplaces with nearly a dozen R.A.P.I.E.R.s each to get to orbit, when a few larger versions of the same thing would probably be better suited to their mission profiles.

Indeed; with large spaceplanes especially it makes me wonder if they're better off using traditional rocket engines after a certain point. Once you're using 13+ RAPIER engines which are far from the lightest or most efficient, how much mass are you really saving? Especially since you end up needing additional whiplash engines for atmospheric flight eventually anyway; a RAPIER Sr. could be larger and even more efficient because of "Improvements" made using data from the previous engines if that was desired.

I wouldn't mind the RAPIER sent to a lower node at all; it just feels more suited as the final product of all Hypersonics development rather than a true endgame part.

[GameTourist]

On 7/10/2019 at 6:25 PM, Fearless Son said:

Here's an example of a small spaceplane as a proof-of-concept piece I called the "Flechette":

Two Whiplash engines give this thing a great acceleration force as it rises up through the atmosphere.  The aerospike engine is activated before the Whiplashes flame out, and the additional thrust from it forces more of the increasingly thin air into the Whiplashes, allowing them to continue to contribute thrust longer than they would alone.  The LF/O engine continues to burn throughout the parabola of the launch, and cuts out at about the time it reaches the apoapsis or shortly thereafter.  It doesn't even need a separate circularization burn, the initial one is enough.  It does get a little nail-bitey though since it's not clear if you have enough delta-v to make orbit right up until the point you actually make orbit.

Gorgeous
Thanks!

What about for big SSTOs?

Edited July 23, 2019 by GameTourist
add question

[Aelipse]

I am a sucker for SSTO spaceplanes, and back in the day when I was still playing stock, I managed to build a pretty decent SSTO. 4 Rapiers, 2 Whiplashes and 2 Nervs mounted on an Mk2 fuselage, wet mass was around 80 tonnes. The thing got easily into orbit even with some little payload stuck to its back, on its own had some 2,800 m/s delta v left after reaching the LKO, not to mention its mining capability making it a "all planets in one mission" candidate.

The absolute key to a successful SSTO is perfect aerodynamics. You cannot afford any unnecessary drag, since you spend much more time in the atmosphere than a rocket would. That means no clipping parts, no fancy airbrakes or parachutes (a well balanced spaceplane can easily bleed off speed by pitching up and down), and you have to be particularly careful when attaching fuselage parts to cargo bays - it has to snap to the cargo bay itself, not the contents of it. Also, putting the nose cones backwards on the rapiers (creating the famous rapier-spikes) decreases the drag a lot (and I suggest lowering their gimbal limit as they have a tendency to incinerate the nose cones when doing harsh maneuvers).

Long story short, if you pitch up to 10° to 15° and continue accelerating past the sound barrier, you've won. 

Rapiers are IMHO very cool engines and it's only fair that their ability to get you to high velocities within the atmosphere is offset by a rather low and crappy ISP in closed cycle.

Edited July 23, 2019 by Aelipse

[KerikBalm]

yea, but its so crappy that one doesn't really use it.

I'd be fine if it was just a faster version of the whiplash - but somehow giving it closed cycle, but making it really crappy in the regime in which its "supposed" to be used seems... worse than not having it at all.

[Aelipse]

Oh you'd be amazed! In combination with the atomic Nervs, rapiers are absolutely fine. For main burns you use nervs and switch the rapiers on only when you need extra thrust, like for example taking off from Duna or performing a suicide burn on the Mun.

[KerikBalm]

yes, I know, they are used for extra thrust, but the closed cycle should be used as little as possible, because it has horrible vacuum stats, because its got the Isp stats of an engine meant to be used at 1 atm, not .1 to 0. atms.... for... some reason...

[Empiro]

It's true the ISP sucks, but again, remember that it's a specialized engine designed for an SSTO spaceplane in stock Kerbin. For most ascent profiles, you only need around 1600 m/s delta-V to get into orbit, do some maneuvers, and then land. The ISP doesn't matter too much in that realm. If I want to go somewhere farther, I'd use use a cargo SSTO powered by Rapiers to get into orbit, and then deploy a craft equipped with very efficient vacuum engines. That way, you don't haul unnecessary engines and wings around.

[IronMaiden]

On 7/10/2019 at 9:50 PM, Incarnation of Chaos said:

Indeed; with large spaceplanes especially it makes me wonder if they're better off using traditional rocket engines after a certain point. Once you're using 13+ RAPIER engines which are far from the lightest or most efficient, how much mass are you really saving? Especially since you end up needing additional whiplash engines for atmospheric flight eventually anyway; a RAPIER Sr. could be larger and even more efficient because of "Improvements" made using data from the previous engines if that was desired.

I wouldn't mind the RAPIER sent to a lower node at all; it just feels more suited as the final product of all Hypersonics development rather than a true endgame part.

The advantages of spaceplanes are mainly the reusability and payload fraction. A plane powered by Whiplashes or even Panthers will still easily have a higher payload fraction than a traditional rocket. Number of engines doesn't really matter, it's all a ratio. If you're using 13+ RAPIERs then you've got a space plane that's 450+ tons, over half of which can be payload. To send a similar payload to orbit with conventional rocket engines your vehicle will be pushing 900 tons, so you're saving a lot of mass.

Whiplashes don't really augment RAPIERs all that well, Panthers have much better TWR at mach 0 and better drag characteristics. It doesn't matter if they flame out earlier than the whiplashes, because at that point the RAPIERs are in beast mode. The most demanding part of an SSTO launch is getting it off the ground and supersonic, once you've made it over that hump a single RAPIER will very comfortably push 45-50tons past mach 5 at an Isp of 3200s, Whiplashes don't really add anything here.

RAPIERs are already one of if not the best engine in the game. It's upper atmospheric performance is unrivaled, and the basically free built in rocket engine (no matter its faults) is the cherry on top. Making it more accessible, higher powered, better closed cycle efficiency, etc would devalue many other engines. It's in a fine spot as is.

[Incarnation of Chaos]

9 hours ago, IronMaiden said:

The advantages of spaceplanes are mainly the reusability and payload fraction. A plane powered by Whiplashes or even Panthers will still easily have a higher payload fraction than a traditional rocket. Number of engines doesn't really matter, it's all a ratio. If you're using 13+ RAPIERs then you've got a space plane that's 450+ tons, over half of which can be payload. To send a similar payload to orbit with conventional rocket engines your vehicle will be pushing 900 tons, so you're saving a lot of mass.

Whiplashes don't really augment RAPIERs all that well, Panthers have much better TWR at mach 0 and better drag characteristics. It doesn't matter if they flame out earlier than the whiplashes, because at that point the RAPIERs are in beast mode. The most demanding part of an SSTO launch is getting it off the ground and supersonic, once you've made it over that hump a single RAPIER will very comfortably push 45-50tons past mach 5 at an Isp of 3200s, Whiplashes don't really add anything here.

RAPIERs are already one of if not the best engine in the game. It's upper atmospheric performance is unrivaled, and the basically free built in rocket engine (no matter its faults) is the cherry on top. Making it more accessible, higher powered, better closed cycle efficiency, etc would devalue many other engines. It's in a fine spot as is.

Note i don't want the RAPIER it's self made more powerful; that would the job of a entirely new part. But i would like to move it down one node; because by the time you get it you're launching 3.75/2.5M rockets and can make them fully reusable. Payload fractions aren't important; i can build a fully reusable rocket that's larger in a tenth of the time it takes to fine-tune an SSTO and get the same payload to orbit and the same reusablity.  And 900 ton rockets don't really mean anything to me at this point; i was launching 2-300 ton stacks of asparagus to the moon and minmus in my current career save after only a week or 2 (The entire stack weighed about 300 tons; what made it to the mun was around 30-60 tons). And i frequently exceed the megaton range now ; fuel is silly cheap compared to engines/crew and reuse means essentially i'm only paying fuel costs.

That being said i find your arguement in favor of the Panther interesting; pushing whiplashes to Mach 1 wastes a lot of fuel before hitting the efficiency curve where they're actually doing work. Panthers would start out decent and only get better before flaming out; i may be running some tests fairly soon because of you xD.