Un-Nerf the Wolfhound. (And the Skiff)

[petlahk]

I'm not sure if this has been said already but I'm actually kinda upset that they nerfed the Wolfhound. That 412 vaccuum ISP was very important to my spaceplane design and allowed me to do things that just weren't possible before. The Wolfhounds weren't good enough to get a LF/OX spaceplane or rocket all the way out to another planetary system, but they were perfect for Kerbin SOI rockets and SSTOs. They were the perfect intermediary between the other engines and the NERV. Please un-nerf the wolfhound.

Edit: Similarly with the skiff.

Edited December 21, 2018 by petlahk
Forgot something. It's better to stick it in this post than make another one.

[RealKerbal3x]

The Wolfhound was just too overpowered and didn't fit in with the base game engines. Quoted from @Maxsimal's devblog:

Quote

Wolfhound: The Wolfhound is amazing in every category that matters - an ISP that's 20% higher than any other LFO engine, great TWR, unlocks relatively early, and is the cheapest cost/kn for an LFO engine.  Sorry rocketeers - the Wolfhound needed adjustment to have some valid trade-offs vs other vacuum engines.  It's still an amazingly efficient LFO engine, without having the sort of abysmal thrust & cost of a NERV, but now it doesn't completely overshadow every other LFO vacuum engine.  As a more specialized, high efficiency engine, its moved back in the tech tree with the other Making History Apollo-class parts as well.

It just had too many benefits over other engines, without having many downsides.

As for the Skiff, it was too good in too many categories. Maxsimal explains it best.

Quote

 

Skiff: The Skiff's tuning is closer to ideal , but it turned out to be *too* good in too many categories categories - more efficient, better TWR, and lower cost/kN than other engines.  It occurs later in the tech tree, so we've chosen to keep its high efficiency at the cost TWR and cost. Now it’s a great sustainer-category engine - its ASL ISP and cost won't justify its use as a main engine anymore, but it’s fantastic as the center stage with some SRBs or Kodiak-powered boosters.

EDIT: Link to the full devblog: here

Edited December 21, 2018 by RealKerbal3x

[Superfluous J]

I understand your unhappiness. I really do. However, the Wolfhound made your spaceplane possible when no other part did for exactly the same reason that they nerfed it: It was ridiculously, unrealistically (for the Kerbal universe), gamebreakingly good.

Spaceplanes really shouldn't be viable at all. That they are in any way is a bonus of the game and putting the Wolfhound (and to a lesser extend the Skiff) back in line with the other engines doesn't break spaceplanes, but instead makes them appropriately hard again.

[petlahk]

5 minutes ago, 5thHorseman said:

I understand your unhappiness. I really do. However, the Wolfhound made your spaceplane possible when no other part did for exactly the same reason that they nerfed it: It was ridiculously, unrealistically (for the Kerbal universe), gamebreakingly good.

Spaceplanes really shouldn't be viable at all. That they are in any way is a bonus of the game and putting the Wolfhound (and to a lesser extend the Skiff) back in line with the other engines doesn't break spaceplanes, but instead makes them appropriately hard again.

I understand what you're saying. Bascially I'm just really annoyed that I finally I got my spaceplanes to having an Delta-V* in Kerbin SOI that I'm happy with, and now I need to rebalance/add more fuel all over again. *sigh*. Back to using stupid NERV's I guess.  The issue with the Nervs though is that they're absolutely atrocious for that last little boost from the atmosphere. At least the Wolfhounds still have 380 though

Addendum: Also, I'm optimistic that one day IRL we'll have SSTO's and SSTO spaceplanes but that it may not be within our life times. A universe without SSTO's anywhere in it is an infinitely more boring one.

 

Edited December 21, 2018 by petlahk

[l0kki]

How was the Skiff considered OP though? In my experience it doesnt have the TWR or the gimbal to be used even in center stages on most rockets i have and they cant even be clustered that well since they are so big.

[Superfluous J]

5 minutes ago, l0kki said:

How was the Skiff considered OP though? In my experience it doesnt have the TWR or the gimbal to be used even in center stages on most rockets i have and they cant even be clustered that well since they are so big.

It had fantasitc TWR and great ISP and only weighed a ton. I'll admit to not using it much but I feel that's likely due to not playing KSP much in the past year or so.

[l0kki]

49 minutes ago, 5thHorseman said:

It had fantasitc TWR and great ISP and only weighed a ton. I'll admit to not using it much but I feel that's likely due to not playing KSP much in the past year or so.

Great ISP sure but in most rockets i have built using skiffs has given me too low of a TWR, mayby i was just using it wrong but oh well

Edited December 21, 2018 by l0kki

[magnemoe]

4 hours ago, 5thHorseman said:

I understand your unhappiness. I really do. However, the Wolfhound made your spaceplane possible when no other part did for exactly the same reason that they nerfed it: It was ridiculously, unrealistically (for the Kerbal universe), gamebreakingly good.

Spaceplanes really shouldn't be viable at all. That they are in any way is a bonus of the game and putting the Wolfhound (and to a lesser extend the Skiff) back in line with the other engines doesn't break spaceplanes, but instead makes them appropriately hard again.

Spaceplanes make sense in KSP, because the low orbital velocity, you can reach half the orbital velocity using jet engines. 
Now this is an issue with existing games like my Tylo SSTO using it and that mission is still underway to Jool. 
Not used it much for other stuff but here it was priceless and yes OP, 

[Bill Phil]

I understand that there’ll be issues. Of course, you can change the stats yourself or go back to a previous version. Many people use previous game versions for mod compatibility, for example. 

The problem is that, in light of such a huge ISP and very few downsides, the engine is superior to just about everything else. Indeed, as a core engine with strap-on boosters it was excellent before it was nerfed. And as far as I recall, it was intended as a vacuum engine... the kind a service module would have.

[Jestersage]

I posted this somewhere else, but for sake of discussion of Wolfhound:

The nice thing about this nerf is that it actually give me a reason to not use WolfHound in a good way. See, when I design stuff, I usually create multiple variants; some for looks, other for performance. Wolfhound is ugly, but due to its high TRW and high ISP, for Space Capsules (and space tugs), I ended up creating variants simply for the sake of function even though it does not fit in looks (eg: An Orion MPCV with Wolfhound -- how is it different from an Apollo-clone except it have less fuel?)

With such focus on TRW (based on KER), it gives good reason to actually check when to use WolfHound and when not to. It actually give Cheetah a real purpose, as the AJ10 Kerbal Analogue is the Cheetah -- 2x AJ10 gives one Centaur, which is what Poodle is now based on.

[Kerbart]

5 hours ago, petlahk said:

I'm not sure if this has been said already but I'm actually kinda upset that they nerfed the Wolfhound.(...) Please un-nerf the wolfhound.

Edit: Similarly with the skiff.

I can feel your grief, but right from its introduction the complaint about the Wolfhound was that it was ridiculously overpowered, and many had the suspicion that it was a case of mixed up engine stats.

I didn't use the wolfhound at all for that particular reason; it just felt wrong to use it.

While it's not nice to see an engine with such wonderful stats being nerfed, chances that it gets undone are slim; and it's likely that you're in a small minority. The good news is that someone (you?) can make a MM mod to fix the problem.

[Jesusthebird]

As another stated...a simple MM config could revert to previous stats. Or just play an older version. I personally liked the wolfhound for its opness..but wuth updates..comes balancing...

Thankfully ksp is a game thats easily tweaked. If you dont want to make an MM config..you could revert to previous version..copy the cfg in its entirety and make that the cfg for the woldhound in the current version. Not sure how much its been changed(v1.6 engine overhauls) but as long as there isnt any new textures/variants/geometry...copy/paste cfgs should work just fine and dandy

[Snark]

Moving to Making History Discussions.

[Vanamonde]

Some posts have been removed. If you think a post is a problem for some reason, please don't take it upon yourself to scold other forum members, especially if they're newbies to the forum. Rather, simply report the post and let us moderators deal with it. 

[KerikBalm]

On 12/21/2018 at 7:59 PM, petlahk said:

I'm not sure if this has been said already but I'm actually kinda upset that they nerfed the Wolfhound. That 412 vaccuum ISP was very important to my spaceplane design and allowed me to do things that just weren't possible before. The Wolfhounds weren't good enough to get a LF/OX spaceplane or rocket all the way out to another planetary system, but they were perfect for Kerbin SOI rockets and SSTOs. They were the perfect intermediary between the other engines and the NERV. Please un-nerf the wolfhound.
 

Ummm, I can make rapier only SSTOs that can get to Jool... Ones that can deliver a 100 ton payload to Jool intercept (granted, the return journey to Kerbin is a long one). LV-Ns improve things, but whatever.

More generally, SSTOs, sans wolfhounds, can get over 50% payload fraction. 40% is more realistic, 30% is fairly easy. If your spaceplane started at 100 tons, and got to orbit with 30 tons of "payload" left over, and used 50 tons of fuel to do it, it would have a dry mass of 20 tons. If that "payload" was just 30 tons of fuel, that means you could have a wet:dry ratio of 50:20, and with just an aerospike, you could get 3,055 m/s of dV. That's more than enough to go to duna orbit and come back... with enough margin that you could even deploy a small lander to get to the surface and redock.

Throw in LV-N into the mix, and you've got the ability to go to Laythe and back. I'm really curious what you think you could do with the wolfhound that you can't do now...

Sure the wolfhound was OP, and made things easier, but if the nerf doesn't make things impossible that were previously possible, then this is just a request for OP parts to make the game easier.

On 12/21/2018 at 11:26 PM, l0kki said:

How was the Skiff considered OP though? In my experience it doesnt have the TWR or the gimbal to be used even in center stages on most rockets i have and they cant even be clustered that well since they are so big.

It had a 30:1 TWR (vacuum), the highest of any LFO ingine in the game, and it had gimbal. Highest TWR engine with above average vacuum Isp, gimbal? It was great for reusable tylo landers.

[Aeroboi]

On 2/5/2019 at 10:32 AM, KerikBalm said:

Ummm, I can make rapier only SSTOs that can get to Jool... Ones that can deliver a 100 ton payload to Jool intercept (granted, the return journey to Kerbin is a long one). LV-Ns improve things, but whatever.

More generally, SSTOs, sans wolfhounds, can get over 50% payload fraction. 40% is more realistic, 30% is fairly easy. If your spaceplane started at 100 tons, and got to orbit with 30 tons of "payload" left over, and used 50 tons of fuel to do it, it would have a dry mass of 20 tons. If that "payload" was just 30 tons of fuel, that means you could have a wet:dry ratio of 50:20, and with just an aerospike, you could get 3,055 m/s of dV. That's more than enough to go to duna orbit and come back... with enough margin that you could even deploy a small lander to get to the surface and redock.

Throw in LV-N into the mix, and you've got the ability to go to Laythe and back. I'm really curious what you think you could do with the wolfhound that you can't do now...

Sure the wolfhound was OP, and made things easier, but if the nerf doesn't make things impossible that were previously possible, then this is just a request for OP parts to make the game easier.

It had a 30:1 TWR (vacuum), the highest of any LFO ingine in the game, and it had gimbal. Highest TWR engine with above average vacuum Isp, gimbal? It was great for reusable tylo landers.

I'm not much against the old Wolfhound stats. Most parts that were formerly introduced had it's complaints like the Vector for instance. Somehow the Wolfhound is stats beyond believe. I'm not against it since it is a replica and most modded engines have stats based on prototypes that are leaps ahead like atomic engines and sorts alike.

So how would the old Wolfhound cope outside the realm of KSP? If anything, a engine with similar stats (415ISP) that is a Hydrolox engine would be considered realistically scaled, it's just that there is only LF/OX in the game making such a engine arbitrary. If that is what a Wolfhound engine is to represent Squad should listen more closely as the community clearly expects some alternative fuel resource to backup the engine stats.

As of now I see the Wolfhound as a exceptional engine for Tylo SSTO's. As it happens I made a few for a soon to come Grand tour science mode
While being OP, having the old Wolfhound stats could justify other applications for this engine, as being said this is SSTO's with higher payload fraction especially on early SSTO's and better Eve upper stages.
If this is against the wishes of many players they can just not use them or leave them out of the challenge rules.

[KerikBalm]

1 hour ago, Aeroboi said:

So how would the old Wolfhound cope outside the realm of KSP? If anything, a engine with similar stats (415ISP) that is a Hydrolox engine would be considered realistically scaled, 

Outside the realm of KSP isn't really relevant, this is inside the realm of KSP. Isp directly scales to available dV. Realistically (assuming a 10:1 scaling), all dV requirements should be sqrt(10) x higher. So to have proportionally realistic Isps, you should take real Isp s for O/H2 engines (415-455, or thereabouts) and divide them by 3.16... so like 143s... of course then the dry masses of the tanks are way too high and engine TWRs are way too low... so lets forget about "real" values.

They already nerfed all the LFO engines from a max of 390 to 350 when 1.0 came out, for balance reasons... then somehow they decide to go even higher than that with the wolfhound? now with the nerf, its not so bad, but its still so much better than other LFO engines, I think its imbalanced.

As for the vector, its mostly fine, its stats are basically the same as a mammoth... its main "OP" feature is its high thrust to cross section ratio.

[Aeroboi]

21 minutes ago, KerikBalm said:

Outside the realm of KSP isn't really relevant, this is inside the realm of KSP. Isp directly scales to available dV. Realistically (assuming a 10:1 scaling), all dV requirements should be sqrt(10) x higher. So to have proportionally realistic Isps, you should take real Isp s for O/H2 engines (415-455, or thereabouts) and divide them by 3.16... so like 143s... of course then the dry masses of the tanks are way too high and engine TWRs are way too low... so lets forget about "real" values.

They already nerfed all the LFO engines from a max of 390 to 350 when 1.0 came out, for balance reasons... then somehow they decide to go even higher than that with the wolfhound? now with the nerf, its not so bad, but its still so much better than other LFO engines, I think its imbalanced.

As for the vector, its mostly fine, its stats are basically the same as a mammoth... its main "OP" feature is its high thrust to cross section ratio.

Right, didn't know about the old values but I'm from v0.90

As for the vector, I ment it's cross to thrust section. Also that it combines with superior gimbal while being overthrusted for a mk3 space shuttle replica so what is it? On top of that it has very little drag and can be surface mounted without a nose cone on top (clipped or non clipped) with very little drag.
On top of that they're great as Submarine ballast while being able to increase ballast through staging.

All in all my reference to the Vector OP'ness is based on this and whatever they do with the Vector it should remain a superior class engine, to my taste only having lower thrust of about 500kN.

[KerikBalm]

On 4/21/2019 at 1:46 PM, Aeroboi said:

Right, didn't know about the old values but I'm from v0.90

Then you should have been around when the aerospikes and poodles got 390 Isp.... When the soup-o-sphere meant that you needed 4,500 m/s (give or take 50 m/s) to get to orbit. Then aero-changes dropped that number to about 3200, and they thus nerfed engine Isp across the board. The result was that smaller rockets could get to orbit, but bigger rockets in orbit were needed to go places.

Quote

As for the vector, I ment it's cross to thrust section. Also that it combines with superior gimbal while being overthrusted for a mk3 space shuttle replica so what is it? On top of that it has very little drag and can be surface mounted without a nose cone on top (clipped or non clipped) with very little drag.
On top of that they're great as Submarine ballast while being able to increase ballast through staging.

All in all my reference to the Vector OP'ness is based on this and whatever they do with the Vector it should remain a superior class engine, to my taste only having lower thrust of about 500kN.

And yes, the vector has some OP qualities that basically disappear once in space. Once in space, thrust to cross section ratio is pretty irrelevant (but its very relevant for aerodynamics inside atmospheres)... so I don't mind it sooo much, since this is a space game and it only really matters for Kerbin/Eve/Laythe... and for Kerbin and Laythe, airbreathing engines are so much better, that its hard to consider any LFO rocket overpowered for its atmospheric qualities in an O2 containing atmosphere.

So in the end, the vector is the king of 1st stage eve ascent engines... which is a niche I'm OK with. I'm also ok with its other properties that make it a good shuttle engine, but I'd rather see 1.875 or 2.5m SRBs and weaker vectors. Also, surface attach might as well be present for any engine, as it saves part count (otherwise, you can surface attach a nosecone, and then stick an engine below its node).

Considering this is a space game, I'm most concerned with vacuum performance, and the wolfhound's was way too good, and rightfully nerfed.

I think the Skiff was maybe nerfed to hard... its TWR is lackluster, and so is its vacuum Isp. Worse TWR and Isp than the Rhino. Worse TWR (19 vs 22) than the Skipper for only 10 more Isp. Barely better TWR (19.1 vs 18.3) than the aerospike for 10 less Isp. It should get a thrust increase... but its hard to make it not redundant with the skipper. The skipper has better atmo Isp and the skiff better vacuum Isp, so I guess maybe buff the thrust of both of them (the skipper is pretter lackluster too, and since its better suited as a 1st stage engine/1st stage with some side booster assist, it could always use more thrust).. with the skipper getting proportionately a bit more of a buff...

I'd bring the skiff up to the TWR of the current KR-2L (its still got a lower Isp), and then bring the skipper TWR to halfway between the KR-2L and the mainsail. Masses would stay the same, TWR buffer by increasing thrust.

[Aeroboi]

7 minutes ago, KerikBalm said:

Then you should have been around when the aerospikes and poodles got 390 Isp.... When the soup-o-sphere meant that you needed 4,500 m/s (give or take 50 m/s) to get to orbit. Then aero-changes dropped that number to about 3200, and they thus nerfed engine Isp across the board. The result was that smaller rockets could get to orbit, but bigger rockets in orbit were needed to go places.

Yes.

8 minutes ago, KerikBalm said:

And yes, the vector has some OP qualities that basically disappear once in space. Once in space, thrust to cross section ratio is pretty irrelevant (but its very relevant for aerodynamics inside atmospheres)... so I don't mind it sooo much, since this is a space game and it only really matters for Kerbin/Eve/Laythe... and for Kerbin and Laythe, airbreathing engines are so much better, that its hard to consider any LFO rocket overpowered for its atmospheric qualities in an O2 containing atmosphere.

So in the end, the vector is the king of 1st stage eve ascent engines... which is a niche I'm OK with. I'm also ok with its other properties that make it a good shuttle engine, but I'd rather see 1.875 or 2.5m SRBs and weaker vectors. Also, surface attach might as well be present for any engine, as it saves part count (otherwise, you can surface attach a nosecone, and then stick an engine below its node).

Considering this is a space game, I'm most concerned with vacuum performance, and the wolfhound's was way too good, and rightfully nerfed.

I think the Skiff was maybe nerfed to hard... its TWR is lackluster, and so is its vacuum Isp. Worse TWR and Isp than the Rhino. Worse TWR (19 vs 22) than the Skipper for only 10 more Isp. Barely better TWR (19.1 vs 18.3) than the aerospike for 10 less Isp. It should get a thrust increase... but its hard to make it not redundant with the skipper. The skipper has better atmo Isp and the skiff better vacuum Isp, so I guess maybe buff the thrust of both of them (the skipper is pretter lackluster too, and since its better suited as a 1st stage engine/1st stage with some side booster assist, it could always use more thrust).. with the skipper getting proportionately a bit more of a buff...

I'd bring the skiff up to the TWR of the current KR-2L (its still got a lower Isp), and then bring the skipper TWR to halfway between the KR-2L and the mainsail. Masses would stay the same, TWR buffer by increasing thrust.

I almost never use a Vector in vacuum unless I bring it with me to where I need to be, like a eve lander that has their engine vector free from blockage.

I have rightfully used the Skiff on upper stages, even some of my Eve lifters used them because of their relatively good TWR before they got nerfed. The Skiff having a TWR of 19.1 as it is now used to be less heavy so I could use plenty to get the required thrust, now when I do use them I cut the Delta-V budget on the way up so much that I could use many alternative engines.
The skipper is a very good example you gave there, adding to that the Skipper has a Atmospheric ISP of 280 vs 265 on the Skiff. So by all means, skipper engines can be throttled sooner on the ascent curve compared to Skiffs while having more thrust per mass. So what is the Skiff good for I wonder if it's intended application is tackled by a pre v0.90 stock engine?

Because of what I said the buffed skiff engine is kind of out line because the Skiff is a upper stage engine. In theory, any rocket using upper stage skipper engines will do better. I wouldn't compare to a Rhino directly since it's primarily a vacuum or interplanetary stage engine only while the Skiff or Skipper can be upper stage engines. As for the aerospike, it's still a atmospheric/vacuum optimized engine that has the best ISP range throughout a flight profile. To my humble opinion, the aerospike is a case in and of itself. On eve it is a winner and proper assist engine, especially useful if you light aerospikes within eve's atmosphere all the way to orbit without staging them. This allows to make proper use of the engine stats as a aerospike is put to best use if operated throughout the ascent entirely. When not on Eve aerospikes nearly never win the race, albeit on space planes having them. Because of a aerospike lower thrust to cross section and having no gimbal you need quite a few + extra control surfaces or reaction mass/wheels that would add dry mass to take with you. On space planes using wings you need less thrust and thus less aerospike engines. In that case the LF/O is put to best use when using a aerospike. However, who would want to build a aerospike space plane when you got a better alternative, jet engines. In case of another aerospike application, it should make the furthest reaching rocket space plane capable on a atmospheric world, which is handy if you want to hop science equipment on eve using ISRU.

 

[KerikBalm]

16 minutes ago, Aeroboi said:

Because of what I said the buffed skiff engine is kind of out line because the Skiff is a upper stage engine. In theory, any rocket using upper stage skipper engines will do better.

Why, because of the better thrust to cross section ratio? By the time you get to a 2nd stage, you're getting nearly vacuum Isp, so the Skiff should be getting equal or better Isp at the start of its burn (as a 2nd stage), and certainly better on average throughout the burn.

Quote

I wouldn't compare to a Rhino directly since it's primarily a vacuum or interplanetary stage engine only while the Skiff or Skipper can be upper stage engines.

I would disagree here. The Rhino is an excellent 2nd stage engine, I've even used it as a 1st stage engine coupled with SRBs (although that was pre-vectors, and I think it got nerfed a bit since then)

2nd stage engines that start from 5km up might as well be vacuum optimized. So the only real difference between a 2nd stage engine, and a vacuum/interplanetary engine is the focus on raw power/TWR, and the Rhino's pretty good in that regard. I used it for both purposes. I mainly play 3x rescale now, and I find a mammoth first stage, a rhino 2nd stage works pretty well. Of course I'll also use gargantuan launchers/spaceplanes to take a Rhino to orbit and use it as an ejection stage engine for large interplanetary missions, but I think the Rhino definitely has a role as a 2nd stage engine

Some pics of craft that worked with the Rhino firing from sea level to orbit, stock KSP size:

Spoiler

^this one had a flyback booster, it was a bit difficult to get the timing right switching btween the craft and within KSPs atmospheric switching/deletion limits, but it worked. Payload was barely increased relative to taking the whole thing to orbit as an SSTO. Yes, I had to throttle back the mammoth using the slider as fuel burned off.

^This was before the vector was introduced I think. It could just barely make orbit with an orange tank, I think it needed to use the monoprop to fully circularize and then deorbit. I'd often fail to get the PE high enough if I didn't fly the ascent right... but again, its certainly not serving as an interplanetary transfer engine. Its surface level Isp isn't great (and I think it went through some nerfs and then rebuffs in the meantime), but once it gets a few km up with the help of SRBs, it does quite well.

 

Quote

As for the aerospike, it's still a atmospheric/vacuum optimized engine that has the best ISP range throughout a flight profile. To my humble opinion, the aerospike is a case in and of itself....

I agree with much of what you said, especially having it fire throughout the ascent to make maximum use of its properties. My eve landers are often 2 vectors with an aerospike core (and mk1 crew cabins to increase kerbal capacity, keeping the frontal area down relative to 2.5m crewed parts)... but the vector is also a bit like that even in the game.

315 is... respectable... (its not like 305 or 290 that some engines get, losing 25 Isp relative to an aerospike isn't so bad given its TWR and cross section vs thrust). Good Atmo performance, decent vaccum... the aerospike beats its Isp at eve sea level (but loses within a certain altitude range) and vacuum, but its not so far off.

And it should be, aerospikes aren't the only altitude compensating nozzle designs. The SSMEs were such an engine type, and its modeled after those.

https://en.wikipedia.org/wiki/Altitude_compensating_nozzle

We could make the aerospike far from a class of its own, with an engine moddling nozzle extensions/expanding nozzles:  https://en.wikipedia.org/wiki/Expanding_nozzle and https://en.wikipedia.org/wiki/Nozzle_extension . Then there's also these https://en.wikipedia.org/wiki/Stepped_nozzle. And the SSME : https://en.wikipedia.org/wiki/Space_Shuttle_main_engine#Nozzle.

Quote

The RS-25 nozzle has an unusually large expansion ratio (about 77.5:1) for the chamber pressure.^[12] At sea level, a nozzle of this ratio would normally undergo flow separation of the jet from the nozzle, which would cause control difficulties and could even mechanically damage the vehicle. However, to aid the engine's operation Rocketdyne engineers varied the angle of the nozzle walls from the theoretical optimum for thrust, reducing it near the exit. This raises the pressure just around the rim to an absolute pressure between 4.6 and 5.7 psi (32 and 39 kPa), and prevents flow separation.

Large expansion ratio for vacuum operation, without flow separation... its not as good as an aerospike, but I think it got close, and was more conventional.

Last but not least are these:

https://en.wikipedia.org/wiki/Plug_nozzle#In_rockets

https://en.wikipedia.org/wiki/Expansion_deflection_nozzle

the last of which is proposed for use in the Sabre engine, so if the rapier is modled on it, its closed cycle thrust curve should be similar to an aerospike's

[Aeroboi]

2 hours ago, KerikBalm said:

Why, because of the better thrust to cross section ratio? By the time you get to a 2nd stage, you're getting nearly vacuum Isp, so the Skiff should be getting equal or better Isp at the start of its burn (as a 2nd stage), and certainly better on average throughout the burn.

Because of the thrust but that isn't the primary reason. It is useful on Eve when your 2nd, 3rd or any subsequent stage using Skiffs is above a certain altitude. But that doesn't make it a best case, just a good option depending on the ascent curve. Some use a very shallow ascent profile and the Skiff is best used only at altitude when the rocket already goes very fast and you'll do most of the orbit insertion higher up in which case a terrier, cheetah, poodle or even rhino is a better choice.

2 hours ago, KerikBalm said:

I would disagree here. The Rhino is an excellent 2nd stage engine, I've even used it as a 1st stage engine coupled with SRBs (although that was pre-vectors, and I think it got nerfed a bit since then)

2nd stage engines that start from 5km up might as well be vacuum optimized. So the only real difference between a 2nd stage engine, and a vacuum/interplanetary engine is the focus on raw power/TWR, and the Rhino's pretty good in that regard. I used it for both purposes. I mainly play 3x rescale now, and I find a mammoth first stage, a rhino 2nd stage works pretty well. Of course I'll also use gargantuan launchers/spaceplanes to take a Rhino to orbit and use it as an ejection stage engine for large interplanetary missions, but I think the Rhino definitely has a role as a 2nd stage engine

Some pics of craft that worked with the Rhino firing from sea level to orbit, stock KSP size:

Well, your showcasing a shuttle, which is 2 stages, in a way it's a tsto discarding the srb's. While I wasn't clear my comment basis is referenced based on a multistage rocket design where each dedicated stage is used and where a dedicated engine is used in the upper atmosphere potentially leaving a rhino in orbit if you so desire to go elsewhere. Some multistage rocket have their 3rd stage that I put Skiff's on prior, but I could already fire them by dropping 1st stage boosters to free up their thrust vector. Also, the difference between 5km and 10km does make a big difference even although 5km has the thickest air. This does involve a ISP difference of ~10 even ~15 on some engines, mind you it is a 50ISP difference for the aerospike.
Also know that ISP scales with thrust, so the thrust at sea level is lower. So a better engine means more thrust per mass over the entire timespan to orbit, so it's a tiny more thrust to battle gravity losses next to having a little more Delta-V. In the end a more specific engine combination always proves to be better. But since the game is gamey a dedicated Kerbin launch rocket doesn't require sophisticated stages to get most things into orbit to begin with, so on Kerbin I'm usually not that nitpicking, as long as the engine performs efficiently at the places I want to go. For that matter I enjoyed the old Skiff much on some Eve ascent vessels, but that is it.

Furthermore, if it is a 2 stage design and the upper stage (shuttle) is meant to go elsewhere or do stuff in orbit and uses a Rhino to assist of the launchpad, then I would do that also in which case it's useful. Either way, having use for a Rhino as long as it ends up in orbit to do stuff there is a viable reason to put one on your rocket, even if it is less efficient then any other option. For that matter I got a few TSTO's using a Rhino.