KerikBalm

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

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I've often been annoyed by the poor vacuum performance stats of the rapier. Its an engine that is optimized for high altitude and spaceflight, not low altitude flight in the thick atmosphere. Even its airbreathing mode is optimized for very low atmospheric pressures. I've often thought its Isp curve should be similar to the Rhino, Skiff, or Aerospike.

Its Terrible as a closed cycle engine. Worst TWR of the LFO engines, worst vacuum Isp of any of the inline engines (only tiny radials are worse) except the Twin Boar booster which (discounting the integrated fuel tank mass) has a massive TWR and good atmospheric Isp. To me all this didn't make sense for an engine mode that one would only use in vacuum and near vacuum conditions, that is based on a real design with an altitude compensating nozzle and great vacuum Isp.

But recently, I wonder if my concerns are misplaced, and arguing for better vacuum Isp would make it OP, (likewise, the vector, based on the space shuttle engines, has relatively poor vacuum stats) when I was designing a Rald VTOL. *for that story, see the bottom

If we consider the Rapier airbreathing mode to be of roughly equal value to a whiplash, then we get the closed cycle mode on top of that for only 0.2 tons. Considering the closed cycle mod as a seperate 0.2 ton engine "addon", gives that "addon" a kerbin relative TWR of 91!!! by far the highest in the game.

Taking a whiplash design and adding rocket engines with an equivalent closed cycled thrust would use much more mass. Is that 91:1 TWR "addon" OP'd?

On the other hand, rapiers often give much more closed cycle thrust than needed for spaceplanes (keeping in mind LF only planes exist and will get to orbit with the anemic thrust of LV-Ns). The actual mass savings are often much smaller: consider 2 whiplashes and an aerospike (4.6 tons) vs 2 Rapiers (4 tons).

When I start looking at VTOL designs, I also find the rapiers to be rather bad. So many engines are needed, whiplashes are far better (or panthers). You basically don't need rapiers for airbreathing mode either (they are heavy with low static thrust for the VTOL portion, and whiplashes will get you very high and fast, adding a few rapiers won't get you much higher or faster in the small time they'd operate before cutout if you only have a few of them), that it seems to be better to use an engine with better vacuum Isp.

I don't know... it seems simultaneously that the closed cycle mode is OP by giving you a lot of rocket thrust for a little tiny additional mass per engine, and at the same time, it seems like it kind of sucks because of its low Isp and the tyranny of the rocket equation.

* Rald VTOL design:

Spoiler

I was designing a VTOL for my mod planet Rald and 3x modded system (as soon as kopernicus updates). Rald has a thin atmosphere (around 0.2 Atms), and relatively high gravity (just above 0.5). As jet thrust decreases with air pressure (or is it density? the graphs here show air pressure, but I'm pretty sure its density in the files https://wiki.kerbalspaceprogram.com/wiki/Jet_engine), and this decrease isn't the same for all the jets, the panthers are no longer the go-to engine.... producing less than 40% of the static thrust at 0.2 atms as they do on at 1 atm. Its worse at high altitudes on Rald, such as the Karsis plateau, or if I want to land on the top of the volcano corresponding to Olympus Mons.

A turboramjet still produces over 50% of its normal static thrust at 0.2 atms, and is useful for reaching orbit. Thus it still gets around 7:1 static TWR on Rald... at tleast around the lowlands. The Rapier does even better than the Whiplash at lower atmospheric pressure (but the whiplash still does better in the 0.08-0.2 atm range). Still, jet TWR was lower than on Kerbin, and making a jet VTOL was going to use a lot of engines. Meawhile, the low atmospheric pressure helps rocket engines, and the low gravity means that the rapier closed cycle gets a respectable 18:1 TWR on Rald. I thought maybe I could pakc on a few hundred extra dV in closed cycle mode, and take off and land in closed cycle mode. Then I figured if I was packing so much closed cycle dV, and doing VTOL in closed cycle, maybe it would also work on 3x Duna (with some other tweaks) which I have orbit Rald. I could have a single craft for Rald-Duna-Ike, using just Rapiers.

A problem on Rald was the transition from closed cycle to airbreathing, as the airbreathing mode needed time to spool up after the switch.

I was using 4 rotating engine pods of 2 rapiers each, so 8 rapiers, but then I figured... I don't really need 8 rapiers for horizontal airbreathing flight. The big limitation was VTOL flight TWR. I could ditch half the rapiers for aerospikes (same thrust as aerospike closed cycle), and save 4 tons, while gaining a lot of Isp. It would also enable me to maximize the last bit or airbreathing thrust by keeping the rapiers in airbreathing mode as I light the arospikes for orbital insertion. Rapier closed cycle would only be supplemental for vertical liftoff and landing.

Then I figured another option was a single vector on each side instead of 2 aerospikes: Far better lifting power, better thrust vectoring control in VTOL mode, and the rapier closed cycle mode was not needed at all. I could switch the rapiers for whiplashes, and I could have airbreathing supplementary thrust even in VTOL mode. This system would weigh less than the pure rapier model (0.8 tons save by switching 4 rapiers to 4 whiplashes), and have far more closed cycle thrust (enabling lifting more fuel by VTOL mode). The 2 vectors would supply 2000 kN compared to the 1,440 kN of 8 rapiers. On Rald (but not Duna, but Duna's gravity is lower), the 4 whiplashes would add about 200 kN.

So I'd have 2200 kN for vertical thrust vs 1440 kN (can lift 52% more mass vertically). I'd have 315 vacuum Isp vs 305, I'd have better thrust vector control, fewer parts, and 0.8 tons less mass. The cost would be a weaker airbreathing mode with a lower top speed.

The aerospike variant would have the same vertical thrust, 340 Isp vs 305, and a significant 4 tons less mass, for a weaker air-breathing mode with a bit lower top speed (same engine limit, just less engines)

 

 

 

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For a spaceplane reaching low Kerbin orbit the rocket delta-V requirement is quite modest, about 1000 m/s I think. When delta-V needs are low, specific impulse is relatively less important (within reason) and engine TWR more important. The mass saving from a lighter engine will more than offset needing a slightly higher fuel fraction.

But if you're building a spaceplane for a larger planet from a mod, or one to go beyond LKO, then you need a lot more rocket delta-V. That's when the specific impulse counts for more, hence the popularity of Nerv spaceplanes.

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5 hours ago, cantab said:

For a spaceplane reaching low Kerbin orbit the rocket delta-V requirement is quite modest, about 1000 m/s I think. When delta-V needs are low, specific impulse is relatively less important (within reason) and engine TWR more important. The mass saving from a lighter engine will more than offset needing a slightly higher fuel fraction.

That's why I thought it would be good for VTOL, low dV requirement, but high thrust needed for takeoff and landing. This ends up giving much more air breathing engine mass than needed, and a combo of vector and whiplash seems to outdo it.

Quote

But if you're building a spaceplane for a larger planet from a mod, or one to go beyond LKO, then you need a lot more rocket delta-V. That's when the specific impulse counts for more, hence the popularity of Nerv spaceplanes.

Well, its in a 3x scaled up system (by sigma dimensions), the mod planet by itself is smaller than kerbin. With the 3x rescale, orbital velocity is about the same as stock Kerbin, thanks to the lower gravity and proportionately smaller radius.

Duna... 3x rescaling increases dV requirements by about sqrt(3), or 1.73x, so its like 2km of dV needed. Maybe a combined Duna drop ship and Rald dropship is too ambitions for a 3x rescale with stock parts.

But still, I'm finding, very often, the answer to optimization is to use something other than the rapier closed cycle mode, stock planets or modded... yet it does get you a lot of extra closed cycle thrust for a little extra weight.

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It's a balance of utility.  The R.A.P.I.E.R. isn't a very good (lower) atmosphere air breathing engine, nor is it a very good vacuum LFO engine.  However, it does both of those things for a very modest amount of mass compared to mounting multiple engines that do only one of those things.  And nothing else quite compares to it for building surface-tangential speed during higher atmospheric flight.

I do admit sometimes being a bit frustrated at it's lack of thrust in the lower atmosphere, which often requires either lots more R.A.P.I.E.R.s than strictly necessary for most of it's flight or additional Whiplashes to help push it into it's ideal envelope, both of which involve a big tradeoff in mass, or giving it a RATO system.  But hey!  If it didn't have some disadvantages, we wouldn't need to find interesting ways to engineer around it. :)

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Posted (edited)
3 hours ago, Fearless Son said:

I do admit sometimes being a bit frustrated at it's lack of thrust in the lower atmosphere, which often requires either lots more R.A.P.I.E.R.s than strictly necessary for most of it's flight or additional Whiplashes to help push it into it's ideal envelope, both of which involve a big tradeoff in mass, or giving it a RATO system.  But hey!  If it didn't have some disadvantages, we wouldn't need to find interesting ways to engineer around it. :)

This^.

I personally don't have much use for SSTO spaceplanes.  To my tastes, they take WAY too much development time for the little bit they can get to LKO to be worth the hassle.  20 minutes before you know if it will work or not, make a minor design tweak, then another 20-minute failed launch, repeat ad infinitim.  I could, instead, spend 5 minutes slapping together a conventional rocket for the same job, know it will work the 1st time, and get it to LKO in 3 minutes, so I can then devote most of my limited playing time to the actual payload mission instead of spending it all developing the launch vehicle.

HOWEVER, every once in a while I find a real need for an SSTO spaceplane.  NEVER at Kerbin, but at some other atmospheric planet, stock or mod, where I want to make multiple trips to and from orbit.  And if it's got O2, then the frustrating aspects of the RAPIER really bug me, when compared to the ease of building conventional rockets.  So usually I work around them by finding some OP mod parts :)

Edited by Geschosskopf

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This all seems like a very detailed explanation as to why the RAPIER is, in fact, a pretty well-balanced engine when considering the rest of the game..

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4 hours ago, Fearless Son said:

However, it does both of those things for a very modest amount of mass compared to mounting multiple engines that do only one of those things.

Agreed.

I would argue that the savings is really in cross-sectional area (and therefore drag and construction constraints) rather than mass, although the mass savings are significant as well.  For anything that needs more than 6 or so engines (vacuum + airbreather) the rapier is the way to go -  you're going to run out of places to put them all without increasing drag a good bit.

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7 hours ago, Fearless Son said:

Whiplashes to help push it into it's ideal envelope, both of which involve a big tradeoff in mass, or giving it a RATO system.  But hey!  If it didn't have some disadvantages, we wouldn't need to find interesting ways to engineer around it. :)

I find you can just substitute many rapiers with whiplashes, and get a weight savings, not a penalty. The whiplashes overlap with the rapiers for quite a bit, and adding more rapiers at the high end of the envelope often doesn't help much because thrust drops so rapidly when you get to the edge of the envelope.

And of course many engines have disadvantages that the way you "engineer around them" is to not use them except in their niche. You don't engineer around a terrier's poor atmospheric Isp and TWR to make a 1st stage using them... you don't use them in that regime. Just like you don't use rapiers to go to Moho/Eve/Duna/Dres/Eeloo/Jool (though you may bring them there for Laythe)/Eeloo

4 hours ago, Geschosskopf said:

for the little bit they can get to LKO to be worth the hassle. 

Spaceplane SSTOs can get over 50% payload fraction, and carry things that fit inside the largest 3.75m fairings... probably 5m fairings as well, but i haven't tried with them so much since they are sort of new.

Modding the system to 3x... that fraction drops to 5-18%, depending on how heavy into LV-Ns you want to go, and then you end up taking a long time to get to orbit, true. That's why I recently dropped SSTOs from my 3x lineup, and instead use a 2 stage spaceplane. The first stage (have tried with Whiplashes and rapiers), climbs at 30-40 degrees, with a lot of airbreathing engines for great TWR so it can attain high speeds while still climbing steeply. Then the 2nd stage engine lights, cross feeding from the 1st stage fuel tanks, (I tried a Rhino but its TWR was too low, and now use a mammoth.... 315 Isp is still better than 305) to get the apoapsis high enough that I can switch back to the spaceplane after orbit (no mods needed to recover it). Once the apoapsis is high enough /as the 1st stage oxygen depletes, the 2nd stage releases, and gets to orbit without the dead weight of the rapiers/whiplashes, and with minimal wing area. I Switch back, turn the 1st stage around, light the whiplashes/rapiers again, and fly it back home. Then the 2nd stage releases its payload, and deorbits for a horizontal landing at KSC. With this method, I can get 15-20% payload fraction... and I don't need the rapier closed cycle (although, I admit, on the rapier design, I do briefly use it to augment TWR for the first couple hundred m/s before release)

4 hours ago, I_Killed_Jeb said:

This all seems like a very detailed explanation as to why the RAPIER is, in fact, a pretty well-balanced engine when considering the rest of the game..

I was almost going to say something similar, if you can't tell if it needs a nerf or a buff, its probably in a good place... but the engine has only 1 role. If your craft isn't going to be operating in an O2 atmosphere, there is no situation, ever, where the rapier is the right choice.

Within the airbreathing niche,  its got competition with the Whiplash, which I prefer for long range atmospheric flight (better Isp, better low altitude performance at the destination, and an alternator which is very usefull since high speed flight prevents deployment of solar panels and tends to fry the fixed ones).

So that really leaves its niche as restricted just to airbreathing spaceplanes. Its good at that niche primarily thanks to its high speed and altitude air breathing performance. I still think the closed cycle mode with quite "meh", and often when optimizing, it doesn't get used... although the rapier may be able to work as a balanced engine with no closed cycle at all.

3 hours ago, natsirt721 said:

I would argue that the savings is really in cross-sectional area (and therefore drag and construction constraints) rather than mass, although the mass savings are significant as well.  For anything that needs more than 6 or so engines (vacuum + airbreather) the rapier is the way to go -  you're going to run out of places to put them all without increasing drag a good bit.

When I go with rapier only designs, its often for this reason. But I've started looking at incorporating Vectors into the design (although vectors are often cited as OP), and they have a great thrust to cross sectional ratio, and their surface attachment means that you can really pack them on. Many times the weight savings are much less than what they may initially seem because you need to pack on more rapiers for the low altitude airbreathing part than other engines.

Also when switching to closed cycle, its often done well before airbreathing thrust has completely disappeared.... so the result is stopping a 50 kN airbreathing engine and starting a 180kN closed cycle engine... only a 130kN increase in thrust. Adding vectors for the closed cycle thrust allows you to run the rapiers (or whiplashes, or a combination) until flameout.

In stock, I find the drag not to be too bad (especially since by the time one is going fast, the climb rate is often quite high as apopasis starts increasing, and the air thins out pretty fast). When I play 3x and orbital velocity is over 4 km/sec, drag is much more important, but so is Isp and the tyranny of the rocket equation really hurts with the low Rapier Isp. I did a challenge of sorts once, and basically the way to increase payload fraction was to pack on as many LV-Ns and as much LF as the rapiers could carry.

I dunno, I think it should be the premier solution for a winged airbreathing spaceplane, but more often than not, the closed cycle mode is left behind and another engine is added. I think that should change, and it could easily change without making the rapier in any way competitive with other engines when an O2 atmosphere is taken out of the equation.

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6 hours ago, KerikBalm said:

Spaceplane SSTOs can get over 50% payload fraction, and carry things that fit inside the largest 3.75m fairings... probably 5m fairings as well, but i haven't tried with them so much since they are sort of new.

But the hassle and amount of time (mostly due to the 20+ minutes per test flight) required to perfect a spaceplane design increases as the square of payload size, it seems :mad:,  I know some folks groove on solving this sort of problem but I don't.  Thus, my SSTO spaceplanes are all small personnel carriers.  But that's challenge enough because instead of landing on the KSC runway, they land on the lumpy ground of other planets, so need enough wing for a safely slow approach speed.  RAPIERS seem to have problems with this sort of thing.

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Panthers and whiplashes are not even close to being in the same ball park.  It has been a while since I have made a space plane but I remember them having a top speed in the 1200 range at 17ish km.  I typically hit 1800 at 22ish km with air breathing with the rapiers.  This means you have basically doubled your dv needed to reach orbit.  I do kind of feel Rapiers with reverse nose cone clipped in are OP.  They really should just remove the tail node.

I would agree that this engine is clearly based off the saber engine and should have really good vacuum isp but how would you balance it?  In RO you have to use hydrogen which would double or triple your tank mass and you need 6000 dv out of the LOX stage.  Do you nerf all air breathing by making the speed of sound 70 m/s thus giving the rapier a top air breathing speed of 420 m/s and a ceiling of 8km?

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Posted (edited)
35 minutes ago, Geschosskopf said:

But the hassle and amount of time (mostly due to the 20+ minutes per test flight) required to perfect a spaceplane design increases as the square of payload size, it seems :mad:,  I know some folks groove on solving this sort of problem but I don't.  Thus, my SSTO spaceplanes are all small personnel carriers.  But that's challenge enough because instead of landing on the KSC runway, they land on the lumpy ground of other planets, so need enough wing for a safely slow approach speed.  RAPIERS seem to have problems with this sort of thing.

I have noticed everyone tends to land 2x as fast as they should be.  You should be pitched up to 5-10 degrees when your wheels make contact.  Learning to land on an uphill is quite challenging but it can greatly reduce your stopping distance.

But yes if I am making a SSTO plane I generally plan for at least 4 hours to put it together.  My hardest part tends to be reentry getting a case of the flippies, then lawn dart, then flippies as I make adjustments.

Edited by Nich

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1 hour ago, Nich said:

It has been a while since I have made a space plane but I remember them having a top speed in the 1200 range at 17ish km.  I typically hit 1800 at 22ish km with air breathing with the rapiers.

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

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9 hours ago, KerikBalm said:

I find you can just substitute many rapiers with whiplashes, and get a weight savings, not a penalty. The whiplashes overlap with the rapiers for quite a bit, and adding more rapiers at the high end of the envelope often doesn't help much because thrust drops so rapidly when you get to the edge of the envelope.

And of course many engines have disadvantages that the way you "engineer around them" is to not use them except in their niche. You don't engineer around a terrier's poor atmospheric Isp and TWR to make a 1st stage using them... you don't use them in that regime. Just like you don't use rapiers to go to Moho/Eve/Duna/Dres/Eeloo/Jool (though you may bring them there for Laythe)/Eeloo

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.  It would burn up if it had to fly like that most of the time, but since it gets out of the atmosphere quickly, that's less of an issue for it.  A small radiator can help bleed the heat off one it escapes the air.

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Posted (edited)

@KerikBalm I didn't do many pure rapier designs to be honest.  Most of my stuff was meant for Minimus refueling so it had a nuke stage.  Lite nuke (or nuke cluster) at 20km around 1600 m/s.  Flame out at 24 km around 1800 m/s and switch to LFO mode to raise orbit to a 70x30km orbit and burn the nuke to finish with a 75x75 orbit.  I always had melting problems with the rapiers if I was too low and thrust problems if I got too high.  I used a shock spike in front (antenna) and reversed shock cones clipped into the rapier.  With the low enough drag I never had an issue breaking the sound barrier at sea level but getting off the runway was difficult.  I felt like 1650 is just the point stuff started melting at 22km. But like I said it has been multiple versions since I have made SSTO's  1.3 or 1.4 I am guessing

Edited by Nich

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25 minutes ago, 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.  It would burn up if it had to fly like that most of the time, but since it gets out of the atmosphere quickly, that's less of an issue for it.  A small radiator can help bleed the heat off one it escapes the air.

For this type of assent wouldn't panthers be better

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4 hours ago, Nich said:

I have noticed everyone tends to land 2x as fast as they should be.  You should be pitched up to 5-10 degrees when your wheels make contact.  Learning to land on an uphill is quite challenging but it can greatly reduce your stopping distance.

Not me.  I've always been a STOL guy (MUCH simpler than VTOL).  STOL and SSTO don't play well together but I still try to keep my spaceplane stall speeds down around 50m/s at most, thinking ahead to landing on any bumpy place that I want to.

 

2 hours ago, KerikBalm said:

These numbers are quite off in my experience.

RAPIERS flame out at 22km now?  Wow.  Last time I played, back in 1.3.1, they (and the other jets) were konking out at 18ikm, which was a real pain.

 

2 hours ago, 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.  It would burn up if it had to fly like that most of the time, but since it gets out of the atmosphere quickly, that's less of an issue for it.  A small radiator can help bleed the heat off one it escapes the air.

This is the general direction I'm trending toward in my own designs.  I WANT to use the RAPIER for spaceplanes because that's its whole purpose., but there seem to be rather better options.

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Leave RAPIERs alone, they're fine.

I love SSTO spaceplanes. In career mode I only really use them for shuttling stuff to and from orbit (Kerbin or Laythe), and RAPIERs are ideal for that purpose.

With long-distance planes that carry NERVs, RAPIERs are still great because they can provide a quick burst of hard acceleration when landing or taking off from an airless body. NERVs have such terrible TWR that they can be highly useful for both, even if most of the dV is produced with the NERVs.

In other words, from where I'm at RAPIERs are just about perfectly balanced. They rule at powering SSTO spaceplanes and are kind of mediocre at everything else. That's exactly how a specialised design should be.

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45 minutes ago, Geschosskopf said:

RAPIERS flame out at 22km now?  Wow.  Last time I played, back in 1.3.1, they (and the other jets) were konking out at 18ikm, which was a real pain.

26km, generally.

3 hours ago, Fearless Son said:

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.

I design mine so that when I get up to speedrun territory, I can hit SRF PRO, and be completely hands-off except for action groups. If you've got a long nuke burn to raise Apo, you can walk away from the computer for a considerable amount of time. I've even had a few that can do that from the runway.

I really need to rewrite my generalized spaceplane ascent script and integrate it into MechJeb. It could handle everything from a plane with a jet TWR of 4+ on the runway to something that would barely limp to orbit. Maybe people would like spaceplanes more if they didn't have to fly them.

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3 hours ago, Nich said:

For this type of assent wouldn't panthers be better

Maybe.  I haven't tested it.  But I do know that the Whiplash has a better TWR at high speed than the Panther does.  I tend to use Panthers more in situations where I need a plane with a high gimble range on the engines or the ability to rapidly raise their thrust at a moment's notice.  For something where you just push the throttle to max and go in a straight line at high altitude and high speed, my instincts tend toward the Whiplash.

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@Fearless Son True but do you even get over mach 2.5 at 45 degrees.  Feel like that would give you a really high AP.

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1 hour ago, FleshJeb said:

26km, generally.

Wow, that certainly makes a difference.  IIRC once they finally settled on an atmosphere model in the immediate post-1.0 chaos, most jets would die at about 22km and the RAPIER at about 24km, which was the main reason for using it.  Getting higher let you get faster on the jet.  But then apparently SSTOs became too easy so they knocked everything down below 20km.  Then it was a REAL pain making an SSTO with any engine combination.  You needed way more rocket fuel and started using it at an altitude where neither launch nor vacuum engines (or RAPIERS) worked well.  Prior to this, I'd kinda gotten the hang of SSTOs and,, while I still didn't enjoy them, I no longer dreaded building them.  The low-altitude period, however, totally turned me off from them.  Glad to hear the pendulum has swung back the other way.

 

1 hour ago, FleshJeb said:

Maybe people would like spaceplanes more if they didn't have to fly them.

Flying them or not, it still takes the same amount of time on the clock for them to get to space, which is limited for most folks who have a life.  That's my main objection to them.  The only way to speed this up significantly is to build what is essentially a jet-powered ICBM whose wings serve little purpose except when landing.  Which kinda defeats the spaceplane concept.  With that much power, you might as well omit the wings and just do vertical launches and landings :)

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Posted (edited)

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. 

I also just plain like flying them.

I think the reason people don't like spaceplanes is because most people's spaceplanes have marginal TWR, possibly sub-optimal aerodynamics, and therefore take a lot of finesse to get to orbit. Conversely, if you've given them enough power, you can take off, point them up at 12 degrees, set surface prograde, go for a coffee, and come back for the circularisation burn. (Mild exaggeration.)

The thing with spaceplanes is that ones with marginal TWR aren't even efficient. You're burning more fuel at low altitudes getting into the ramjet zone, and after your acceleration you likely end up with less jet-powered speed and/or going really flat when the jets flame out, which means you burn more Lf/Ox getting to orbit. Add another engine or two and you might find that you have more dV left when you're up there, with the same starting fuel load.

Your spaceplane should have enough power to crack 1500 m/s on jets without sweating it, and should stay pointed at surface prograde for most of the run, with only very rare control adjustments. (Control adjustments kill speed.)

This is kind of the opposite of rockets where you really want to get both the TWR and the dV just right if you don't want to end up with a ridiculously big rocket.

They do take longer to design and tune in the first place and getting around payload dimension constraints is much harder than just slapping a fairing on a rocket. So overall they're definitely harder to get right than rockets, but when you do, they're extremely satisfying to fly. Also easy to fly to orbit every time and 100% recoverable.

Edited by Brikoleur

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1 hour ago, Brikoleur said:

The thing with spaceplanes is that ones with marginal TWR aren't even efficient. You're burning more fuel at low altitudes getting into the ramjet zone, and after your acceleration you likely end up with less jet-powered speed and/or going really flat when the jets flame out, which means you burn more Lf/Ox getting to orbit. Add another engine or two and you might find that you have more dV left when you're up there, with the same starting fuel load.

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.

 

10 hours ago, Brikoleur said:

 They rule at powering SSTO spaceplanes and are kind of mediocre at everything else. That's exactly how a specialised design should be.

"kind of mediocre at everything else"? They are terrible at everything else. 9.18 vacuum TWR with 305 Isp... even the much maligned early 1.25m engines (reliant, swivel) are better than that... far better.

And... that's fine, but for SSTO spaceplanes, the reason they rule seems to be their airbreathign performance. Even so, most of the time, you're better off with another engine for circularization. Considering their very narrow niche (air breathing surface to LKO spaceplane or laythe spaceplane), I wouldn't mind if they were the optimum engine type for that regime, such that they'd be the *only* engine type needed for that regime.

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

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21 minutes ago, KerikBalm said:

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

The vacuum Isp is fine. If you need to burn them in closed cycle long enough that it becomes a problem you need to redesign.

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Posted (edited)
9 minutes ago, Brikoleur said:

If you need to burn them in closed cycle long enough that it becomes a problem you need to redesign.

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.

Edited by KerikBalm

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