Are R.A.P.I.E.R.s overrated

[regex]

It's almost like designing a craft in KSP is a process based on personal considerations and requirements.

[MaxL_1023]

Jet engines produce no thrust at all above a certain speed, making whatever that top speed is (1600m/s or so for Rapiers, less for other engines) as fast as you can go without going closed-cycle. This extra atmospheric velocity makes a significant difference in the amount of fuel needed to circularize, as well as the amount of TWR you need. At 1500m/s in air, you should be able to get to 200+ km on a ballistic trajectory. This gives you plenty of time to use NERVs or even Ion Engines to circularize - you may not need the Rapier rocket mode at all. This is much more difficult with other engines, as your lower top speed leads to a lower ballistic arc, less time in space and requires you to carry more rocket engines. 

This is why Spaceplanes are very inefficient in RSS/RO - Jet engines top out at only about 1/5th or orbital velocity and maybe 1/5th of the minimum orbit altitude. 1500m/s and 30km is not that much of the way to orbit - you usually get a better mass fraction just making a rocket SSTO as opposed to a plane. 

KSP needs a Scramjet engine - basically a jet engine which has a thrust curve stretched to high velocities and low atmospheric pressures. It would produce very little thrust (or none) at low speeds like a ramjet, pick up power near Mach 3 (instead of peaking there) and have peak thrust at Mach 6 or 7, near orbital velocity. It would have the best performance thrust/drag wise near 30km altitude.  One has already been tested (X-51 I think) at near Mach 10 and 30+ km - it worked and provided thrust for several seconds.  A Scramjet powered SSTO would be able to basically reach orbit on air breathing thrust, only needing to circularize when out of the atmosphere. The engine could be expensive and heavy, high in the tech tree, but it would be ideal for SSTOs especially in scaled-up systems. 

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

Edited January 7, 2017 by MaxL_1023

[LameLefty]

45 minutes ago, regex said:

It's almost like designing a craft in KSP is a process based on personal considerations and requirements.

I think that's too subtle for this audience ...

[theend3r]

53 minutes ago, sardia said:

If you had a pair of docking ports, you could decouple your engines, and be 1-2 tons lighter.I've had more success with KIS/KAS and removing the unnecessary parts once I'm docked in space to the space station. I store the excess parts there.

Refitting engines in space doesn't fall under my RP policies but it sound awesome nonetheless. As far as mods are concerned, I've made an infinite dV spaceplane with crazy TWR with the Interstellar mod. That's a bit over the top but it's difficult to find the fine lines of modding in threads like these so I think my post still applies.

[Fearless Son]

I find that different designs of spaceplane require different ascent profiles, in regards to their engine choice.  A R.A.P.I.E.R.-driven craft will usually have a fairly shallow ascent profile, trying to extract the maximum amount of horizontal speed possible before burning its rockets.  A Whiplash-driven hybrid on the other hand tends to (in my experience) favor a more aggressive profile, as it gets most of its speed in thicker air.  This means it gets out of the atmosphere quicker, but also has a longer burn to circularize, requiring a very efficient space engine.  

A big part of this has to do with heating.  Whiplashes generate so much thrust in relatively thick atmosphere that a spaceplane that tries to exploit their full horizontal potential for as long as possible will find itself burning up in the atmospheric compression (and spending fuel fighting induced drag) long before it tops out its air-breathing thrust.  Its ascent needs to be aggressive because it has to get beyond the point where it is building up heat faster than it can shed it without losing too much velocity due to drag and declining thrust.  

Edited January 9, 2017 by Fearless Son

[obi_juan]

 

RAPIERS...  I think that they are the best Kerbal are the best approach to SABRE (Synergistic Air-Breathing Rocket Engine), low ISP in the atmosphere and in the space, a part of near future for the last 35 years, there are a possibility that we will see it in the next 10 years (I was reading about that in the last 20 years). I use them a lot.

But not always, Whiplash and Panther are also a good options. Usually when I use more than 2 air-breather engines in an SSTO/SSTE model several of them are Whiplash.
When I play in a Kopernikus mod with several planets with an oxygen atmosphere make several  SSTE ships are great.

As a lot of people said here. Are you alone who decide how to make your ships. Almost every part have its advantages and Its backwards...

I tried in the last two months make a SSTO ship of 100Tn to reach Minmus and connect my surface bases there. After several failures I think that I made yesterday. Now I am trying to land, if fly well i present it.  

Of course, every one uses RAPIERs is a good way to understand the SSTO. I think that the person that started this thread is now at the next level of making a SSTO.

But I present to you my Rescue Ship, like those that seems that you hate, in my career game I went directly to RAPIERs.  

2 RAPIERs an one nuke with a delta-v in orbit of 1100-1400m/s depending in my capacity of make the ascent. I rescue more than 5 stranded Kerbals in low orbit in a single fly... A good profit for a ship that only cost 84.000 credits. 

 

2 hours ago, MaxL_1023 said:

KSP needs a Scramjet engine - basically a jet engine which has a thrust curve stretched to high velocities and low atmospheric pressures. It would produce very little thrust (or none) at low speeds like a ramjet, pick up power near Mach 3 (instead of peaking there) and have peak thrust at Mach 6 or 7, near orbital velocity. It would have the best performance thrust/drag wise near 30km altitude.  One has already been tested (X-51 I think) at near Mach 10 and 30+ km

I know it... But I think that KSP doesn't need a Scramjet.

Because Kerbin had the 1/10 size of the Earth.

Here we need a Scramjet to increase the velocity from mach 3 to mach 15-20 using the oxygen on the atmosphere to lower the fuel combustion to reach the space... Because for the size of the earth a low orbit need the velocity of mach 28-26. 

In Kerbin for the size of the planet you can orbit it at only mach 7. If you want a Scramjet, use a Rapier without change the the mode.

But if you are using a real size earth to play I could recommend the mod  "mk2 expansion" it have a good collections of Scramjet. 

Edited January 7, 2017 by obi_juan
I am not a good English writer...

[Aegolius13]

Sure, the Whiplash is usable on a spaceplane.  But I don't see any compelling advantage compared to a Rapier.  Pros of the Whiplash:

• Earlier in the tech tree.  This means it makes sense to use before you get the Rapier, but does not matter for comparison purposes once you have both. 
• Cheaper.  I do use the Whiplash for this reason on disposable boosters.  But on an SSTO, if you're planning on landing on the runway, part cost is 100% refundable.  So cost does not really matter as long as you can afford the up-front hit. 
• Has better ISP.  This means you burn a little less LF on your way up.  But this is likely overshadowed by the longer rocket burn required when the Whiplash flames out earlier - since even good LFO rockets will consume fuel about 10 times more than the Rapier in jet mode.  
• Is 0.2 tons lighter.  Always nice to be light.  But again, you pay for that with the weight of additional fuel burned in relatively inefficient rockets to get to orbit, plus the weight of the additional rocket engine(s) you need.  
• Has better low-speed thrust.  This can be helpful for getting past the sound barrier, especially if you want a relatively quick ascent. To me, this is the most convincing reason to use the Whiplash, but is generally not that big of a deal. 
• Has a cooler looking exhaust plume.  Nuff said.

I don't see any of those factors beating out the Rapier for general use, given that the Rapier has:

• A substantially greater altitude and speed limit, which decreases the amount of rocket thrust you'll need to get to orbit.
• A rocket mode, which saves engine weight and, if designed appropriately, the drag of another rocket stack or two.  
• A rear node, which is not that important, but nice if you're, say, sending a plane to Laythe as part of a Jool 5. 

I used to think the Whiplash might make more sense for super long distance SSTOs (e.g., to Laythe), since you might want a more efficient rocket engine for the vacuum stuff.  But I don't think even that pencils out.  If you want to go top efficiency, that means nukes.  And with nukes, it's difficult, if not impossible, to get to orbit with only jets and the nukes (and that's even with the Rapiers as your jets).  So with a Whiplash + nukes, you'd likely need yet another rocket engine to bridge the TWR gap after your jets shut off.  No good!  Maybe there are intermediate delta-v destinations where something like a Whiplash + aerospike combo would work well, but I generally don't see it.  For short hauls to LKO, you generally want the Rapier as your only engine. For long hauls, you want nukes, and that also means Rapiers.  

So in light of all that, I don't think the Rapier is overrated.   That's not to say you can't use the Whiplash effectively, the Rapier is just the more useful part for most jobs.

[memes in space]

I NEVER use spaceplanes  but RAPIERS are cooler so just use them. Planes are already hard enough to fly, having them get to orbit is impossible! (it actually is though, every time I fly either the stock ones or my own ones, (my own ones suck though) they get to 50k and start dropping. Usually into places where it's impossible to land such as mountains or ocean. I can land in grassland though incase I get off target when flying regular planes.)

[suicidejunkie]

I haven't researched RAPIERs yet, but I'm not particularly interested in getting them either.

Having to balance different fuels makes the RAPIER less appealing when my current SSTO plane is Oxidizer-free with Whiplash + LV-N.

I fly to 23km/1600 orbital on the whips and then light the nukes for a long burn and accelerate in a ~50m/s climb until reaching 45km/2300 and then coasting to space.  Almost 2km/s left in the tanks at that point for a trip to Minmus flats and back, or a one-way to Mun midlands with a refuel at Munbase Alpha for the return.

 

RAPIER could help with additional TWR for landing and getting to orbit, but I've already got enough for both muns, and the requirement for fuel and oxidizer management; ugh.  I think I'd have to include an ISRU and ore instead of tanks so I could adjust the ratios in flight, and then with a drill too, it would be SSTE instead of SSTA

 

Whiplash + Reliant is also a drop-in replacement option for the LV-N if the main tanks are switched over to LFO, but it only makes it to LKO.  Although the tech level difference is tremendous, RAPIER could help there by combining the two engines and I should try it from this perspective some day.

[sardia]

46 minutes ago, suicidejunkie said:

I haven't researched RAPIERs yet, but I'm not particularly interested in getting them either.

Having to balance different fuels makes the RAPIER less appealing when my current SSTO plane is Oxidizer-free with Whiplash + LV-N.

I fly to 23km/1600 orbital on the whips and then light the nukes for a long burn and accelerate in a ~50m/s climb until reaching 45km/2300 and then coasting to space.  Almost 2km/s left in the tanks at that point for a trip to Minmus flats and back, or a one-way to Mun midlands with a refuel at Munbase Alpha for the return.

 

RAPIER could help with additional TWR for landing and getting to orbit, but I've already got enough for both muns, and the requirement for fuel and oxidizer management; ugh.  I think I'd have to include an ISRU and ore instead of tanks so I could adjust the ratios in flight, and then with a drill too, it would be SSTE instead of SSTA

 

Whiplash + Reliant is also a drop-in replacement option for the LV-N if the main tanks are switched over to LFO, but it only makes it to LKO.  Although the tech level difference is tremendous, RAPIER could help there by combining the two engines and I should try it from this perspective some day.

You can use the rapier as an upgraded version of the whip lash. It goes higher and faster. That's really helpful since it pushes your plane to a more efficient burn with your Nerva. 

Edited January 9, 2017 by sardia

[suicidejunkie]

1 hour ago, sardia said:

You can use the rapier as an upgraded version of the whip lash. It goes higher and faster. That's really helpful since it pushes your plane to a more efficient burn with your Nerva. 

Heh.  Good point.  Rocket mode is completely optional.

[MitchS]

I don't know what all the contention is about. I learned on Whiplashes and made some really efficient SSTOs. Then I applied that to RAPIERs and made some really efficient SSTOs. Whiplashes get you to your speedrun a little more quickly and have much higher Isp in atmo, but RAPIERs result in a higher speed (500m/s higher!) and altitude (5-8km higher!) before it's necessary to switch to lower Isp and/or oxidizer-using vacuum engines. Sure, these bonuses come after a rather plodding ascent, but if you're in such a hurry that 15 minutes of powered flight for launch is unacceptable, then there's a VAB right down the street from the SPH, and you can make tall, pointy things that get to orbit as quickly as it suits you. 

Me, I'll just keep building things that suit me.

[sardia]

On 1/9/2017 at 2:29 PM, suicidejunkie said:

Heh.  Good point.  Rocket mode is completely optional.

The limit to this is your ship is going to be really small, unless you construct your interplanetary stage piece by piece. At that point, you need oxidizer. 

[Daripuff]

37 minutes ago, sardia said:

The limit to this is your ship is going to be really small, unless you construct your interplanetary stage piece by piece. At that point, you need oxidizer. 

Nope,  not with a NERVA as your vacuum motor. 

You can do an oxidizer free SSTO spaceplane quite readily with a RAPIER / nuke combo. 

Not as easily as you used to,  but it can be done. 

[sardia]

Are you referring to a interplanetary stage that is constructed piece by piece via docking, or are you referring to refueling the ssto and sending it interplanetary?

57 minutes ago, Daripuff said:

Nope,  not with a NERVA as your vacuum motor. 

You can do an oxidizer free SSTO spaceplane quite readily with a RAPIER / nuke combo. 

Not as easily as you used to,  but it can be done. 

 

[suicidejunkie]

1 hour ago, sardia said:

The limit to this is your ship is going to be really small, unless you construct your interplanetary stage piece by piece. At that point, you need oxidizer. 

The craft is already an oxidizer free SSTO spaceplane with whiplash/nuke combo.

And it gets to orbit with almost 2km/s to spare at 1/3rd on the LF gauge.

The only reason it doesn't go interplanetary after a refuel is because nothing I've ever sent interplanetary has had time to arrive yet.  Also because the life support would run out unless it was a solo trip.

[GoSlash27]

On 1/9/2017 at 0:53 PM, sardia said:

You can use the rapier as an upgraded version of the whip lash. It goes higher and faster. That's really helpful since it pushes your plane to a more efficient burn with your Nerva. 

 Something else to consider: If you're just going to rendezvous in LKO and land, it's generally not worth it to add more efficient rockets. Adding rockets carries a mass and drag penalty that's rarely paid off in the (roughly) 2 km/sec you actually use them. It's better for this type of spaceplane to simply use the closed cycle mode of the RAPIER itself.

Best,
-Slashy

[KevinW42]

Hmm, initially I saw the lackluster I_sp and didn't really give them a second thought and just used the whiplash instead, but I guess with the higher top speed I'll have to reconsider using them.

[sardia]

15 hours ago, GoSlash27 said:

 Something else to consider: If you're just going to rendezvous in LKO and land, it's generally not worth it to add more efficient rockets. Adding rockets carries a mass and drag penalty that's rarely paid off in the (roughly) 2 km/sec you actually use them. It's better for this type of spaceplane to simply use the closed cycle mode of the RAPIER itself.

Best,
-Slashy

a nuke and two jet engines is less efficient than a single rapier? So if I built two planes, the single rapier can carry more cargo (or fuel) to a station? If so, I may add a glorified single stack space dump truck rapier plane to my ssto roster. 

[GoSlash27]

12 minutes ago, sardia said:

a nuke and two jet engines is less efficient than a single rapier? So if I built two planes, the single rapier can carry more cargo (or fuel) to a station? If so, I may add a glorified single stack space dump truck rapier plane to my ssto roster. 

sardia,

 Yes, if you define efficiency as dollars per tonne delivered to orbit, the RAPIER is more efficient than any jet/ LV-N combo.

There are some combos that can get close, though (at least in the closed cycle portion). Jet/aerospike and jet/Poodle are both pretty good in 1:1 ratios. The downside is that the additional nodes are much draggier, especially with Poodles.

Best,
-Slashy

[fourfa]

1 hour ago, sardia said:

a nuke and two jet engines is less efficient than a single rapier? So if I built two planes, the single rapier can carry more cargo (or fuel) to a station? If so, I may add a glorified single stack space dump truck rapier plane to my ssto roster. 

The problem is, where do you put them?  A single Mk1 fuselage will crush a triple Mk1 fuselage for drag, or a Mk2 with a bicoupler.  The drag adds up and can be a much bigger factor than the raw ISP.

But let's not focus on splitting hairs over efficiency to the exclusion of all else.  As far as career play is concerned, fuel is practically free.  As long as you recover your expensive spaceplanes for 100%, it's never going to make a difference to your overall career success if you ran your cargo-to-LKO mission with 100 LFO, or 5000.  Other factors (ease of piloting, physics lag on complex ships, elapsed time to orbit, reentry reliability, return flight characteristics) may dwarf the core efficiency for your overall career path.

[Rune]

On 6/1/2017 at 4:51 PM, GoSlash27 said:

There is a situation where you would want to employ a hybrid design instead: You haven't unlocked the RAPIER yet. Other than that, the RAPIER is the way to go.

Quoted for truthinness.

The RAPIER is the best (airbreathing) SSTO engine because it achieves the highest payload fraction in a run to LKO. The reasons for that are varied, but given that, the only reason to put other engines on a plane is to take it outside of LKO. And then you put RAPIERs to take it to LKO in the first place, so the in-space propulsion is just part of the payload fraction to orbit.

 

Rune. Other alternatives are viable, just not as good.

Edited January 13, 2017 by Rune

[Foxster]

"Are R.A.P.I.E.R.s overrated?" 

No.