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Razorforce7

Why use JX-4 Whiplash over Rapier in spaceplane designs?

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Posted (edited)

I've been building spaceplanes for years now in KSP and the topic question has been pondering me for quite a while.
I just want other peoples opinions to see how I could put the Whiplash to better use.

All my spaceplane SSTO's have rapiers only, and Nervas ofcourse if I want to go elsewhere, but never any whiplash, except for some career saves.

However, I see alot of designs out there with whiplash engines, always in combination with rapiers and or nervs unless it's just a supersonic aircraft ofcourse.
I see it on forum posts, KerbalX, videos and such.
Because they use a combination of both engines it means that peoples spaceplane designs are not limited to Whiplash only designs (due to i.e. unmet science node requirements in career mode)
Yet they use a combination of both engines while my supposedly vast or limited experience says that the rapier is the only choice you can make.
The answer is probably that they're less knowledged? Or am I?


Let me simply ask, why do people do this? I can only think about 2 reasons, but hell, I'm asking here so please feel free to expand my list.

1:Aesthetics
2:Flying on laythe with better airbreathing ISP over a Rapier in airbreathing mode. (or Kerbin for that matter, but those usualy aren't spaceplanes)

But furthermore, I simply don't know of other reasons.

When you build "efficient" spaceplanes the Whiplash limits you, or atleast in all my designs. If that shouldn't happen then please tell me in what kind of spaceplane designs the Whiplash is useful with it's higher efficiency.

Yes its lighter and more efficient then the rapier, but that is it.
It has less thrust overall and thrust falls steeply when reaching 18-22km or thereabout and usually can never go faster then a rapier anyway. This also means you will need rocket engines sooner. Then why would you care if you have spared a few units of LF for the cost of inefficient LF/O expenditure at altitudes up to 10km lower then with a Rapier setup.
Do you not want a spaceplane that is very aerodynamic and can go as fast and high as possible before switching to rocket engines?

The Whiplashes don't help in that regard. Then I'd rather have 200kg per engine more in dead weight but can go 300+ m/s faster and 5-10km higher
While it is (A) less efficient it also has (B) no rocket motors like tha Rapier does.
So while the rapier seems less efficient it doesn't require secondary rocket motors to get any further and also goes higher and faster.
The Whiplash is definitely in game for a reason. Even under current understandings it will serve always a role in career play or for fooling around with atmospheric aircraft.
But if you only go to space, and have unlocked the rapier engine then are you not destined to never ever touch the Whiplash?

Blow me!


 

 

Edited by Razorforce7

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Posted (edited)

I was thinking.
Wouldn't it be a good test to make two "LF" only spaceplanes.
They're both identical, but,,, "your guessing it" one has rapiers and the other has whiplash engines at its engine attachment nodes (just switches engine type basically)
In this test the "rapier" may not enter rocket mode obviously, and fly it to orbit with a assisting nerva.
Obviously the spaceplane design must be able to get to orbit with both whiplash(s) engines or rapier(s)

@AeroGav

I see what your saying. However, I have a rapier only cargo SSTO. It's the biggest SSTO I got (about 1360Tons with cargo and it's stock)
I tried your advice by eventually removing some of the rapiers. I went from 70 rapiers to 60 rapiers and 12 Whiplashes. I did had to put in extra oxidizer but not much though.
Yes the spaceplane goes through mach 1 faster and spends less time in the lower atmosphere and saves precious LF in the process.
However, it takes more time to burn that LFO because I got less Rapiers and it doesn't go as high and fast as with the rapiers only.

Like I said, I had to add oxidizer to make the rapier/whiplash combo work, meaning more weight.
The mass to orbit was eventually identical. But for the Rapier/Whiplash combo I had to add weight in oxidizer. So the weight fraction on the rapier only design is higher.
Obviously my aim with this thread is to clarify the difference in total weight to orbit by using both engine types.
It is nearly the same in my test. But the starting weight of the Rapier+Whiplash design needed more oxidizer because the rapiers needed to burn longer.
So on this particular design I use rapiers only.
Obviously this spaceplane used rocket mode on the rapiers. So it doesn't serve the test I devised written above. Something that I want to test tommorow. It will also be my first LF only spaceplane, yay! If I can pull it of lolz never failed


 


 

 

Edited by Razorforce7

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10 minutes ago, Razorforce7 said:

Let me simply ask, why do people do this? I can only think about 2 reasons, but hell, I'm asking here so please feel free to expand my list.

1:Aesthetics
2:Flying on laythe with better airbreathing ISP over a Rapier in airbreathing mode. (or Kerbin for that matter, but those usualy aren't spaceplanes)

3. Available earlier in the tech tree

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Beyond that, the Rapier has very bad performance at very low (takeoff) speeds, and very bad performance right around Mach 1. And unless you use the reversed-cone trick, it also has significantly more drag. So sometimes the Whiplash is preferable for its low speed performance to get you off the ground and into a high-acceleration supersonic mode.

Then comes the question of heating. Yes, it is true that the Rapier will get you an extra 200 m/s of speed over the Whiplash in airbreathing mode -- boosting you from the 1300 m/s range into the 1500 m/s range. Depending on your flight profile after you light the rockets, the extra 200m/s may get you going too fast by the time you get to the dangerous heating region in the atmosphere. In which case, you have to slow down -- so you've lost any possible benefit from the Rapiers.

Then comes the question of whether you're carrying oxidizer or whether you're going pure-LF. The biggest advantage of all of the Rapiers is that you don't need any additional rocket engines in designs that carry oxidizer. If you have no ox, then all those rapiers are basically a lot of additional dead weight from 27km to your destination and back.

And they cost a lot more, in designs that decouple one stage of engines.

 

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I often find that the number of Rapiers necessary to take off and maintain a good climb rate early on, is too many once you switch over to rocket mode (I can usually get by only switching half).  Whiplashes can save a little mass, a little drag, a little money, a little fuel on the way up... it can add up if you're careful.

If you're OK with a little creative clipping - a fun combination is the Whip-spike: a Whiplash on top of an Aerospike and clipped together.  Best of both worlds

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I would say the main advantage is low end thrust,  to help get supersonic and into the Rapier sweet spot.  But I've used them less as I've gotten better at streamlined design and more patient with my ascent profile. 

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1 hour ago, fourfa said:

I often find that the number of Rapiers necessary to take off and maintain a good climb rate early on, is too many once you switch over to rocket mode (I can usually get by only switching half).  Whiplashes can save a little mass, a little drag, a little money, a little fuel on the way up... it can add up if you're careful.

If you're OK with a little creative clipping - a fun combination is the Whip-spike: a Whiplash on top of an Aerospike and clipped together.  Best of both worlds

This.

As a rule of thumb,  a well streamlined, well flown Spaceplane only needs 60kn of non-airbreathing propulsion per 15 tons of mass.   

One RAPIER puts out 180kn in closed cycle mode, which is therefore enough for 45 tons.   However, one RAPIER can only get about 30 tons of craft through mach 1.   

So, for example, a 90 ton SSTO   would be fine with 2 RAPIERs in terms of close cycle power, but would need an additional whiplash engine to get it through mach 1.    Why not just fit 3 rapiers?   Well,  the Whiplash has stronger low speed performance, making it easier to get through mach 1,  and it is lighter,   which gives more delta v once we go rocket mode.

 

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@Razorforce7

Good grief 70 engines !    It must be huge !

In such a large design there is a lot of scope to create extra drag with your engine attachment method, due to the way the stock game calculates drag, hope that doesn't skew results .

In a low drag design you are going to be far into the RAPIER's  point of diminishing returns on a 90 ton ship whether 3 rapier or 2 rapier 1 whiplash.  You're likely to be exceeding 1500-1600m/s air breathing either way,  by which point thrust is declining very rapidly and bringing 3 R won't add more than 10-15 m/s to Vmax.

As regards to the All Whiplash design,  it's not optimal.   Whiplash peak at mach 3 not mach 3.75,  and thrust declines more quickly if you exceed mach 3 than the rapier does if you go past it's max.  With one Whiplash per 30 tons on an all-whiplash design you're getting 1100-1200m/s tops .  In terms of altitude it looses out too, on the RAPIER, thrust declines more slowly than air density up to 22km,  on the Whiplash it's 17km.

400px-J-X4_Whiplash_Turbo_Ramjet_Engine_400px-CR-7_R.A.P.I.E.R._Engine_velocity_

Rapier has fallen to half of peak thrust by mach 5.25,  Whiplash has fallen to half of peak thrust by mach 4.5

All liquid fuel design ?

30 Tons  2 Nukes  1 Rapier

90 Tons  6 Nukes   2 Rapier 1 Whiplash

With all LF designs,  use lots of wing and keep drag low (cones on the back of every engine!).  Big S strakes are also your friend, and a good thing to build your tail fin(s) out of (free LF capacity).  Learn to love the fact you only have one fuel type on your vessel, "how much oxidizer / jet fuel should i bring " is no longer something you have to think about.

Some of my LF only ships -

https://kerbalx.com/AeroGav/freedom-fighter

https://kerbalx.com/AeroGav/Auto-Ray

https://kerbalx.com/AeroGav/Firefox-II

20161216174338_1_zpstbdmwv9i.jpg

https://kerbalx.com/AeroGav/K133--Curlew

 

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On Monday, March 20, 2017 at 8:44 PM, AeroGav said:

This.

As a rule of thumb,  a well streamlined, well flown Spaceplane only needs 60kn of non-airbreathing propulsion per 15 tons of mass.   

One RAPIER puts out 180kn in closed cycle mode, which is therefore enough for 45 tons.   However, one RAPIER can only get about 30 tons of craft through mach 1.   

So, for example, a 90 ton SSTO   would be fine with 2 RAPIERs in terms of close cycle power, but would need an additional whiplash engine to get it through mach 1.    Why not just fit 3 rapiers?   Well,  the Whiplash has stronger low speed performance, making it easier to get through mach 1,  and it is lighter,   which gives more delta v once we go rocket mode.

 

Woah, you launch a 90 ton plane to orbit with 3 engines? I would need at least six for that kind of weight! 

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Different strokes.... my planes get to orbit in a couple of minutes. If I test fly one of @AeroGav's planes, I put on a kettle :)

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Posted (edited)

R.A.P.I.E.R.s are boring. One engine that works both as a jet and a mediocre rocket; where's the fun in that? I want to get into orbit on Junos and Puffs!

(Note: I don't know if it's actually possible to get into orbit on Junos and Puffs, but I'm suddenly tempted to try.)

Edited by Whisky Tango Foxtrot

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Posted (edited)

@AeroGav

I've been doing some tests to compare with your data. I guess your spot on, but the differences between both engines are not necessarily minor but hard to implement in a design where I find that they are really making difference except LF only designs that is.
I've been trying to build my own LF only planes yesterday and it's definitely the only real reason (for me atleast) to make whiplash designs. I do feel I'm limited to certain spaceplane sizes to limit part count because you'll soon need alot of nervas to assist all that extra weight if you go bigger. But they're very good for cool crew transfer vessels from KSC to LKO and back.

@Whisky Tango Foxtrot

A rapier hater,... guess we are all different then.

I made a TSTO with Wheesleys/Junos and LVT30 Reliant rocket engines.

Got me out of the atmosphere and then took a 2nd rocket assisted cargo or plane to orbit.

With TSTO designs using normal jet engines I can actually put a reasonable amount of cargo in LKO.
Only level 1 runway/sph and 90 science node requirements needed. Do you accept 300+ part count it could probably be done with Junos only, albeit you'd need a SPH upgrade.

It can actually glide 200km back to the KSC at mach 1.5 only several clicks from the runway.

Oh and don't let me Hijack my own thread:d

Edited by Razorforce7

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5 minutes ago, Razorforce7 said:

I made a TSTO

So did I -

It was based on the premise that real jet engines are unlikely to ever exceed 20% of orbital velocity, so i made a TSTO limiting myself to wheesley tech.    The launcher has 5 Wheesleys,  the upper stage two NERVs and a Terrier.   The terrier doesn't have that much ox, but helps us to mach 3.

3000dV in LKO.    Someone else borrowed this craft and made a video with it.  He was pulling the nose up further off prograde (not needed on this design because of large wing area and the fact wings have built in incidence angle) - the extra drag cost about 15% delta V but it got the mission done.  And he makes much better videos than i do !

 

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@AeroGav

Nice:d

Mine was from a career save so I have no Nervs yet to pull that stunt of.
I think yours is alot cheaper even although I can recover my 1st stage. Unless you use stage recovery that is.

It is pleasing to see that you can basically go anywhere with a TSTO design. Personally I would want things recoverable, always. So that makes things a bit more difficult for my personal playstyle.

Gorgeous wings btw.

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1 hour ago, Razorforce7 said:

I do feel I'm limited to certain spaceplane sizes to limit part count because you'll soon need alot of nervas to assist all that extra weight if you go bigger. But they're very good for cool crew transfer vessels from KSC to LKO and back.

NERVs really are for stuff that goes beyond LKO, which  is very possible.

Re: size.  If you have an interplanetary NERV powered spaceplane with mining equipment, it becomes pointless building beyond a certain size even though you can.    I mean, you could make one lift an orange tank , but why the hell would you?  Just send the nerv spaceplane to wherever that poodle/orange tank ship was going.  Or just use its own drill to support remote ops.   I suppose you might want it to carry a lander for somewhere like tylo, eve or moho, that's about it.

Are you using any kind of stage recovery mod?

If  not, then whiplashes do make good droppable boosters, being only a third the cost of a rapier.

 

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Posted (edited)

@AeroGav

Hmm, I know what your saying and all.
But here comes the point and reason why I won't go there.

I place several restrictions on my gameplay. One of them is to never use drills and ISRU, even for tylo and such places. While I have made them I don't like to use them. And they are part of other savegames where i'd like to add near future/future mods.
But yes, it will work that way obviously.
And no, I don't use stage recovery on this particular install of KSP although I certainly used it at times. Although I'd rather invent a way to get them back in stock.

But you couldn't possibly know all that:D

Although I still find your suggestion about dropping whiplashes useful. They are indeed dirt cheap. And my terms are not necessarily to recover everything as long as it is financially efficient. In fact, I think I would end up with quite a bit of surplus vaccuum Delta V if I were to do that. Anyway, you caught my interest about LF only spaceplanes and thats what I'll work on later today. I'll sure share some pics once I'm done.

 



 

Edited by Razorforce7

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I think if you're setting yourself the handicap of doing interplanetary with a 90% re-usable ship,  then allowing yourself to ISRU is acceptable.   It's pretty easy to go anywhere if you're asparagus staging 3.75m boosters to send a little mk1 pod to space.

It is still possible to go interplanetary without ISRU of course, check out this vid (not mine!) - 

 

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16 hours ago, Whisky Tango Foxtrot said:

R.A.P.I.E.R.s are boring. One engine that works both as a jet and a mediocre rocket; where's the fun in that? I want to get into orbit on Junos and Puffs!

(Note: I don't know if it's actually possible to get into orbit on Junos and Puffs, but I'm suddenly tempted to try.)

Link to the challenge thread, please

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But you can't make a challenge of it until you're at least reasonably sure it's possible.

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Posted (edited)
On 22 March 2017 at 3:10 PM, Whisky Tango Foxtrot said:

R.A.P.I.E.R.s are boring. One engine that works both as a jet and a mediocre rocket; where's the fun in that? I want to get into orbit on Junos and Puffs!

(Note: I don't know if it's actually possible to get into orbit on Junos and Puffs, but I'm suddenly tempted to try.)

Close: https://kerbalx.com/Wanderfound/Kerbodrone

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5wBF7Xq.jpg

S0kS0RF.jpg

Edited by Wanderfound

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