Rapier in 1.0.2 completely useless

[RocketBlam]

I have made two different space planes recently that do a great job getting into orbit. One has four Rapiers, one has two with a turbojet. The key to the Rapiers is that they want to be going as fast as possible. When I take off, I accelerate until I start to see the mach/compression effect. Basically, if you aren't seeing those white glow effects, your not going fast enough. Both of these ships achieve orbit handily, and I don't even have to level off, let alone dip to gain speed. I can post them if you want.

[Frank_G]

Your designs don't have to be big...

I totally agree. And i like your designs!

I managed to get this into a 100 km circular orbit today. Not as small as i like to have it, but i am getting close.

[Falkenherz]

May I re-introduce my "Fuel Dragon", a mainly Mk-2 plane for all those career modes which are still left dreaming of finally getting the Rapier unlocked. This baby is about 100 tons mass and brings 1440 units of fuel into LKO (i.e. half an "orange tank").

I hope to improve its efficiency later on by adding precoolers and am very curious of what is going to happen once and if I replace the 2 or 4 of the Whiplash with Rapier and/or Shock Cones for RAM intakes.

Edited June 4, 2015 by Falkenherz

[Levelord]

I hope to improve its efficiency later on by adding precoolers...

[Falkenherz]

As you can see on this picture, my engine nacelles are pretty much heated to the limit. This also true to the front part of the wing.

Each nacelles is built with RAM intake, engine nacelle, LFtank, engine nacelle, turbojet. I cannot believe that the precooler will yield just the same result as the lower-tech engine nacelle and hope that those forum reports are true which says that the pre-cooler will swallow and radiate heat away much more faster, i.e. into the other wing parts and thus relieve the engines a little bit more of the current heat stress, so that in the end I can stay in thicker air longer and get even some more speed.

[Red Iron Crown]

The precooler radiates a bit better, but that's really only significant during the cooldown phase as the amount of heat radiated by the most emissive parts is dwarfed by the heat generated by an engine. As for transferring heat to other parts, that's conduction rather than radiation and it doesn't seem to be any better in that regard. Best to attach your engines to parts with more thermal mass, like larger tanks.

[mikegarrison]

The spaceplanes I've flown so far never needed to dip down in order to gain speed. That wastes a lot of fuel and uneccesarily prolongs the flight to orbit... and also increases fuel requirements.

I know this an old post, but I thought it might be interesting to point out that the minimum fuel, optimum climb profile for the Concorde was to climb subsonically, dive down through Mach 1, and then start climbing again. This didn't "waste fuel", this saved fuel.

The reason why is that there is one optimum climb profile (in terms of altitude and speed) for subsonic flight, and another optimum climb profile for supersonic flight. They don't intersect, so you have to cross from one to the other. The way to cross from one optimum line to the other with as little fuel cost as possible is sometimes by making a dive. It's better to spend more of your time at optimum even if you have to climb through the same altitude twice than it is to spend a lot of time far away from optimum trying to cross from one optimum line to the other.

Edited June 4, 2015 by mikegarrison

[herbal space program]

I know this an old post, but I thought it might be interesting to point out that the minimum fuel, optimum climb profile for the Concorde was to climb subsonically, dive down through Mach 1, and then start climbing again. This didn't "waste fuel", this saved fuel.

The reason why is that there is one optimum climb profile (in terms of altitude and speed) for subsonic flight, and another optimum climb profile for supersonic flight. They don't intersect, so you have to cross from one to the other. The way to cross from one optimum line to the other with as little fuel cost as possible is sometimes by making a dive. It's better to spend more of your time at optimum even if you have to climb through the same altitude twice than it is to spend a lot of time far away from optimum trying to cross from one optimum line to the other.

I'd say it really depends on how slowly you're climbing to your highest subsonic altitude and how much of a dive you have to take to punch through. The best efficiency point probably lies on a profile that requires around 1000m of altitude loss before pulling up. More than that and I think your ascent to orbit will take too long for the reduced weight of shipping less engine power to have a real benefit. Also, it's just kind of a pain in the end that should not point towards space to have to go through such contortions to get to supersonic flight.

[NathanKell]

However, using that profile means you can have a far lower thrust:drag ratio (fewer engines, lower dry mass) and get a higher payload fraction. In order to go supersonic in level flight, let alone climb, you need rather more engine.

[mikegarrison]

I'd say it really depends on how slowly you're climbing to your highest subsonic altitude and how much of a dive you have to take to punch through.

The optimum climb profile is different for every airplane. It depends on lots of factors.

Commercial airplanes will actually fly themselves based on what Boeing calls a "cost index". The pilot tells the plane how much to value fuel versus how much to value time, and the airplane will choose the flight profile that reduces total cost. (If you don't care about time, you just tell it that fuel is infinitely more valuable then time. In that case it will try to fly a minimum fuel profile.) You can also tell the airplane that you want to be at the destination at a certain time, and it will try to find the minimum fuel profile that fits the schedule.

http://www.boeing.com/commercial/aeromagazine/articles/qtr_2_07/AERO_Q207_article5.pdf

Edited June 4, 2015 by mikegarrison

[herbal space program]

However, using that profile means you can have a far lower thrust:drag ratio (fewer engines, lower dry mass) and get a higher payload fraction. In order to go supersonic in level flight, let alone climb, you need rather more engine.

And you have to balance that benefit against the greater amount of time it takes you to get up there with less thrust. In my experience flying with different amounts of RAPIER engines, I found that all the planes where I had to climb slowly to 13km then dive to 10km to get above 420m/s ended up with considerably less fuel on orbit than the ones where all I had to do was level off around 11km, in spite of the added weight of the engines. My solution to get the best of both worlds was to add like 1 turbo per 3 rapiers to the mix, because their improved performance near Mach 1 offset their worse performance near flameout. Based on doing that over and over, I came to the conclusion that the ideal profile to get the greatest payload on orbit per ton of vessel would be to lose like 1km of altitude going trans-sonic, but I have not yet found a combination of available parts that yields that profile.

- - - Updated - - -

The optimum climb profile is different for every airplane. It depends on lots of factors.

Commercial airplanes will actually fly themselves based on what Boeing calls a "cost index". The pilot tells the plane how much to value fuel versus how much to value time, and the airplane will choose the flight profile that reduces total cost. (If you don't care about time, you just tell it that fuel is infinitely more valuable then time. In that case it will try to fly a minimum fuel profile.) You can also tell the airplane that you want to be at the destination at a certain time, and it will try to find the minimum fuel profile that fits the schedule.

http://www.boeing.com/commercial/aeromagazine/articles/qtr_2_07/AERO_Q207_article5.pdf

This is of course true, but the limited number of configurations available in KSP, combined with the implied objective of getting the maximum payload fraction to orbit, constrains the design parameters quite significantly. I made the estimate I did based on those constraints, although there is no doubt some wiggle room even within them.

[NikkyD]

Question, is there a CoM issue with rapiers ? When i build a spaceplane with two flt100 tanks and a remote control in the middle. If i add a rapier at the end and a shock cone at the front, the CoM is on the cone side... HOW ? it weighs 10% of the rapier

[Red Iron Crown]

CoMs for jets and RAPIERs are offset forward of the actual part. Not sure what the exact reasoning for that was, perhaps it makes planes easier to balance.

[NikkyD]

thats total BS... messes up every small design

[Mic_n]

certainly does.

However if you think about it, those "engines" are really just nozzles. An actual turbojet engine is way longer than that. If you want to make those small designs "right", you should put a nacelle, pre-cooler, structural intake or just a chunk of structural fuselage in front of them. Together, they're "about right". Or, you could use a fuel tank and just ignore that you're violating fundamental laws of science having both fuel and engine occupying the same space and time by pretending that fuel's actually in the wings, which is something we generally (with a couple of notable exceptions) can't do.

[herbal space program]

CoMs for jets and RAPIERs are offset forward of the actual part. Not sure what the exact reasoning for that was, perhaps it makes planes easier to balance.

(facepalm) So THAT's why I seem to be able to get away with all these outrageously tail-heavy designs! should have known something was not quite right. Although I suspect as you said, they did it for playability, I guess you could reason that all the turbines and such, which constitute the bulk of the mass, would actually be significantly forward of the part you place in the SPH.

[mikegarrison]

(facepalm) So THAT's why I seem to be able to get away with all these outrageously tail-heavy designs! should have known something was not quite right. Although I suspect as you said, they did it for playability, I guess you could reason that all the turbines and such, which constitute the bulk of the mass, would actually be significantly forward of the part you place in the SPH.

Exactly. The part that you place is only the nozzle. The idea is that the actual engine is hidden somewhere forward of the nozzle.

[CptRichardson]

There's always the option of more POOOOWAH! I just use a 4-rapier cluster to punch the sound barrier until it gives me its lunch money and I POWAH climb into orbit that way.

[NikkyD]

Why don't they just make the engine + nacelle one unit ?! Like this it is totally misleading!

[Tankman101]

Maybe they, I don't know, want people to be able to slap the engines where they want without a garish tube sticking out of the back?

Also it isn't misleading in the slightest, just look at actual turbofan/jets.