Best use of rapier engine

[gilflo]

Given a mass to set to orbit with SSTO around Kerbin I am wondering what is the best use of the Rapier Engine

-The weight of the Rapier is the weight of 400 LF

-The Rapier gives his best TWR and thrust when accelerating in low atmosphere to reach around 1600 m/s between 22 and 25000 before switching to Closed Cycle

-I am trying a MK1 SSTO with cabin crew and 2 Rapiers only,  to reach orbit and Get 2000DV once establish

The choices are:

- Add a lot of LF and and a very long acceleration in low atmosphere to take advantage of the Rapiers and reach 1600m/s around 22000m before switching

-Build  TSTO with 2  Rapiers more or maybe 4 with 2 or 4 MK1 LF, this stage breeding the 2 Rapiers of the SSTO part and its own Rapiers and being decouple when switching to closed cycle on the SSTO. Here the acceleration would be crazy, you don't need to stay in low atmosphere and climb very quickly to 25000m reaching 1600m/s with higher Apo before decoupling and switching. I tested TWR up to 4.5 but it's very not to explode du to heat

In this this TSTO only 4 MK1 tank with only 280 LF in each are enough to reach 25000m

 

[Gargamel]

Moved to gameplay questions 

[magnemoe]

Well you have way to many engines. I tend to make an light SSTO, it has two rapiers and two air intakes.  Larger wings as I want better handling at low speeds for landing. 
Yes I know I could do with one intake but that looks stupid. My rockets and bases looks stupid enough. I tend to go for an around 10 degree climb angle once I pass 400 m/s.
I say it accelerates fast enough once above speed of sound, you could use an extra jet engine who has high trust below and into the speed of sound to speed that up and stop one past that. 
That engine would also be nice for landing. 

[18Watt]

As @magnemoe said, that's too many engines for a plane that size.  Two Rapiers would be overkill, a single Rapier would probably work.  With the weight of all those Rapiers and the fuel for them, you may need more wing area too.  Although losing most of the Rapiers will reduce the need for more wing area.

I believe in the latest version of KSP, one Shock-Cone intake will still feed up to 3.5 Rapiers.  I personally would use a single intake for two Rapiers, unless the 'looks' of it bothers you.  (Which is fine!)

One of the most important things to consider to reach orbit with a spaceplane is the flight profile.  In general, you want to:

• Get supersonic as quickly as possible.  (~350 m/s or better)
• Gain as much speed as you can in air-breathing mode.
• Try not to overheat your nose parts.  

[gilflo]

Thank you for answers. This TSTO give me 2500 DV left, once 80 km orbit is reached……i don.t knowhow to  get so many DV left with only 1 Rapier

[magnemoe]

1 hour ago, 18Watt said:

As @magnemoe said, that's too many engines for a plane that size.  Two Rapiers would be overkill, a single Rapier would probably work.  With the weight of all those Rapiers and the fuel for them, you may need more wing area too.  Although losing most of the Rapiers will reduce the need for more wing area.

I believe in the latest version of KSP, one Shock-Cone intake will still feed up to 3.5 Rapiers.  I personally would use a single intake for two Rapiers, unless the 'looks' of it bothers you.  (Which is fine!)

One of the most important things to consider to reach orbit with a spaceplane is the flight profile.  In general, you want to:

• Get supersonic as quickly as possible.  (~350 m/s or better)
• Gain as much speed as you can in air-breathing mode.
• Try not to overheat your nose parts.  

Yes my standard space plane is a 1.25 meter base with nose, cockpit and 3 passenger cabins modules and an rear port who might be replaced with an nosecone if not needed. 
An small wing segment and an  shock cone, fuel then an equipment bay, part storage and the raptor, wings outside this. Kind of an SR-71 with longer engine pods. 
Main benefit is that if I put landing wheels on the pods and an extra set of wheels at the rear who is only used for landing I have something who can land like an bush plane. 

1 minute ago, gilflo said:

Thank you for answers. This TSTO give me 2500 DV left, once 80 km orbit is reached……i don.t knowhow to  get so many DV left with only 1 Rapier

Now that might be the difference. I use space planes as an cheap (fully reusable) way to get into low orbit to deliver or return / rescue kerbals or bring up parts, Yes I has taken them to Laythe but then on docked at the  front of an base. I tend to have 5-600 m/s in leo who is more than enough to dock and option to do an secondary objective like rescue an stranded one and return. 

One option is to dump the wings and raptors then out of the atmosphere and use an LV-N, drop the oxidizer tanks with the raptors but keep the fuel ones except you might want landing engines for landing on the belly. 

[miklkit]

Is your objective to build Rapier only spaceplanes?  That is pretty inefficient once in orbit.  NERVs are way better once you get up there.

[jimmymcgoochie]

You have too many engines, only one or two are necessary for a plane that size. Get rid of the wingtip pods as they just add mass and drag. The wings are also very strange- the canards in the middle will do almost nothing and the wing area is too small for a plane that heavy, add more wings near the rear of the plane and shift the canards to the front.

Rapiers are very good as an air-breathing engine as they can produce more thrust the faster you go, however their efficiency in rocket mode isn’t that good and a vacuum-optimised engine like the Terrier or even a NERV could give you better performance once out of the atmosphere. It’s really hard to make an SSTO that can then go somewhere else once in orbit

[KerikBalm]

13 hours ago, miklkit said:

Is your objective to build Rapier only spaceplanes?  That is pretty inefficient once in orbit.  NERVs are way better once you get up there.

Indeed, rapiers are terrible once in orbit.

Bad TWR (high dry mass)

Bad I_sp

Terrible

Going to oribt? for only 0.2 tons more than a Whiplash, you get 180kn of closed cyclethrust. Plus you get higher and faster so that you don't need as much closed cycle dV. If going for maximum SSTO payload fraction to orbit, that's hard to beat. Sure, other engines have higher closed cycle I_sp, but how many do you need to supply enough thrust? how much dry mass do they add? what does that dry mass do to your dV?

This is all fine and good when you're talking <1,000 m/s of closed cycle dV from a single stage. Going beyond that - forget it. I have almost always left the rapiers in LKO or suborbital (exception: going to laythe).

Get to orbit, decouple a spacecraft from the deadweight of the rapiers, intakes, wings, etc (ideally in the form of a very light plane that you can recover)- go where you want- possibly coming back to redock with your wings and airbreathing engines to land back on kerbin.

I find the easiest way to do 100% reusable to Mun-surface and back is to take a multi-stage approach: SSTO to LKO, undock the transfer stage and lander>go to low mun orbit> detach lander, land, redock in low Mun orbit> go back to LKO (using aerobraking)> dock with the SSTO plane> land

[miklkit]

Too complicated.  If you are going to do all that, then just use rockets. 

You are using too many Rapiers anyway.  You could put a MK3 into orbit with 6 Rapiers.

[gilflo]

Well guys, thank you for all your answers. in fact you're right! too many Rapiers

I just work on a new plane which is a TSTM: Two Stages To Run, The first stage is an atmospheric stage that feed the 3 Rapiers, Then when I switch to closed cycle, the atmospheric stage is decouple and one in orbit I got 2700-2800 DV left with the Space Plane, much more lighter an much more endurance!

But I am working on a version with 2 Terriers giving same DV once in orbit and only 21T at take off. The atmospheric stage is 2 Rapiers with 200 kg of LF

 

[magnemoe]

It my go to SSTO, this was designed for doing science on Laythe, the parashutes because I was not sure how easy it was to land there, no problem so they got dropped. 
Rear docking port as it was docked to an Laythe base and its transfer stage for the trip. 
Yes wing area is large, this is an bush plane, this is also the reason for the rear landing legs, they are used for landing and taxing. They are raised for takeoff but they still help against engine strikes. You also only need one shock cone but like the SR-71 like look. 

Use an simplified version as an general SSTO on Kerbin for LEO rescue and smaller crew transfer.
Note then using cargo or equipment bays, as I understand stuff not on the two internal center nodes create drag, so make an pillar here and attach stuff to pillar not to the surface of bay. 
Some correct me if wrong here but saw much better performance then changing this.