Scramjets or me?

[Joltout]

(This is in career mode)

So I decided to finally tackle trying SSTO's out instead of regular old rockets. Spent all day building them, flying them and for the most part enjoying myself. Until I tried getting them to space.
I recently accepted a mission to grab a 2.5m/2.5m/1.5m object in low Kerbin orbit and bring it back to the surface. Not a problem. Except every design I make for an SSTO i can't get above 14kms

The highest/most advanced SSTO engine I have unlocked is the Ramjet (Whiplash) and despite watching tutorials and builds all day long to try and figure out whats wrong, I just can't get my finger on it. It doesn't seem to be weight distribution, doesn't seem to be a shifting COM problem, aerodynamics seem perfectly fine, yet the plane can't go above 350ish m/s and above 10kms alt becomes impossible to control if I so much as touch WASDQE. So what am I doing wrong?
 

Dropbox link for the .craft file (tell me if it doesn't work): https://www.dropbox.com/s/7mf1gvq6jntsf4r/Whitebird EXP.craft?dl=0

Edited February 16, 2017 by Joltout

[Reactordrone]

Looks like you're running into a drag problem. Get the monopropellant containers inside the cargo bay, swap the thruster quads for place anywhere thrusters an get intakes on the front of those pre-coolers. With those modifications I nearly got into orbit with it. May need a touch more oxidiser.

Edited February 16, 2017 by Reactordrone

[Joltout]

10 minutes ago, Reactordrone said:

Looks like you're running into a drag problem. Get the monopropellant containers inside the cargo bay, swap the thruster quads for place anywhere thrusters an get intakes on the front of those pre-coolers. With those modifications I nearly got into orbit with it. May need a touch more oxidiser.

Thanks for the help!
I left the mono-tanks outside because I wasn't exactly sure how much room I had for the part I was going to be grabbing. After testing out the pre-coolers as well just now, I thought they had to be openly exposed from the front to gain as much intake air as possible. As for the quad thrusters, i didn't know they caused drag actually. I thought the smaller parts didn't cause drag, just added weight to the object they were connected too O_o.

Not sure where to put more oxidiser tho, but i think it's safe to that assume weight isn't the problem?

[Birdco_Space]

Drop the intercoolers and put a shock cones instead.  The flat surface of the intercooler is really draggy.  this may also be impacting your controllability.

[foamyesque]

Major issues:

1. No caps on the precoolers. I'd swap them for ramp or shock intakes, or, at least, put a nose cone on the precooler.

2. Nothing inside the cargobay is shielded from flow because of how you've structured the attachments. Detach the -8 cargo bay, detach the claw-tug's very back docking port, directly attach the cargo bay to the cockpit, then attach the tug's docking port to the cargo bay.

3. You have a lot of stuff hanging off the outside of the craft. The fuel lines are unneeded, the drogue chutes are unneeded, you have a lot of RCS units that could do with some trimming, and the RCS tankage may be able to be moved inside a cargobay or moved into stack tanks and put at the head of the whiplash nacelles.

[Joltout]

23 minutes ago, foamyesque said:

Major issues:

1. No caps on the precoolers. I'd swap them for ramp or shock intakes, or, at least, put a nose cone on the precooler.

2. Nothing inside the cargobay is shielded from flow because of how you've structured the attachments. Detach the -8 cargo bay, detach the claw-tug's very back docking port, directly attach the cargo bay to the cockpit, then attach the tug's docking port to the cargo bay.

3. You have a lot of stuff hanging off the outside of the craft. The fuel lines are unneeded, the drogue chutes are unneeded, you have a lot of RCS units that could do with some trimming, and the RCS tankage may be able to be moved inside a cargobay or moved into stack tanks and put at the head of the whiplash nacelles.

I'm not sure what it was on this list that did it, but you fixed it and made me an SSTO pilot, thanks! I did everything you suggested and I'm willing to bet that the pre-coolers drag + whatever magic I did with the cargo bay in the first place was the cause of my problems.

Edited February 16, 2017 by Joltout

[AeroGav]

I actually like precoolers, they are slightly lower drag than the shock cone and adjustable ramp intake.   The shock cone can feed 2 rapiers of course, but it's not hard to find room for 2 coolers on a ship.  They are also a part you can stick at the front ahead of the cockpit to keep it further back from re-entry heating, or to form an engine nacelle without adding too much fuel mass ahead of cg.  Just put nose cones on the front !

1 hour ago, Reactordrone said:

 Get the monopropellant containers inside the cargo bay, swap the thruster quads for place anywhere thrusters

I'd probably go further and put the RCS ports themselves inside the cargo bay.    Below 70km, you do not need to translate up/down/left or right.    You can point the nose with aerodynamic controls, thrust vectoring, and reaction wheels.  Once out of the atmosphere feel free to open the cargo bay, this will allow the RCS to work and you can translate up/down for docking.

Swap that pointy cockpit for an inline.   During re-entry, it's going to be very prone to overheating.   During launch, if you're flying an efficient profile and getting the most possible speed out of airbreathers, it might also overheat.

Finally, two swivels might be too much rocket engine for a ship this size.  It will work but won't break any payload to orbit or delta v records.

One would be enough, but actually, as you have the Whiplash, you also have the Dart Aerospike.   I'd pick a single Dart, it has a much better vacuum ISP.  Put a reaction wheel in the cargo bay to make up for the lack of gimbal. 

[bewing]

3 hours ago, Joltout said:

 After testing out the pre-coolers as well just now, I thought they had to be openly exposed from the front to gain as much intake air as possible.

Just FYI -- the precoolers get their air entirely from the sides of the unit. They do not take in any air at all from the front. They are sort of like radial air intakes.

[foamyesque]

7 hours ago, AeroGav said:

I actually like precoolers, they are slightly lower drag than the shock cone and adjustable ramp intake.   The shock cone can feed 2 rapiers of course, but it's not hard to find room for 2 coolers on a ship.

Swap that pointy cockpit for an inline.   During re-entry, it's going to be very prone to overheating.   During launch, if you're flying an efficient profile and getting the most possible speed out of airbreathers, it might also overheat.

Eh.... for one, a single shock cone can power six RAPIERs, not just two, and secondly precooler + nosecap doesn't really have much drag advantage and is very bulky compared to a single shock intake. The smidgen of extra fuel it carries is functionally irrelevant. The only real advantage it has is a superior static draw, but that only really matters if you're driving a pile of airbreathers at speeds under 5m/s.

The Mk 2 nose cockpit's thermal tolerances are perfectly adequate for a Whiplash launch and if you manage to blow it up on reentry, it's because you screwed up the profile, not because the part can't handle it. The plane already has a rear-ward CoM shift, so I don't think adding more fuel to the nose is a good plan. Cutting one of the Swivels might help with that.

[AeroGav]

2 hours ago, foamyesque said:

Eh.... for one, a single shock cone can power six RAPIERs, not just two, and secondly precooler + nosecap doesn't really have much drag advantage and is very bulky compared to a single shock intake. The smidgen of extra fuel it carries is functionally irrelevant. The only real advantage it has is a superior static draw, but that only really matters if you're driving a pile of airbreathers at speeds under 5m/s.

Understood, on a drag per rapier fed basis the Shock cone is superior,  but i don't really do designs with one shock cone on the nose feeding multiple rapiers at the back or on nacelles,  i just don't like the look much.

I normally go with inline cockpits and want something to put in front that keeps the cockpit far back from the heat.  On a mk1, engine pre-coolers fit the bill nicely.   I can't use a fuel tank because there is already usually more fuel ahead of CoM than behind it, due to the way engine mass tends towards the rear which makes the CoM aft of the halfway point on your fuselage.

(pre-cooler ahead of cockpit here)

For mk2 designs,  i often put my nervs out on nacelles.  I want the NERVs near CoM, but that means if i use a fuel tank as the root part for the nacelle i've got fuel mass ahead of CG again, with the resultant full/empty CoM shifting.   Pre-cooler makes a nice convenient root.

  

(pre cooler engine nacelles)

[foamyesque]

@AeroGav:

I usually avoid NERVs, to be honest; RAPIERs in rocket mode are perfectly adequate for the standard spaceplane use case and involve less design complexity in general (the biggest one being tuning the oxidizer amount).

Honestly, you're too paranoid about heat; two parts is ample to keep the skin heat down enough so that it explodes only after your nosecone would've anyway, and the thermal soak isn't a problem unless you hang around at 15km trying to squeeze every last m/s out of things. If you just climb with reasonable authority instead, sure, you're not quite as efficient in your fuel use but it's much less of a hassle to design and fly; the extra tankage is pretty easy to squeeze in, since you're not TWR-limited the way rockets are.

Having your cockpit forward also assists in counterbalancing engine masses, which further helps with simplifying design.

 

 

[Andetch]

I have an SSTO that can handle re entry on a MkI pointy cockpit..... It is just a case of slowing down onto the correct sub orbital trajectory and keeping the nose gently moving around the prograde. Do not try aerobraking with it.

Admittedly when I jump the pilot out of the cockpit for some freefallin' they are glowing (you can drop kerbals into the sea quite safely from around 20,000 metres if you're subsonic, just sometimes I occasionally do if I miss the KSC on landing, purely for amusement)..... But SSTO is a lot about your piloting skills!

Also, why everyone make everything so big?

Edited February 16, 2017 by Andetch
Forgot to add soemthing.

[foamyesque]

2 hours ago, Andetch said:

Also, why everyone make everything so big?

Nothing in this thread is big, IMO.

 

You want big? Try this:

 

[Andetch]

Yeah that is quite a lot bigger than anything in this thread, or that I have made to fly. I tend to use a plane made from less than 20 parts, maybe 15... I haven't counted. KISS principles. However, I must admit my planes are useless for things other than LKO and air breathing speed runs!

[Wanderfound]

15 minutes ago, Andetch said:

Yeah that is quite a lot bigger than anything in this thread, or that I have made to fly. I tend to use a plane made from less than 20 parts, maybe 15... I haven't counted. KISS principles. However, I must admit my planes are useless for things other than LKO and air breathing speed runs!

You don't need to go big to go far, BTW.

...and the big 'uns are most useful for bulk delivery to LKO, anyway. Space-station construction and fuel tankers are where MkIII spaceplanes really shine.

 

 

[Andetch]

10 minutes ago, Wanderfound said:

You don't need to go big to go far, BTW.

I have already noted this. I have a design that goes to Jool and nearly returns to Kerbin for a flyby, and the main vessel is only about 10 parts, four of which are solar panels, 1 is a battery.....1 fuel tank, 1 mkI capsule, 1 engine, 1 parachute and 1 antennae. Sometimes I add small science items.

It was a major headf**k getting over the fact that more fuel and power doesn't mean going further!