Back to front rockets

[AeroGav]

Up till now, I've built rockets that look like real ones **cough, asparagus staging, cough** 

ok, well, the top bit looked conventional anyway.

You'd have an upper stage that looks like this  -

Parachute

Capsule  

Heat Shield

Separator

Fuel Tank

Upper Stage Engine

 

However, I ran into a problem trying to fulfill the contract to launch a load of space tourists on sub-orbital trajectories.   

Firstly,  if you economise on the rocket part and just go straight up without adding much horizontal velocity,  you re-enter at a very steep angle and smack into the ground before slowing down enough to get the chute out.   Secondly, if you do re-enter from an orbital trajectory, adding a mk1 crew cabin below the capsule can muck up the aerodynamics so it no longer enters heatshield first, and burns up.

Now, as someone who built spaceplanes in KSP long before he managed to figure out rocket flights, the solution sounds obvious.    Put some steerable fins on the re-entry vehicle so that

a) in sub-orbital flights, you can angle the thing away from prograde once you've slowed down enough to not burn up, that way you can get the chutes out in time.

b) keep your heatshield facing the right way before that point.

However, since the heatshield is at the bottom of the re-entry vehicle this necessitates putting the fins at the top, and since the re-entry vehicle sits at the very top of the rocket stack,  it means you have one set of fins at the very tip of the rocket, which makes it aerodynamically unstable.

The solution was to reorganise the upper stage so the fins can be attached nearer the stack's CG, and to re-enter nose first.

New layout -

 

Nose Cone

Fuel Tank

Fuel Tank

Inline cockpit

Mk 1 Crew Cabin  

Upper Stage Engine

Since it is the final propulsive stage, there is no additional delta v to be gained by separating the upper stage's fuel tanks and engine from the vehicle, unlike with spent lower stages.  If you like living dangerously, you can in fact skip the heatshield altogether and watch as the nose cone, then one fuel tank after another overheat and explode.   BOOM .....  red bar..   BOOM !  red bar....      will you run out of disposable parts before completing re-entry?   However,  i figured out the recovery value of the tanks is more than the cost of a heat shield, so i eventually added one between the nose cone and first tank.

Also, the fuel tanks actually give body lift which is helpful with arresting the sub orbital trajectory plunge and gives a little cross-range capability.  You can still do sub-staging, by putting stack separators between tanks, to jettison empty tanks/useless aero nose cones, once out of the atmo.

A picture is worth a thousand words, so..

 

Why aren't real ships built this way?  Well for a start, I suppose it means propellant lines running through crew cabins, with possibility of leaks etc.   Kerbals complain that monopropellant makes their snacks taste funny.

[steuben]

leaving aside the structural issue of two tanks of mass pushing down on the capsule at whatever g's at launch as well. I think it makes the control system math more interesting as well. I'd have to stare at a blackboard for a while to be sure. And more interesting when dealing with this amount of energy is a Bad Thing^TM

As for an in game reason why not. none at all. I built early mun/minus landers that way, before I unlocked ladders.

[RizzoTheRat]

Personally I'd rearrange the fins a bit on that...

[Snark]

As you've surmised:  in real life, there are oodles of difficult engineering considerations when building rockets.  It's not like KSP, where there aren't any actual micro-level engineering problems at all, just macro-level problems like overall ship mass, rigidity, etc.  In KSP you can just slap pieces together like Lego and they work every time with 100% reliability.  Real-world rockets have to deal with lots more complications.

For example:  Real-world rocket fuel is typically cryogenic, i.e. liquid H₂ and O₂ that are chilled hundreds of degrees below room temperature.  Keeping them cold is a huge problem, because typically the way one keeps things super-cold is to slather lots of insulation on, and that's something that's hard to do on a rocket where space and mass are at a premium.  That's why rockets typically stand empty on the launchpad for days until liftoff is near, and they refuel them at the last minute-- because every second the fuel is actually on the rocket, it's boiling away and you're losing fuel even if you're not running the engine.  And the worst possible geometry, if you're trying to limit heat flow, is a long skinny fuel line.  You want big spherical tanks (or, at least, as close to a sphere as you can have, given the constraints of the rocket's shape).  You want to keep the length of fuel lines to an absolute minimum, meaning keep the tanks close to the engines.

(Yes, there are cases of non-cryogenic rocket fuels, such as the Saturn V's first stage, which IIRC used kerosene.  But even there, you need an oxidizer.)

Then there's the question of running fuel lines through a decoupler.  In KSP, decouplers are magic, you just hit space bar and they fire perfectly every time, and never misfire, or get stuck, or blow up any other nearby pieces, or anything.  In real life, they're hard.  It's easy for things to go wrong.  Yes, it's a conceptually simple piece of equipment, but there are so many ways for them to go wrong, either due to human error during assembly or just engineering-tolerance problems.  Many spacecraft have been lost or severely damaged because something went wrong with a decoupler.  You want to keep the physical arrangement around the decoupler as simple as humanly possible, to minimize the number of moving parts in case something goes wrong.  Which means that you really, really don't want to be running fuel lines or anything else through them.

...And so on, and so forth.  The fact remains, KSP is a game, it's why you can play it without needing a PhD in aeronautical engineering, it's why NASA doesn't let 12-year-olds design their rockets. 

By the way-- my own preferred solution to the "put tourists on a reentry vehicle" is to just build a lawn dart with a Mk1 command pod on the front, as many Mk1 crew cabins strung behind it as necessary, and some AV-R8 winglets on the back for steering.  Bring it in prograde, up-angled like a spaceplane.  Gets tons of body lift, comes in gentle as a feather by the time it gets within a few kilometers of the ground.  If I have to come in much faster than LKO speeds (e.g. from Minmus or somewhere), stick a jettisonable heat shield on top with a nosecone, for getting through the hot part of reentry.

Edited January 27, 2016 by Snark