Recoverable Booster Launch Profile

[AeroGav]

Taking a break from all my spaceplanes, I'm trying my hand at making a stock, recoverable two stage rocket.

The booster stage has wings and will glide back to the space centre, before landing on parachutes.  

The upper stage also has stub wings (mainly to help it aim and slow down) and parachutes.

The goal is to come up with something you can put on KerbalX and have the average joe just be able to fly it, without having to install mods or tweak physics range etc.

As you can imagine, physics range is the bugbear.    I keep finding one or other part of my rocket has gotten auto deleted before I can deal with the other.    Each time  i  want to tweak separation altitude/ascent profile, means a complete redesign, moving propellant from one stage to the other then having to rebalance the aerodynamics so each component is stable on its own.      If you guys can help with that, it will save a lot of time building iterations of this rocket.

Attempt 1

My first iteration only had about 1000dv on the lower stage, but with 8 aerospikes the lower stage was pretty powerful (TWR 2.75) and launched steeply.       Separation altitude was 20km and due to the steep ascent, the first stage didn't have a long glide to get back to the space centre.    The upper stage had about 1900dv but only 0.5twr and i think i should have boosted that, in retrospect.

The problem is, these engines only run for as long as i'm flying that vessel.    They do take the upper stage's AP out of the atmosphere, but as i understand it, I have to switch back to the lower stage before it gets more than 2.5km away.   

EDIT - ok, it appears the non-focused stage is ok so long as it stays above 25km OR is within 2.5km of the flown vessel.

When i switch to the lower stage, the upper stage's engines immediately stop and it is still well below orbital velocity.   It completes the sub-orbital arc and falls below 25km and gets deleted before i can land the lower stage.

 

Attempt 2

I tried adding extra fuel to lower stage and removing from upper.     The lower stage now has 1700dv.  Separation was at 950m/s at 35km in a 65 degree climb.  The booster reached 65km before falling back.    I still wasn't able to put enough velocity on the upper stage before having to switch back to the booster, and it fell back below 25km again just as the booster was opening its parachutes.

 

I don't like adding more fuel to the booster, it means either a very inefficient lofted trajectory or being too far from the space centre to glide back at separation.

So,  it seems i need to give the upper stage some serious TWR so it can get close to orbital velocity much quicker.    

BTW, what happens if the upper stage is an SRB?  Does that magically extinguish itself when i switch away from the vessel ?   Just thinking out loud.

[Human Person]

15 minutes ago, AeroGav said:

As you can imagine, physics range is the bugbear.    I keep finding one or other part of my rocket has gotten auto deleted before I can deal with the other

If you want to avoid this, you have to do stage sep in space. You could choose a very high trajectory, so the second stage can circularize before the first stage enters the atmosphere again.

For stock KSP, I'd recommend building an SSTO Rocket without wings. Simply put some airbrakes and some parachutes on and build the stage with a little bit of extra delta v. 

Edited September 30, 2017 by Physics Student

[Palaceviking]

P It's possible but,

1:first stage must make at least 75k apoapsis 

2: burn back must start over 21 k

3: 2nd stage needs at least TWR < 0.8

4: the burn back and landing needs to be fast!

[Spricigo]

7 hours ago, AeroGav said:

The goal is to come up with something you can put on KerbalX and have the average joe just be able to fly it, without having to install mods or tweak physics range etc.

Dunno if Mr Average have anything  serious against Stage Rocovery or FMRS. OTOH too complicated (hard to follow) flight profile may easily turn him away.

As you probably noticed by now is a question of time.  Increasing the time ther lower stage stay out of the 'despawn zone' and decreasing the time the  upper stage need to get up to orbital velocity. 

[MajorMushroom]

This is exactly what I wanted to do, except without all the bother of exact precision. 

Good luck! 

[Spricigo]

36 minutes ago, MajorMushroom said:

This is exactly what I wanted to do, except without all the bother of exact precision. 

Good luck! 

You'll need a lot less precision if you go with a SSTO (rocket or plane)  since you don't need to deal with all the switching shenanigans. Also mods, if your ethos allow it.

[bewing]

There is no efficient way to do it. You either have to OP the first or second stage.

The way I do it is to launch straight up, with a triplet of kickbacks. The kickbacks get the upper stage to about a 250km Ap, and then fall straight back down. After 1st stage burnout, the upper stage has just enough time to circularize -- and then you switch back to the kickback triplet just as they hit atmosphere.

The kickback triplet is built with just enough aerodynamics so that it "flies" with body lift. Then you can either try to land it very very gently in the water, or you use a pair of landing gear and some parachutes over land. But since it is coming straight back down over the KSC in the first place, you don't have to fly it far.

 

[Helmetman]

I made a few iterations of SpaceX styles recoverable rockets myself. Like Bewing says, there is no way without OP'ing any of both stages.

I generally make a TSTO recoverable SpaceX style rocket by having decent TWR on the 2nd stage. With the first stage it's not about efficiency but by launching that into a suborbital trajectory where the 2nd stage has a greater amount of seconds to Apoapsis then the time it takes to land the 1st stage once passing 70km.

The 2nd stage still needs to be under Apo or there at during the time you switch vessels once you landed the 1st stage. So you'll have to sacrifice efficiency for that.
Basically, the best thing one could do is make the most efficient 1st stage concerning a predefined payload for the 2nd stage that is able to land as quikly as possible so you'll have spare seconds to Apoapsis on the 2nd stage.

Usually I aim for 80km on the 1st stage so I have time to accelerate the 2nd stage to greater apo (hundred(s) km'(s) before the 1st stage drops below 70km at which time I switch to it.

However, how this is executed depends per player. So if your goal is a foolproof method of a 2 stage recoverable rocket it kinda depends on how foolproof the end user is. The end user that picks this up from KerbalX must be educated on the physics rules as to actually know how to fly a recoverable rocket in KSP. People aren't educated at that by default since it's a ludicrous system and you'll only know it if you've played the game for some time.

So good luck building a fool proof recoverable space X style rocket for newbs, they still need to know the physics range rules. At best you can properly verify them to this fact in the description on KerbalX so they don't bury their hands in the sand while launching it. 

 

Edited September 30, 2017 by Helmetman