How to improve SSTO/TSTO Refueler

[GuyWithGlasses]

Hi fellows,

Ive been working around the construction of a SSTO/TSTO Rocket to refuel my Kerbin Space Station (150x220Km aprox).

My requirements to the deliver to the KSS:

- 750 of monop

- At least one complete 7200 kerbodine tank, preferably two.

- Enough PL + oxygen + monop to land at KSC.

 

This pic: https://imgur.com/RoMf2QX is the evolution of an older SSTO rocket I built and Ive added more kerbodyne tanks, the SRBs and another 750 monop tank..  it can deliver what Im looking for, but I want to upgrade it, but how? I

- I dont have rapiers, so I dont know if I using air-breathe engines will be a good solution..

- Shall I add two nuclear engines for the space migration to the station fron LKO?

- I dont know I can improve de drag-related lossess...

 

Any Tip?

Regards.

 

Pd:

The parts: 1 shield clamprotron + three 1k batteries + LC 101 core probe + adapter + 7 kerbodyne 7200 tanks + 1 mammoth engine  + 2x 750 monop tanks + 16 aerobrakes + 4 solar panel + 12 RCS thrusters + 1 antenna + 8 SRBs + 4 radial chutes + 2 big control wheel

Edited October 15, 2017 by GuyWithGlasses
ImproBe!! My eyes!!

[Rocket In My Pocket]

That seems like an insane amount of Monoprop to me...do you really need so much?

[Foxster]

I agree that this maybe needs a lateral approach where the need for all the monoprop is removed. 

Personally I never use the stuff - well, except for a little bit for docking and I avoid that wherever at all possible. 

[luizopiloto]

I do refueling missions using a general purpose SSTO I have... Depending of the craft I takes like 5 travels to fully refuel it... But the SSTO is so practical, it can be done without much effort...

[bewing]

It's simply a matter of learning how to dock and land with less. Replace all the airbrakes with two little wings. Get rid of all the RCS thrusters on the ship (except for a couple of tiny retrothrusters) -- use the reaction wheel to dock and be patient. Get rid of the extra reaction wheel. Get rid of 3 of the batteries. Replace all the gigantors with 2 Ox-Stats. Replace the shielded docking port with an advanced nosecone on top of a standard clamp-o-tron.

Edited October 13, 2017 by bewing

[Aegolius13]

It would help a bit to see the bottom of the ship - I assume there's a Mammoth at the bottom?  And I assume by TSTO, do you mean you discard the SRBs but the rest is reusable?  

As far as engines go, in my experience the Mammoth and Vector are your best bests fot this kind of thing.  The real bottleneck on your thrust is at liftoff.  As you get higher your TWR increases as fuel burns; your ISP gets better; drag gets lower; and as you get closer to orbit gravity losses go down.  All those things mean you want to use an engine with good TWR and ISP at sea level.  The Mammoth and Vector are tops in that regard (and the Mammoth has a small TWR advantage over the Vector).    

As far as  airbreathing engines - I'm skeptical for this application.  Yeah, they will add some fuel-efficient thrust at low altitude (which as mentioned above is the bottleneck), but once they cut out they're dead weight.  Also, I have not had great luck getting good thrust output on rockets with jet engines.  Since they climb more quickly than planes, the decrease in air pressure tends to counteract the increase in speed, meaning the Whiplash, Rapier, etc. will have trouble getting into their high-thrust butter zones.  Thus, their TWR just does not compete with the likes of the Mammoth.  As a general rule, I only use airbreathers when I'm doing something plane-like, or if I really, really need to cut out mass for a challenge. 

I have not used nukes very much on SSTO-type rockets, but again I'm skeptical.  Nukes are dead weight at low elevations, when your TWR need is the highest.  Once you get high enough and switch to the nukes, your chemical engines (which had to be big and heavy enough to lift those heavy nukes) are now dead weight.  And your total vacuum delta-v need is not that high; nukes tend to do better when you need many thousands of m/s out of a stage.  Finally, due to their form factor, nukes (and their LF-only tanks, if your're playing stock) might be a pain to attach to your design. 

TLDR: you're probably using the right engine already.  If you need more delta-v, your choices are probably lose unnecessary dry mass, as @bewing says, or make your rocket even bigger so that a larger percentage of your total mass is fuel.   A Falcon Heavy-looking thing with three stacks of tanks and Mammoths might work.  You can also use heftier boosters (TwinBoars seem like an option at this scale), but that's getting further away from your reusable concept.

 

 

 

[Spricigo]

As other said, reduce dead-weight.

I agree with Aegolius, the Mammoth is the only real option for the main engine of a Lifter Rocket of this size. Jets would require an impossible flight trajectory and nuclear engine are just not suited for high thrust aplications.

The core stack can be helped by a pair of Twinboars or Vectors but is at least dubious the weight will not overcome the benefits in the way to orbit. If going to make boosters disposable kickbacks can offer much more bang for your bucks, they can also be combined with 1,25m drop tanks to shave a bit mass from the core stage.

[GuyWithGlasses]

Hello everybody,

I've been working around your tips.

 

First of all, the monop. I carry that innsane amount of monop because I want mechjeb do all the job, it consumes around 150 of monop for docking and all it can spend in landing (to land the nearest posible of KSC). And my station have 4 ships that needs monop too xD

Of course, in future missions Ill cut off the amount of monop to save PL + LO (or change the tank for other things; cargo bays, hichjaker..).

I've working in cutting dead weight;

- I've installed 3 x SP-L 1x6 Photovoltaic Panels instead gigantor panels,  and generates all the energy the ship consumes, perfect (I thoguht only the gigantor panel had the hability to retract).

- I've removed one reaction wheel, and the ship has a good ascent profile (testing with mechjeb), it is slow in the maneoveurs, but I think It'll dock well, so another thing done.

- I've tested differents noses, but I don't know witch has lower drag, two of the test: https://i.imgur.com/2jrdyse.png   https://i.imgur.com/rfuJc2n.png  

- I've just been testing installing fins instead of the airbrakes; they have lower mass and gives the ship to hold retrogade in the landing maneouveur, but when the ship is around 55k (landing), the engine  explodes (I think for overheat or overstress, I dont know).

So I think I'll test adding 4 airbrakes to see what happens... and after, modify the RCS system..

 

Best regards.

 

 

 

[Spricigo]

A few comment:

8 minutes ago, GuyWithGlasses said:

- I've tested differents noses, but I don't know witch has lower drag

As a rule of thumbs it's the more 'pointy' one. I suggest to instead of usign the adapter just put everything that don't need to be ascessible inside a fairing with staging disabled. On the other hand the little extra drag of a shielded docking port will not make a dent on the performance of this rocket.

 

16 minutes ago, GuyWithGlasses said:

but when the ship is around 55k (landing), the engine  explodes (I think for overheat or overstress, I dont know).

It's possible that it can be solved with a different reentry trajectory, either coming higher to slow a bit before facing the worse of the heat, or just diving into the lower atmosphere to slowdown before you heat get you. In any case this is a heavy ship that certainly will not accept any trajectory.

21 minutes ago, GuyWithGlasses said:

I carry that innsane amount of monop because I want mechjeb do all the job, it consumes around 150 of monop for docking and all it can spend in landing

Mj consume a lot of MP to dock because you designed the vessel to use MP to dock. (You need [advanced tweakables] enabled in setting)  remove all the RCS thruster blocks and use either PlaceAnywhere RCS Thruster or  Vernors(For a rocket this size probably the better option) to provide translation only (in the VAB right click on  RCS/Vernors, [show activation toggles], turn off [pitch],[yaw] and [roll])

As for the landing part, you dont need any RCS, make the rocket stable and able to survive reentry an you will have a defined place where you do a deffined reentry burn and it will land at KSC grounds each time.

[Spricigo]

 

At some time this thread made me think: I may design something along those lines. 

Spoiler

I prefer much smaller rockets but sometimes a change of pace is interesting.

 

[Rocket In My Pocket]

17 hours ago, GuyWithGlasses said:

because I want mechjeb do all the job

I think I spotted your issue.

You should learn to walk before you learn to run, and you should definitely learn to do both before you try to program a robot to carry you on it's back while it walks and runs.

You could trim off a lot of dead weight if you pilot the ship yourself.

[Spricigo]

2 hours ago, Rocket In My Pocket said:

I think I spotted your issue.

You should learn to walk before you learn to run, and you should definitely learn to do both before you try to program a robot to carry you on it's back while it walks and runs.

You could trim off a lot of dead weight if you pilot the ship yourself.

Well, the robot in question have a reasonable built-in program that may be smart enough to carry you.

In any case, I agree that without some piloting knowledge/experience  is easy to overlook when the robot is struggling to cope with a flawed design.

And the walking analogy fits perfectly: KSP's experienced pilots started, like anyone else, crawling from the launchpad. nothing impressive but worked. The improvement to a walk was accompanied with the struggle to stand. Now they are in a point of this process where the falls much less likely to happen, they run and make it looks easy.