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Hey all, this was an idea for a career I had, based on the idea of OTRAG, but first, I will probably need to explain what it is/was, for those who don't know about it.

What is/was OTRAG?

OTRAG was one of the earliest private launch vehicle companies, operating in the 1970s and 1980s, but was never successful. Their idea was to have a tall, slender (16m x 0.27m), and mass-produced Common Rocket Propulsion Unit (CRPU) that they would be able to bolt several together in order to achieve any payload to Low Earth Orbit, staging them as needed; for example, a sounding rocket would use around 4, while an orbital launch vehicle would use several dozen or hundred. The CRPU was supposed to be extremely simple, with a pressure-fed engine, no gimbal (steered via differential throttle), and an ablative nozzle, ending up with a quite low efficiency for the engine (around 297s), but this was countered by the intended-to-be cheap cost of production (due to economy of scale), resulting in what's effectively a cheap  but big launch vehicle that looks like a "bundle of drinking straws", as Scott Manley described the designs in a recent video.

The Basic Challenge

  1. After one launch in career mode (since this idea doesn't work too well with just Fleas), design a Common Rocket Propulsion Unit, with fixed amount of fuel, a certain profile, and an engine , and all further rockets must only use that design.
    • For example, 720 units of fuel [4 FL-T400 tanks], a 1.25m profile, and a swivel would work as a CRPU.
    • Also, you can't modify any single CRPU's basic design, even if it's just something like offsetting the engine to allow for landing gear clearance.
  2. There is no limit to how many CRPUs you can use on a rocket (and you can detach them as needed), only the limits enforced by the game itself (mass/part limits).
  3. The tanks on the CRPU may be replaced with longer ones later, but the total amount of fuel and the tank diameter may not be changed.
    • For example, you can switch from 4 1.25m FL-T400 tanks to 2 1.25m FL-T800 tanks on the CRPU for part count reasons, but you can't switch to a 2.5m X200-16 tank. 
  4. You are allowed to change the amount of fuel inside the CRPUs in the VAB, but any empty tanks are not allowed to be removed before launch.
    • For example, for the previously described CRPU, you could lower the fuel down to 180 units, but you can't remove the empty FL-T800 tanks or FL-T400 tanks, since that would require a different assembly line, making the CRPU idea worthless.
  5. Since differential throttling is not super practical in stock KSP, gimbaled engines are recommended, and since monopropellant is not great at propulsion, I'm allowing usage of it for things like docking.
  6. Of course, cheating of any amount is not allowed, unless it's justified (such as in a Kraken attack).
    • Try not to clip non-structural parts (mainly fuel tanks and engines) through each other on CRPUs unless it can be rationalized.
  7. The end-goal is to finish the tech tree. 

Some Further Ideas

I recognize the fact that this idea is extremely narrow, so I've layed out a couple more ideas below that might make the career more fun and less grindy. This is the rough idea for a challenge after all. This challenge is supposed to be in the same vein as the Caveman challenge in that it's highly customizable in difficulty, while still retaining the base idea behind it all.

  • Use 2 CRPUs instead of one throughout the career (presumably a taller one for less efficient lower stages, and a shorter one for more efficient upper stages and/or less tipping over when landing on other worlds)
  • Constrain staging to certain methods of attachment (OTRAG originally used radial attachments only, but fired each stage serially, using no crossfeed)
  • Limit the VAB to tier 2 at most, to limit the part count to 255 (meaning that you can't build too big of a rocket, and must resort to orbital docking for larger missions)
  • Start with 2 non-CRPU launches, to allow for easier access to the 2.5m parts for better CRPUs.
  • Use an SRB for the CRPU (knowing first-hand how difficult it is to work with solid fuel, this probably is quite hard)
  • Tie the number of CRPUs allowed to a building's current level (Example: Upgrading Administration center to level 2 allows creation of extra CRPU for a total of 2 CRPUs)
  • Try landing a kerbal on a hard-to-visit destination (Moho, Eve, Dres, Laythe, Tylo, Eeloo) and return them to Kerbin (this is more of an end-game idea if the challenge ends too fast)

Completers

Since this challenge is finishing the career, I will add any player who finishes this challenge in the blank space below in future edits, with any notes as to what ideas above they used or anything notable about what they did.

@ralanboyle The first completer, via a series of YouTube videos. Used 2 CRPUs, one with 4 FL-T400s and a Swivel engine, and the other with 2 FL-T400s and a Terrier engine. Took a relatively fast approach, doing only a few missions around the Mun and Minmus before going interplanetary with Gilly and Ike landings, culminating in a flyby-only Jool 5 mission. Nice work!

Feedback

Any feedback is welcome and encouraged, as I imagine there's plenty of holes in this that need patching. Note that although I've not actually attempted this challenge, but as a veteran player (since 0.19), I have strong reason to believe that it is feasible; a previous OTRAG challenge on here had the original poster launch a 32 ton payload to LKO using 36 CRPUs, each of which had a 1.25m diameter, 990 units of liquid fuel, and a swivel engine (or reliant, the pictures are a little hard to tell from).

Edited by Beriev
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So, do I have to unlock all of the tech that I want to use with just one launch? Thinking mostly about engines. 
 

With regard to the tech available, is there a difference between the rocket and the payload/ship? For example, can my swivel powered 1.25m tank fed, cluster rocket deliver (to orbit) a high tech Duna lander with high tech transfer stage? Or does it all have to be caveman? 

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11 hours ago, ralanboyle said:

So, do I have to unlock all of the tech that I want to use with just one launch? Thinking mostly about engines. 
 

With regard to the tech available, is there a difference between the rocket and the payload/ship? For example, can my swivel powered 1.25m tank fed, cluster rocket deliver (to orbit) a high tech Duna lander with high tech transfer stage? Or does it all have to be caveman? 

For the first question, I was thinking that it's not super difficult to get 30-40 science from the first launch, which makes the Swivel and Reliant within reach of one launch should you only shoot for them in the tech tree. If you're really pushing it and get like 70 science, you also have the Thud and Terrier within reach, but that's probably stretching the first launch. I would imagine that your best bet for a CRPU would be using a Swivel (for the gimbal) or a Reliant (for the thrust/ISP).

As for the second question, although OTRAG was meant to launch more high tech satellites into LEO, I was thinking closer to your latter suggestion, where the entire rocket is CRPUs, not just the lower stage. The payload can be changed out, of course, and things like RCS, nosecones, and landing gear can be added to CRPUs, but any other propulsion for anything ranging from LKO satellites to manned Laythe landers is not permitted. I do concede that this might just make this harder, so if you want, you can follow the idea to use two CRPUs instead of just one designs, presumably of a terrier engine for better efficiency.

Edited by Beriev
Clarification
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Okay, I've started and may keep after it. 

I made 71 science in the first launch and spent it all on the top line of R&D in order to unlock the terrier. I'm going to do it with 2 CRPUs, one short one with a terrier and one long one with a swivel.  

Edited by ralanboyle
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Best of luck to you @ralanboyle and have fun! Watching your video, I realized one clarification I didn't make that may have big implications for the first launch: you don't have to develop 2 CRPUs after the first launch simultaneously. I was thinking more like creating one CRPU for a few launches and once enough science is gathered via rockets using the first CRPU only, you could design the second CRPU to use.

 

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Here's my first mission. I unlock the first two tech tree levels here.

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Craft is a pretty standard first career craft - a bunch of Fleas and explosive decoupling

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Blastoff

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In space

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Parachuting down

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Tech tree

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@Beriev I have a couple of clarifying questions.

-Is fuel transfer between docked crafts allowed? I don't want all my missions to be direct ascent style

-Am I allowed to launch a fuel tank target vehicle using CRPU's and then dock to the fuel tank in orbit? (Like Gemini/Agena) The fuel tank itself will not have any thruster other than a standard CRPU. This relates to the first question. If the answer to the first question is yes and the answer to the second question is no, then I'd just launch a bunch of CRPU's with docking ports and steal fuel from each of them individually so I don't expect the questions to have different answers.

Edited by camacju
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First OTRAG style launch. I only use one CRPU design because I'm saving the other for a bigger 2.5m rocket. 1200 units of fuel and a Reliant engine. Mission profile is roughly analogous to Apollo 6

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First rocket is two CRPU stacked on top of one another and some stabilizing fins.  Exactly 30 parts

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Grabbing some science

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Taking a relatively steep ascent profile but this design has a lot of delta-v.

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Getting into orbit. I also boost upward to the height of Mun but don't encounter it, as preparation for the next mission.

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Aerobraking

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Chute deployed

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Tech tree summary

Edited by camacju
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Second OTRAG style launch. Analogous to Apollo 8 - going into Munar orbit and collecting some science. I also try to eyeball a Minmus encounter but fail

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Like craft 1 but with strap on boosters and fins added

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Launch

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Korolev cross?

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Eyeballing the Mun transfer

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Bad capture but I have lots of fuel

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Trying to encounter Minmus but I can't quite get there

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Reentry

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Tech tree summary

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OTRAG Launch 3 - Kerbin orbit rendezvous followed by Apollo style landings on Mun and Minmus. After this launch I use my second CRPU slot and make a 2.5m booster, consisting of an orange tank, a half size tank (X200-32), and a Mainsail.

The CRPU I have makes a surprisingly capable single stage lander.

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First stage - six CRPUs

Second stage - three CRPUs

Third stage - one CRPU that acts as a lander

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In orbit

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Same booster, lifting a fuel tank to orbit

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Circularizing in a high orbit

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Docked

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Mun transfer

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Mun descent

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Mun ascent

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Who needs an efficient transfer when you have this much fuel?

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Minmus orbit

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Landed on Minmus

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Rendezvous

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Aerobraking

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At this point I realized I forgot parachutes. Propulsive landing it is

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More science than expected

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Tech tree.

Edited by camacju
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@camacju  I gotta say, I'm enjoying watching you progress through the missions! My only extra thought for future players based on watching your career is that offsetting parts on only one specific CRPU (like on the lander) is likely frowned upon, as in my mind, it's kind of equivalent to changing some core characteristic of the CRPU. I understand why you did it (landing gear clearance), but I feel it may be interesting to see future solutions to counteract this inherent issue of the CRPU (I would personally propose attaching girders to place the landing gear at a lower level on the CRPU, while also allowing for a slightly wider stance).

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OTRAG 4: Duna and Ike

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Crew module, Kerbin->Duna transfer stage, Duna->Kerbin transfer stage, science lab

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In orbit

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Fuel tanker and Duna/Ike lander

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Rendezvous

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1 of 2 Duna transfer burns

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Duna aerocapture

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Lander brakes down to surface while lab transfers to Ike

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Duna landing

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Flag

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Lander transfers to Ike

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Lab arrives after lander

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Rendezvous

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After refueling, lander goes down to Ike and biome hops a bit

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Ike landing

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Short hop between mountains

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Rendezvous

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Kerbin transfer

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Lab is left in orbit, lander and crew module return home

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Chutes deployed

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Lots of science!

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Tech tree. Just over 6000 science remaining. I'll probably complete the whole tech tree in the following mission.

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