What's the heaviest fueled, launchpad setup you've launched into orbit (on a realistic, or near realistic scale) / the lightest?

Fergrim · June 12, 2022

Today I sent up a 20,000 ton rocket into a polar orbit. (this is roughly 7x the weight of the Saturn V fully fueled on the launchpad IRL iirc)

The first stage had approximately 90,000 kilonewtons of thrust and began its ascent at an acceleration still of about 1 m/s. It was more like a slow levitation - though it did get into orbit with about 6000 dv to spare

I feel like I'm totally in the dark as to how make my rockets smaller and more efficient - though. No matter how I move through the tree, it seems like the general size stays about the same.

The Aziz · June 12, 2022

Well, you do have d-v readout available, if you had 6k to spare, it's clear you overengineered it.

I don't count but I sent an orbital station to Moho. Launch vehicle barely fit in the VAB. But I still very much like to use 1.25m rockets, they're perfectly capable of sending small payloads. Simple two stage Swivel+Terrier based rocket can send your cubesat on the way to other planets even.

Fergrim · June 13, 2022

Well the 6000 DV was on purpose, so I'd have the option to follow up with a lunar intercept - or even a large change in inclination, depending on what missions might catch my eye. I was scrounging for science everywhere haha

One way I try to maximize efficiency is once I have a rocket chosen and attached to a tank, I start adjusting the size of that tank based on the stage's thrust/weight ratio.

So if I attach my rocket and I'm looking at 3.0 TWR - I generally increase the size of the tank until that number is closer to one - to squeeze the maximum DV out of it.

But even doing things like that, I feel like I've hit a wall at around 15000 total DV - where no matter how I fiddle with adding and removing stages, or optimizing the vacuum level stages, I just can't seem to really make it much past that number. (this is in career, of course, so not exactly the same issue as if I had 100% access to the whole tech tree)

It's barely enough / not quite enough to get into orbit and then get to the moon. Though if I optimize the timing and everything, it could be enough to head directly into a flyby of venus which is occasionally a 9000+ DV trip if I remember right. Like once a year or so.

Of course, all these numbers are in reference to a full scale universe with realism overhaul- so 1.25m is never an option for me except for some specific suborbital . Also makes it harder to ask for efficiency advice because the realfuels/real scale rockets dont have nearly as broad an audience as stock.

It's worth it though when you light the engines on a rocket that's nearly a kilometer tall. When it's that tall and its first 30 seconds of thrust only accelerate it at like 1-2 m/s and it doesn't even wobble it makes the challenge of full real scale worth it.

Edited June 13, 2022 by Fergrim

jimmymcgoochie · June 13, 2022

2 hours ago, Fergrim said:

But even doing things like that, I feel like I've hit a wall at around 15000 total DV - where no matter how I fiddle with adding and removing stages, or optimizing the vacuum level stages, I just can't seem to really make it much past that number. (this is in career, of course, so not exactly the same issue as if I had 100% access to the whole tech tree)

That’s the rocket equation for you: delta-V has hard limits based on your ISP and mass ratio. You can either try to increase your mass ratio by using lighter, more efficient tanks (e.g. using lightweight aluminium-alloy balloon tanks) or increase your ISP by using different propellants and/or engine types (e.g. switching to hydrolox or even nuclear thermal rockets), but there are problems with both- fancy tanks are expensive and difficult to make, hydrogen is very un-dense and needs larger tanks than other propellants and NTRs are heavy with a relatively low TWR. Ion thrusters using e.g. xenon provide vastly more delta-V due to their extremely high ISP, but their thrust is incredibly puny in comparison.

The largest rocket I’ve ever launched in RO/RP-1 put 650t in low Earth orbit- I actually launched that thing six times carrying the propulsion modules for crewed Mars and Venus missions, plus a one-off mission that had well over 30km/s in total thanks to extremely efficient hydrolox rockets, just to put a one-ton lander on Mercury’s surface. Total launch mass was at least 12kt each time, limited mostly by the size of tooled tanks and how many boosters would fit around the core stage.

king of nowhere · June 13, 2022

I launched a 40kton rocket to get a 1300 ton payload into orbit. such is the price of trying a rss grand tour with kerbalism, stock parts and a large crew. It used 684 vector engines.

as for total deltaV... it only depends on how much mass you can shed before the end. how big is your lander? I'm sure you can do a lot better than 15 km/s.

I had 10 km/s on my rocket, which was barely enough to get into orbit - but making the rocket any bigger would have crashed my pc, so I used up all the payload's fuel and then sent refueling missions.

If I got 1300 tons in orbit with a 40k ton rocket, you can halve its size and get 650 tons into orbit with a 20k ton rocket. You shouldn't have any problem squeezing another 10 km/s (vacuum) from a 650 ton payload. I only needed a payload that big because I needed, like, 800 tons of mining gear

So, I suggest instead of optimizing your rocket, you optimize your orbiter/lander

Edited June 13, 2022 by king of nowhere

MythicalHeFF · June 18, 2022

My heaviest was “only” about 7 kilotons. It was supposed to send a 450 ton submarine to Laythe, but I decided to cancel the mission after reaching orbit.