Kerbooster Two-Stage-To-Orbit "Stack Shuttle"

[Torquemadus]

Hi everyone!

In this thread, I'm going to demonstrate my Kerbooster fly-back stage. Note that the Kerbooster was designed from the outset to be used on high difficulty settings, so some players may find this approach to be overkill, but it should serve to demonstrate how to fly a low-cost space program to those that are interested.

I derived the Kerbooster name from the Starbooster concept featured in Buzz Aldrin's science fiction novel Encounter With Tiber.

I currently play career mode on custom difficulty set to 20% resource gains (compared to 60%) for Hard. This still gives me plenty of resources to do missions, but means that I have to be sensible when it comes to costs. Otherwise, I can easily end up spending more on a mission than I get back in contract rewards.

If you examine fuel tanks and solid rockets in the VAB, you can use tweakables to look at how much they cost with and without fuel. With liquid fuel, most of the cost pays for the fuel tank, not the fuel it contains. With solid fuel, most of the cost pays for the fuel, the empty solid rocket is worth very little. In fact, the recovery value of empty solids is dwarfed by the values of the parachutes needed to recover them.

Solid rockets are cheap and you can afford to throw them away once empty. Liquid fuelled rockets are expensive, but only if you don't recover them. Therefore, I design all of my missions accordingly. I use two-stage-to-orbit launch vehicles that use a cluster of solids as an expendable first stage and a recoverable liquid fuelled rocket as the second stage.

In the early game, science progress is slow and part unlock costs are high, therefore I initially get by with simple parachute recovery methods. Once I have enough tech, I switch to winged fly-back stages, which give me 100% recovery value. If applicable, I also make any upper stages recoverable too. This means that the cost of any mission is reduced to the cost of the fuel I expended and the empty "trash bins full of boom" I discarded.

In my current save, I used my two-stage-to-orbit Kerbooster to send direct ascent landers to the Mun and Minmus that are also capable of acting as long-distance surface rovers. Each lander is loaded with scientific instruments and is capable of traveling long distances over the surface of either moon. I'm not in a hurry to bring either vehicle back, since I keep getting more and more contracts to plant flags and transmit data. Instead of planting a "flag garden", I can easily travel over the surface and plant flags in each new biome I encounter. Once their missions are complete, the landers are able to return to Kerbin and land on the runway (tested previously).

So far, my Kerbooster has been adequate for the launch of missions of any size I need to get the job done. I previously tested heavy and super-heavy variants of the Kerbooster in earlier patches, but I haven't so-far gone any further than test flights with dummy payloads, since I've never needed the extra payload capacity they provide.

The term I use to describe the Kerbooster configuration is a "stack shuttle". Instead of the familiar NASA shuttle configuration, I arrange the shuttle as a stack, with the solids as the first stage, the orbiter as the second stage, and the payload mounted on top. This allows the shuttle to operate in the manner of a conventional launch vehicle without the bother of squeezing the payload into a cargo bay.

These images show a typical Kerbooster launch.

If available, the pilot for the fly-back stage is "rescued" from orbit, which helps to offset the cost of the launch.

Recovery of the fly-back stage takes a bit of practice, but is easily achievable once you get the hang of it. No need for mods to calculate it for you.

Adding big enough wings for a safe unpowered glide landing also adds enough lift to glide long distances if needed. This makes it easier to make corrections if your re-entry isn't as accurate as you hoped.

The direct ascent landers spend a long time on the surface of each moon, each experiencing multiple day/night cycles. Note that on high difficulty settings, the science value of returning the lander to Kerbin is quite low, it makes more sense to remain on the surface and fulfil contracts as they become available and use the Outsourced R&D strategy to earn extra science.