Improved Solids

[sevenperforce]

I've seen this bandied about here and there, but I thought I'd pull several different ideas into one discussion thread for simplicity.

KSP would benefit greatly from the addition of improved solids. It would only take a very small part pack to open up virtually infinite possibilities.

Upper-stage solids

Upper-stage solid kick motors are hugely used across the spaceflight industry, but the form factor and Isp of KSP's small solids just don't fit. Proposal: add one surface-attachable vacuum-optimized kick motor. It would have the same size as the new R-4 Dumpling tank (and could even use the same model, albeit recolored) with a large, protruding nozzle. Vacuum isp of 260, low empty vacuum TWR (~10:1), and very good mass fraction (6:1). So its Isp is comparable to the Puff engine, its TWR is slightly poorer than a Kickback, but its mass fraction is comparable to a Separatron. In-atmo Isp of 100.

Might be worth it to make an 0.625m and 1.25m variant. Probably not worth it to make a larger variant because you can just cluster the smaller ones.

Segmented boosters

Larger boosters with more customization options are sorely needed. To that end, I propose a total of three new parts: a 2.5m solid-fuel segment, a 2.5m Fixed Nozzle, and a 2.5m TVC Nozzle.

Each 2.5m solid segment would function like a separate solid booster and could have its thrust and fuel load adjusted. In order to be fired, however, they'd have to be stacked vertically on top of a nozzle. When stacked on top of a nozzle and fired simultaneously, they'd all have their own individual thrust levels and fuel depletion, but their thrust vectors would be located in the nozzle they were stacked on. You could create complex thrust curves by varying the thrust level (and/or fuel load) in each segment.

The nozzle part would serve as the location of the staging option for the stack (so you can put solid segments into your rocket without a nozzle, but you have no way to stage them). Nozzles have no bottom node. The TVC nozzle has some vectoring ability and is twice the mass of the fixed nozzle. In theory, you can rotate the nozzle to do deflected thrust, though with 2.5m parts this wouldn't be as useful as with smaller SRBs.

[sevenperforce]

On the segmented boosters...

A more expansive way of doing it would be to create a single solid-segment part with a selector similar to the node-count selector on the thrust plates, that cycles size through 0.625m up through 3.5m. Thus that single part would be able to be used to build multisegment boosters of multiple sizes, and you'd only need the nozzle part for each individual size. You could have vacuum or SL nozzles, TVC or fixed nozzles, etc.

But then you'd have to have your thrust limiter on your nozzle, which means you can't very easily build the same de facto thrust curves.

[Ultimate Steve]

Yes. While we are at it, 0.625m atmospheric SRB's would be good as well.

[sevenperforce]

4 minutes ago, Ultimate Steve said:

Yes. While we are at it, 0.625m atmospheric SRB's would be good as well.

I can do a lot of magic with Tweakscale, but it has its limits.

You could still do thrust curves using the selectable segments+various nozzles approach if it was coded such that the thrust would drop as segments "burned out". For example, suppose you stack three segments on top of each other: one with 100% fuel, one with 50% fuel, and one with 25% fuel. Then you'd put an appropriately-sized nozzle underneath. Suppose that with the nozzle size you selected, burntime of a single full segment is 20 seconds. The stack would produce 300% the thrust of a single segment for the first 5 seconds, then 200% for the next 5 seconds, then just 100% for the remaining 10 seconds.

The nozzle thus serves to thrust-limit all of the connected segments, but each of the segments still acts essentially as its own separate SRB, and stacking segments results in more total thrust rather than greater burntime.

To your point -- you could have the following nozzle pack:

• 0.625m fixed vacuum nozzle (vacuum Isp: 250)
• 0.625m fixed SL nozzle (vacuum Isp: 180)
• 1.25m fixed vacuum nozzle (vacuum Isp: 260)
• 1.25m fixed SL nozzle (vacuum Isp: 200)
• 2.5m fixed SL nozzle (vacuum Isp: 190)
• 2.5m TVC SL nozzle (vacuum Isp: 195)
• 3.5m fixed SL nozzle (vacuum Isp: 180)

The nozzles function to limit the thrust of the segments above them. You can freely put an 0.625m nozzle underneath a 2.5m segment, but the thrust will be no greater than if you put it under a 1.25m or 0.625m segment. However, stacking two segments on top of an 0.625m nozzle results in twice as much thrust.