Some good points here; glad to see you kind of agree with the hydrolox - the RL10s were a particular focus of mine as for BDB player's they're the backbone for...quite a few upper stages for quite a long time, so it felt weird that operational RL10s were so far down the line.
As it stands the hydrolox branch in particular would benefit from one or two extra nodes in the progression - if BDB is one of the core facets some spacing out would be ideal as many of the higher end engines in BDB are hydrolox. Maybe 1 or 2 new nodes for hydrolox, then two heavy-lift hydrolox nodes, the first providing, say, the M1 and XLR129 alongside a few others, and the second providing the SSME etc. The heavy-lift hydrolox nodes could have spiked science costs.
Gives a decent flow of progression for powerful hydrolox engines and lets the XLR129 serve its purpose as a 'SSME-but-not-quite' sort of creature. The vaccum node could do with one extra node as well. I agree that for this to really work,
The thing with cryo vs ntr, in my general experience through many BDB playthroughs, is that NTR as a whole has its benefits and tradeoffs that justify continuing to use cryogenics while they exist. For NTR that's pre-aerospike based (aka most up to NTR3 and NTR4) they benefit the most from being on orbit. They have trouble lifting heavy payloads without nearly being all the way there. Cryogenic is heavier but often provides better thrust and is more convenient to use, and frankly there's not a whole lot of options for mounting nuclear engines that aren't the Alamo on anything smaller than 2.5m.
tl;dr 1-2 extra nodes for general cryogenics, for BDB moving the prototype rl10 and the lr87-lh2 to the first node and the rl10-a3 and lr87-lh2 upgrades to the second alongside the first iteration of the j2, to allow for relatively gradual progression of the early game upgrades for hydrolox rockets/upper stages and such
On the topic of tanks, could do with a bit of spacing out as while gameplay function is real, all of the Titan variant tanks unlocking at once feels...odd. Maybe for the 0.625-4.25m there's a couple of extra nodes in between form factors for say, lengthening rather than widening. Consider also segmenting 4.25m and 5m into their own nodes, and putting 6.25 and 7.5 into the final Tank node
Also on a quick, possibly strange note I'd consider moving the Saturn IB tank to one of the earlier nodes as it's a unique case of being 8 clustered redstone tanks and pays the price for it in mass and lack of fuel. Maybe it's an exception rather than the rule but it would work balance wise for people who want to mess with Saturn 1, which was flying by 1961. tl;dr historically accurate and being clustered 1.25 tanks , technically gameplay accurate
Solid point, XLR-129 was intended for SSTOs after all and a lot of shuttle stuff is limited by access to it - that being said a new node like GiganticAirframes might work better for the technological jump from shuttles to SSTOs. And balance wise the MK33 is hilariously optimistically overpowered for what it is so a little of that spacing out could be ideal.
Yeah I think gameplay wise there should be a reason to be making little somewhat strange stations first before jumping to enormous ones. Thanks for potentially looking over the skylab tech stuff.
A couple of miscellanious things - some of the SRB costs should be raised, as it feels strange spending 60 science to unlock the AJ-260 in particular.
ALSO: Pull requests should be coming in the next few days for a few extra compatibility patches - I've personally made patches for:
-Procedural Parts
-Rocket Motor Menagerie
-X-20 Moroz (the Dynasoar)
-EngineIgnition
-Remotetech
-That one Stowaway mod Nertea did that has literally the best centrifuge tethers in all of KSP.
I can shoot you the link to the repo if you'd like - I plan to make quite a few more
Thanks for responding, I really am excited to see how this tree develops. It has a lot of potential to be the perfect mix between CTT and RP-1 and I wish you the best of luck.