In this work I have grouped stages into 'families' of related designs,
and launch vehicles into families based on their first stage type. For
instance, I consider the Thor Agena, Thor Able, and Delta II to be
members of the same family, while Atlas Agena, Atlas Able, and Atlas
Centaur are a second family. Ariane 1 to 4 and Ariane 5 are separate
families - Ariane 1 to 4 share a common first stage design, but Ariane 5
is an entirely different rocket.
It's easier to maintain listings on a per-family basis, and the total
number of first stage families is about 330 - much smaller than the 1500
or so different launch vehicle variants. There are some grey areas in
assigning vehicles to families, but it's much less problematic than
splitting hairs about slight differences between variants.
The rule I've adopted for defining a new family is a change in the
tankage diameter. In some ways this is a superficial thing - to a rocket
engineer, changes in the engine and the avionics will be much more
significant than a change in the external dimensions - but in practise
the diameter criterion works well and reveals important design
commonalities between different launch vehicles. There are some marginal
cases - the Trident II and Trident I missiles are closely related but
have different diameters. The real complication comes in deciding which
stage is the first stage. For orbital vehicles it's pretty clear, on a
Delta II the Thor-derived first stage is a true first stage, it wouldn't
make sense to count the GEM strapons as the core of the vehicle. But for
many sounding rockets the vehicle consists of a booster and sustainer,
with the booster falling off after only a couple of seconds. Is a
Nike-Aerobee (Aerobee 170) a Nike variant or an Aerobee variant? After
much agonizing I have decided to consider it a Nike variant. Similarly,
Skylarks boosted by Cuckoo or Goldfinch motors are under the Cuckoo
family, while single-stage unboosted Skylarks are in the Raven family.