diff --git a/docs/progression_design.md b/docs/progression_design.md
index 518aa02..dc68f65 100644
--- a/docs/progression_design.md
+++ b/docs/progression_design.md
@@ -230,9 +230,37 @@ supporting different fleet doctrines feel structurally different to build.
 - **Rule:** the starting stock suffices for a minimal block loop plus the
   first shipyard — with a little slack for beginner mistakes, but not
   enough to skip the loop entirely.
-- **Rule:** asteroid expansions compete with buildings for the same
-  currency. Expansion pricing makes "more space" a real alternative to
-  "more stuff", not an afterthought.
+- **Rule: the growth curve lives here.** A saturated building produces
+  exactly 1 threat/s, so the player's output curve *is* their
+  building-count curve — shaping growth over a run means shaping the
+  block and space economy, there is nowhere else it can live. Intended
+  shape: exponential bootstrap (block-limited) → ramp
+  (expansion-limited) → asymptotic squeeze as expansion costs outrun
+  income, racing the enemy threat rate throughout.
+- **Rule: escalating expansion costs.** Expansion cost is a formula of
+  the number of expansions already purchased, rising steeply enough that
+  expansions eventually outrun any block income. The starting asteroid
+  is deliberately small — filled within the first boss cycle or two, so
+  the early exponential burst is a satisfying ramp, not a balance hole —
+  and from then on the output curve is the expansion curve. Blocks keep
+  a meaningful sink for the entire run, and "grow vs. army" stays a live
+  decision at every moment.
+- **Rule: designed doubling time.** Block production is a positive
+  feedback loop (blocks buy assemblers, assemblers make blocks); its
+  time constant is a designed quantity, never an accident of quantity
+  choice. The block chain's depth and the per-building costs are tuned
+  against a stated target of the form: "a factory spending X% of its
+  capacity on blocks doubles in ~T minutes."
+- **Rule: growth is limited by economy, never by waiting.** Construction
+  times stay short; the serial build queue must not be used as a growth
+  brake. Waiting for placed buildings to become operational — especially
+  at the start of a run — is frustration, not gameplay. All growth
+  limiting comes from block income and expansion pricing.
+- Note: block income has a structural ceiling — blocks enter the stock
+  through the HQ's single belt port, so income is capped at belt
+  throughput regardless of assembler count. Per-building costs should be
+  high enough that this cap can bind late-game (see the condensed-block
+  idea under Open tasks).
 
 ## Numeric guardrails
 
@@ -260,6 +288,11 @@ Constraints that every recipe must respect, independent of tuning:
   `unlock_at_station_level` ladder, real threat costs and
   `default_modules`, reprocessing weights, and the threat-rate formulas
   according to the rules above.
+- **Condensed building blocks** — a drop-unlockable shortcut-style
+  recipe that packs several blocks' worth of value into one belt item,
+  relieving the HQ intake ceiling (see Building block economy) as a
+  late-game reward. The ceiling is the puzzle, the drop is the fix —
+  same philosophy as shortcut recipes.
 - **Mk2 upgrade recipes** — the deferred design for per-item progression,
   to revisit once the config has stabilized. A duplicate-style drop
   unlocks a distinct `*_mk2` item whose recipe consumes the Mk1 item plus
@@ -292,3 +325,8 @@ in `requirements.md` and the git history). Still open:
    recipe schematic pool eligibility in REQ-DEF-SCHEMATIC-DROP: in
    addition to the existing station-level and output-item checks, all of
    the recipe's input item types must be implicitly unlocked as well.
+4. **Expansion cost formula.** Replace the flat
+   `world.toml [expansion].cost_building_blocks` with a formula of the
+   number of expansions already purchased (x = expansions bought so
+   far), so expansion costs can escalate per the block economy rules.
+   Update REQ-EXP-COST and wherever the UI displays the expansion cost.