ZenRockets is a parametric builder: parts are defined by their dimensions, not dragged into approximate shape. Designs stay exact and repeatable, and parts can reference each other, so a wall-thickness change in one tube can flow through the whole airframe.
Dimensions#
Select a part and its dimensions appear in the details panel: length, outside diameter and wall thickness for a tube; shape, length and shoulder geometry for a nose cone; thickness and core diameter for a centering ring. Nose cones offer seven profile shapes (ogive, conical, parabolic, elliptical, Haack, Von Kármán and power series), in hollow or filled construction.
Selected parts also show dimension annotations on the model itself, so you can read the geometry right on the canvas.
Linked dimensions#
Fit-related dimensions can be linked to another part instead of typed. Supported fields have a small A button: choose Manual (type the value yourself) or one of the candidate sources, each listed with its live value, like "Body Tube – ID: 24.0 mm". Link a coupler's outside diameter to the body tube's inside diameter and it recalculates whenever the tube changes. The field turns read-only and tinted, with the source named in its tooltip.
A few boundaries keep links predictable:
- Only fit-related diameters and wall thicknesses can be linked — lengths never are, and motors, fin sets, launch lugs and rail buttons have no linkable dimensions of their own.
- A link that would create a circular chain is rejected — the offending candidate is disabled in the list.
- Delete the source part and the link falls back to a manual value.
- Linked values are still clamped like typed ones, so a link can't produce impossible geometry — a shoulder wider than its nose cone, say.
Placement references#
Every part is positioned by three settings in its Placement block, and reading them together works like a build instruction:
- Parent
The point on the parent the measurement starts from — its bottom, middle or top.
- Anchor
The point of this part that lands there — its own bottom, middle or top.
- Offset
The signed distance between the two, in millimeters. Positive moves the part toward the nose, negative toward the tail.
"Bottom of the parachute, 20 mm below the top of the nose cone" is Parent: top, Anchor: bottom, Offset: −20 — and it stays correct when surrounding parts are resized. New parts arrive anchored the way they're typically mounted.
Two behaviors worth knowing:
- Switching Parent or Anchor never moves the part. The offset is recomputed so the part stays put — the selectors change what the position means, not where the part is.
- Outer airframe parts don't show Parent and Anchor at all. Nose cones, body tubes and transitions stack nose-to-tail automatically, and their offset nudges a part relative to its stacked seat.
The offset also has its own handle on the canvas: select a part and a dimension line runs from the parent reference point to the part's anchor, with a draggable endpoint. Pull it to adjust the offset directly on the model — snap increments and Shift fine control apply, like any other handle.
Radial patterns#
Parts that live around the airframe rather than on its centerline get radial controls in their Placement block: an instance count, a starting angle, and a distance off the centerline. Set a motor's count to three and you have a cluster — evenly distributed, rendered and simulated as three motors, still edited as one part (Using Motors has the details). The same pattern handles rail buttons, launch lugs and clustered internal structure.
Editing fin outlines#
A fin's profile is a freeform 2D outline you shape point by point, plus a fin count (1–8), material thickness and optional through-the-wall tabs. Trapezoidal, clipped delta, elliptical, or something entirely your own.
Select a fin set and a floating Start editing points button appears over the canvas. In editing mode:
- Drag a point to move it. The two root points stay on the root chord and slide along it; every other point moves freely.
- Drag an edge to move both of its endpoints together.
- Hold Alt and click an edge to add a point at its midpoint.
- Hold ⌘/Ctrl and click a point to remove it — the two root points can't be removed.
- Points snap to the horizontal and vertical positions of nearby points, which makes straight edges and aligned tips easy; the usual snap increments apply otherwise.
While a modifier is held, only the handles it acts on stay visible, and an on-screen hint shows which modifier does what. Through-the-wall tabs get their own handles for width and depth. Done editing exits the mode.
Snapping and precision#
Snap controls at the top of the canvas keep dragged values tidy: switch the magnet on, pick a 1, 5, 10 or 50 mm increment, and every handle drag lands on it. With snapping off, drags still commit at 0.1 mm precision. For the last fraction of a millimeter, hold Shift while dragging — the handle moves at quarter speed. When a drag is the wrong tool, type the exact number in the details panel.