Exercise 9: A rocket that bursts at the top

Purpose. To make one effect fire from another: a rocket that climbs, and a burst of sparks where it ends.

Until now each emitter has worked alone. A firework is two effects, and the second happens wherever the first one has got to. No link can do that, because the rocket is not an object in the workspace. It is a particle. To fire an emitter from a particle, the emitter is put inside the particle’s template. This is nesting.

Before starting

• The workspace holds one new Part with Shape Ball, Size 1, 1, 1 and Material Neon. Name it Rocket.
• The sparks emitter from Exercise 7 is in the workspace, with its link removed. To remove it, select the sparks emitter, then in the Properties panel:
  • Link ▸ Link Dir. ▸ *, then Clear.
  • Link ▸ Link Mode, set to Follow.
  • Link ▸ Link Axes ▸ Pos Y, switched on again.

Procedure

Steps 1 to 4 repeat earlier exercises.

Step	Control	Action	Result
1	Rocket	Select, then press Transform	The HUD and the Properties panel open.
2	Properties ▸ Motion ▸ Speed	Enter 30	No change yet.
3	Properties ▸ Emission ▸ Lifetime	Enter 1.2	No change yet.
4	HUD ▸ Emit button	Press	One ball climbs 36 studs and vanishes. The Emit box already reads 1.
5	Explorer	Expand Rocket	Its children show, among them RenderTemplate.
6	Explorer	Drag the sparks emitter onto RenderTemplate	The sparks emitter is now inside the RenderTemplate of Rocket.
7	The sparks emitter, inside RenderTemplate	Select	The HUD and the Properties panel show the sparks emitter.
8	HUD ▸ Emit, Delay, Duration boxes	Enter 30, 1.1 and 0	No change yet.
9	Rocket	Select, then press Emit on the HUD	The ball climbs. After 1.1 seconds, thirty sparks burst from the ball, wherever it then is. The ball vanishes a moment later. The sparks spread and fall.

NOTE

A nested emitter must be inside the RenderTemplate. An emitter that is inside Rocket but outside its RenderTemplate is not nested. It is a separate emitter that happens to be kept there, and firing Rocket does not fire it.

CAUTION

A nested emitter’s Delay is counted from the moment its parent particle is made. If the parent particle has vanished before the Delay is up, the nested emitter does not fire for that particle. Here the Delay of 1.1 seconds must stay below the rocket’s Lifetime of 1.2.

CAUTION

Counts multiply. A parent that emits 10, each firing a nested burst of 30, makes 300 particles at once.

What the steps did

The RenderTemplate is what every particle is duplicated from. It is also the place to keep whatever each particle should carry. An emitter kept there is fired once for every particle the parent makes.

A nested emitter is fired by the same three numbers as any other, read from its own HUD boxes:

Box, on the nested emitter	Meaning when nested
Emit	Particles made for each parent particle.
Delay	Seconds to wait after the parent particle is made.
Duration	Seconds of stream after the burst, at the nested emitter’s Rate. The stream ends early if the parent particle ends first.

With Emit 0 and Duration 0 a nested emitter never fires. Its Enabled switch plays no part in nesting.

The sparks appear at the parent particle, and until the parent vanishes they are carried along with it. How closely they follow is set by the nested emitter’s own Link Mode, with the same five choices as in Exercise 7. No link needs to be picked: the parent particle is the thing followed. When the parent vanishes, sparks already in flight live out their own Lifetime from where they are.

The nested emitter is still an ordinary emitter. Select it inside the RenderTemplate to change it, exactly as before. A change shows on the next parent particle.

If the result differs

Symptom	Cause	Action
The ball climbs and nothing bursts	The nested Emit box reads 0, or its Delay is not below the rocket’s Lifetime.	Repeat step 8.
The ball climbs and nothing bursts	The sparks emitter is beside RenderTemplate, not inside it.	In the Explorer, drag it onto RenderTemplate.
The sparks burst at the launch point, at once	The nested Delay is 0.	Enter 1.1 in the nested emitter’s Delay box.
The sparks do not rise with the ball	Pos Y is still switched off from Exercise 7.	Switch Pos Y on again under Link ▸ Link Axes.
Sparks fly while nothing is being fired	The nested emitter’s Enabled is on.	Select it and switch Enabled off.

Variations

Change	Result
Nested Motion ▸ SpreadAngle 180	The burst opens in every direction, as a firework does.
Nested Delay 0, Emit 0, Duration 1.2, Emission ▸ Rate 40, Link ▸ Link Mode RigidLocal	A stream of sparks is left behind along the whole climb, like the tail of a rocket.
HUD ▸ Emit 5 on Rocket, with Motion ▸ SpreadAngle 20	Five rockets fan out, and each bursts where it ends.
A second emitter inside the same RenderTemplate	Both fire from each rocket. Give each its own Delay.
An emitter inside the RenderTemplate of the nested sparks	Each spark fires it in turn. Nesting can go as deep as the Particle Cap allows.

Log

New in this exercise	An emitter placed inside a RenderTemplate. Emit, Delay and Duration on a nested emitter.
Done before	Transform (six times). Speed (Exercises 1, 4 and 5). Lifetime (Exercises 2 and 6). The Emit, Delay and Duration boxes (Exercise 3). Link Mode (Exercise 7).
Carried forward	Nothing. Exercise 10 uses the emitters kept from Exercises 4, 5 and 6.