L1 Weight Budget and Ballast Calculations¶
Motor Data: AeroTech H128W¶
Reload Specifications (from ThrustCurve)¶
| Parameter | Value | Notes |
|---|---|---|
| Designation | H128W | White Lightning propellant |
| Motor Type | Reload | Requires RMS-29/180 casing |
| Diameter | 29 mm | Needs 29→38mm adapter |
| Length | 194 mm | |
| Total Weight | 206 g | Complete loaded motor |
| Propellant Weight | 94 g | Burns during flight |
| Average Thrust | 128.0 N | |
| Max Thrust | 168.7 N | |
| Total Impulse | 172.9 N·s | 95% H-class |
| Burn Time | 1.3 s | |
| Delay | 14 s adjustable | Drill to desired delay |
Weight Breakdown¶
| Component | Weight | Notes |
|---|---|---|
| Propellant grains | 94 g | Consumed during burn |
| Nozzle, liner, delay, seals | ~112 g | Reload kit non-propellant |
| Reload kit total | 206 g | As packaged |
RMS-29/180 Hardware¶
| Component | Approx Weight | Notes |
|---|---|---|
| Aluminum casing (tube) | ~60 g | 29mm × ~180mm |
| Forward closure | ~25 g | |
| Aft closure | ~25 g | |
| Hardware set total | ~110 g | Reusable |
Weight Clarification
ThrustCurve's "Total Weight: 206g" is the complete assembled motor (casing + reload). This is what goes in the rocket.
Post-burn motor weight: 206g - 94g = 112g
L1 Configuration Mass Budget¶
Measured Weights¶
| Item | Mass | Method |
|---|---|---|
| Rocket ready to fly (no motor) | 1460 g | Kitchen scale, includes: |
| - Airframe (shortened L1 config) | ||
| - Nose cone | ||
| - Fins | ||
| - Motor mount | ||
| - 48" main parachute (packed) | ||
| - Shock cord | ||
| - Nomex protector | ||
| - Rail buttons | ||
| - 29→38mm motor adapter |
Motor Weight¶
| Item | Mass |
|---|---|
| H128W complete (pre-burn) | 206 g |
| H128W post-burn (casing + hardware) | 112 g |
Pre-Flight Weight (No Ballast)¶
Tripoli 5:1 Thrust-to-Weight Rule¶
Requirement¶
Tripoli RSO (Range Safety Officer) requires:
This ensures adequate velocity off the launch rail for stable flight.
Maximum Allowed Weight¶
Current Status¶
| Parameter | Value | Status |
|---|---|---|
| Current liftoff weight | 1.67 kg | ✓ Under limit |
| Maximum allowed | 2.61 kg | |
| Available margin | 0.94 kg | Room for ballast |
Why Add Ballast?¶
Flight Altitude Concern¶
Lighter rocket = higher altitude = longer recovery walk.
For a certification flight:
- Want predictable, moderate altitude
- Want to see the rocket throughout flight
- Want quick recovery (less walking)
- Want to stay well within field boundaries
Target Weight¶
To fly conservatively low:
This gives:
- Thrust-to-weight ratio: \(\frac{128}{2.5 \times 9.81} = 5.2:1\) ✓
- Lower apogee than minimum weight config
- Still meets 5:1 requirement
Ballast Calculation¶
Required Ballast Mass¶
Ballast Options¶
| Material | Density | Volume for 834g |
|---|---|---|
| Lead shot | 11.3 g/cm³ | 74 cm³ |
| Steel shot | 7.8 g/cm³ | 107 cm³ |
| Sand | 1.6 g/cm³ | 521 cm³ |
| Steel bolts/nuts | 7.8 g/cm³ | 107 cm³ |
Ballast Placement¶
CG/CP Impact
Ballast location affects Center of Gravity!
- Nose ballast: moves CG forward → more stable
- Tail ballast: moves CG aft → less stable
Recommendation: Place ballast in or near nose cone to maintain/increase stability margin.
See Stability Calculations for CG/CP analysis with ballast.
Weight Summary Table¶
| Configuration | Pre-Flight | Post-Burn |
|---|---|---|
| Rocket (no motor, no ballast) | 1460 g | 1460 g |
| + Motor (H128W) | +206 g | +112 g |
| Minimum config | 1666 g | 1572 g |
| + Ballast (target) | +834 g | +834 g |
| With ballast | 2500 g | 2406 g |
OpenRocket Model vs Reality¶
| Source | Mass (no motor) |
|---|---|
| OpenRocket model | 1096 g |
| Actual rocket | 1460 g |
| Difference | +364 g |
The difference is from epoxy, paint, and hardware not fully accounted for in the model.
L1 Config Stability Warning
OpenRocket shows the L1 configuration has negative stability (-0.191 cal) with model mass.
CP (90.8 cm) is forward of CG (92.7 cm) - rocket would tumble!
Nose ballast is mandatory, not optional.
See OpenRocket Simulations for full analysis.
Action Items¶
- [ ] Verify rocket weight measurement (re-weigh)
- [ ] Confirm motor weight when received
- [ ] Update OpenRocket with actual 1460g mass
- [ ] Add nose ballast to achieve positive stability
- [ ] Calculate required ballast for ≥1.5 caliber stability
- [ ] Run flight sim to verify stability throughout flight
- [ ] Source appropriate ballast material (lead shot recommended)