X1 Racing accelerates FAA-certified, deployable flying-car transportation through maximum entertainment. The X1 Cup machine is a series hybrid: an opposed free-piston linear generator (OPFPLG) burns propane, makes electricity, and drives eight lift motors and two forward-flight fans through the mandatory X1 SI Boost layer. A 10 kWh accumulator handles transients and the mandatory power-off landing. It is not a propulsion battery.
That choice is what makes the race format work, and it is the same choice that makes personal air mobility practical. A pure battery vehicle of the same mass cannot finish even the shortest X1 Cup race. A tiltrotor or lift-plus-cruise aircraft cannot take off from a driveway. The X1 architecture combines high-energy-density propulsion with true point-to-point VTOL.
The OPFPLG is the part that decides it: 1,500 hp continuous, 533 kg (the OPFPLG companion paper's first-principles feasibility estimate), with 60% brake thermal efficiency as the program initial target and 67% as the continuous design point. Every figure below comes from one consistent model, stated at the rule book's weight limits: the 3,500 lb maximum recommended takeoff weight expected for early vehicles, and a 2,700 lb goal vehicle (the ~2,500 lb minimum dry weight plus 200 lb of propane).
Series hybrid, 200 lb propane (the rule-book maximum), sea-level standard air. The race mission is the rule-book maximum: 44 minutes plus one lap, flown at a sustained 2× hover power.
| Metric | Initial vehicle (3,500 lb), 60% BTE | Initial vehicle, 67% BTE | Goal vehicle (2,700 lb), 60% BTE | Goal vehicle, 67% BTE |
|---|---|---|---|---|
| Pure hover endurance | 1 h 33 min | 1 h 44 min | 2 h 17 min | 2 h 33 min |
| Race endurance at 2× hover power | 46 min | 52 min | 1 h 08 min | 1 h 16 min |
| Margin beyond the 44-minute race maximum | 2 min | 8 min | 24 min | 32 min |
| Range at 150 mph | 159 mi | 177 mi | 192 mi | 214 mi |
| Range at 125 mph (near max-range speed) | 161 mi | 180 mi | 208 mi | 233 mi |
A vehicle at the full 3,500 lb completes a 44-minute race at sustained full race intensity with about 2 minutes of reserve at the 60% initial target and 8 minutes at 67%. At 50% BTE it cannot (39 minutes). Engine efficiency and weight reduction toward the dry-weight goal are what open up margin. The public speed figure is 200+ mph.
This case is hypothetical and exists only to show why the series uses a hybrid. A battery vehicle of this size is not legal under the rule book, which caps the accumulator at 10 kWh nominal and requires propane for racing. Remove the complete 1,475 lb energy system (OPFPLG 1,175 lb, tank 100 lb, propane 200 lb) and install 1,475 lb of current production battery (73 Wh/lb at pack level, 94% delivery efficiency: 108 kWh nominal, 101 kWh usable). Everything else is unchanged.
| Battery, 3,500 lb | Hybrid 60%, 3,500 lb | Hybrid 67%, 3,500 lb | Battery, 2,700 lb | Hybrid 60%, 2,700 lb | Hybrid 67%, 2,700 lb | |
|---|---|---|---|---|---|---|
| Usable energy at motor input | 101 kWh | 631 kWh | 704 kWh | 101 kWh | 631 kWh | 704 kWh |
| Per lb of energy system | 69 Wh/lb | 428 Wh/lb | 477 Wh/lb | 69 Wh/lb | 428 Wh/lb | 477 Wh/lb |
| Race endurance (2× hover) | 7 min | 46 min | 52 min | 11 min | 1 h 08 min | 1 h 16 min |
| Completes 44 min + lap? | No | Yes | Yes | No | Yes | Yes |
| Hybrid advantage, race time | — | 6× | 7× | — | 6× | 7× |
| Range at 150 mph | 25 mi | 159 mi | 177 mi | 31 mi | 192 mi | 214 mi |
The battery vehicle cannot finish the 22-minute minimum race. The hybrid races 6 to 7× as long from the same mass. Matching the hybrid at its 60% initial target would take about 455 Wh/lb at pack level, roughly 6× today's production density:
| Pack density | Race at 3,500 lb | Race at 2,700 lb |
|---|---|---|
| 73 Wh/lb (current production) | 7 min | 11 min |
| 150 Wh/lb (projected solid-state) | 15 min | 23 min |
| 250 Wh/lb (research chemistries) | 26 min | 38 min |
| ~455 Wh/lb | Matches the hybrid at 60% BTE | Matches the hybrid at 60% BTE |
For reference, the rule book's 10 kWh accumulator alone would power a full-intensity race for about 1 minute. It exists for transients and power-off landing, not propulsion.
The X1 airframe spans 15.5 ft and is 18 ft long in flight, and folds to 53 in high × 90 in wide for transport. Tiltrotor and lift-plus-cruise designs carry wing or rotor spans of roughly 35 ft and up and are built around heliport or vertiport operation. Only a compact ducted-fan layout can take off and land where people park.
| Architecture | Typical span | Built around |
|---|---|---|
| X1 ducted-fan VTOL | 15.5 ft (folds to 53 in × 90 in) | Driveway |
| Lift-plus-cruise eVTOL | ~35 ft and up | Vertiport |
| Tiltrotor | ~35 ft and up | Heliport or vertiport |
Maximum Freedom is the destination: personal air mobility where a driver's license is the only operator requirement. That requires a vehicle that fits the places drivers already use.
Racing is the most energy-intensive way to fly the vehicle: continuous high power, hard acceleration, and climb. It is the sizing case for fuel and efficiency. Race endurance at 2× hover power on 200 lb propane:
| Gross weight | Motor input hp | 50% BTE | 60% BTE | 67% BTE | Propane used in 44 min (60% / 67%) |
|---|---|---|---|---|---|
| 2,700 lb (goal) | 741 | 57 min | 1 h 08 min | 1 h 16 min | 128 / 115 lb |
| 3,000 lb | 868 | 49 min | 58 min | 1 h 05 min | 150 / 135 lb |
| 3,250 lb | 979 | 43 min | 52 min | 58 min | 170 / 152 lb |
| 3,500 lb (max recommended takeoff weight) | 1,094 | 39 min | 46 min | 52 min | 190 / 170 lb |
At maximum recommended takeoff weight the 60% initial target finishes the 44-minute maximum with about 2 minutes, roughly a lap, in hand; 67% leaves 8 minutes. Shorter races (the rule book allows 22 to 44 minutes) and real lap profiles, which are not at full power throughout, widen these margins.
Point-to-point air travel is a different mission. A consumer flight climbs out, cruises near the maximum-range speed, and lands, without sustained full-power maneuvering, and it can be planned more conservatively. The same vehicle burns substantially less fuel per hour traveling than racing:
| Vehicle | Propane burn, racing (2× hover) | Propane burn, 125 mph cruise | Racing burn vs. cruise |
|---|---|---|---|
| Initial (3,500 lb), 60% BTE | 259 lb/h | 155 lb/h | 1.7× |
| Initial (3,500 lb), 67% BTE | 232 lb/h | 139 lb/h | 1.7× |
| Goal (2,700 lb), 60% BTE | 175 lb/h | 120 lb/h | 1.5× |
| Goal (2,700 lb), 67% BTE | 157 lb/h | 107 lb/h | 1.5× |
| Speed | Initial 3,500 lb, 60% | Initial 3,500 lb, 67% | Goal 2,700 lb, 60% | Goal 2,700 lb, 67% |
|---|---|---|---|---|
| 50 mph | 80 mi | 89 mi | 118 mi | 132 mi |
| 75 mph | 119 mi | 132 mi | 172 mi | 192 mi |
| 100 mph | 148 mi | 165 mi | 204 mi | 228 mi |
| 125 mph | 161 mi | 180 mi | 208 mi | 233 mi |
| 150 mph | 159 mi | 177 mi | 192 mi | 214 mi |
| 175 mph | 145 mi | 162 mi | 168 mi | 187 mi |
| 200 mph | 129 mi | 144 mi | 144 mi | 160 mi |
Maximum-range speed is about 115 to 135 mph. Below 100 mph lift power dominates; above 175 mph drag power rises with the cube of speed and dominates. The duct array is the largest drag-cleanup target.
All candidates are complete electrical-output gensets at 1,500 hp continuous, including the generator. A turboshaft produces shaft power, not electricity, so turbine options carry a reduction gearbox, generator, and cooling that the OPFPLG does not need: its linear translator is the generator. Each genset is installed in the initial-vehicle airframe with the same tank and 200 lb of propane.
| Genset | Dry mass | Gross weight | BTE | Race endurance | Range at 150 mph | Range at 200 mph |
|---|---|---|---|---|---|---|
| OPFPLG, design point | 533 kg / 1,175 lb | 3,500 lb | 67% | 52 min | 177 mi | 144 mi |
| OPFPLG, initial target | 533 kg / 1,175 lb | 3,500 lb | 60% | 46 min | 159 mi | 129 mi |
| F1-derived V8 hybrid genset | 380 kg / 838 lb | 3,163 lb | 45% | 41 min | 129 mi | 101 mi |
| Heavy turboshaft + generator | 456 kg / 1,005 lb | 3,330 lb | 30% | 25 min | 83 mi | 66 mi |
| Twin light turboshafts + generators | 532 kg / 1,173 lb | 3,498 lb | 28% | 22 min | 74 mi | 60 mi |
At 533 kg the OPFPLG is not the lightest option. It still wins on every endurance and range figure, because BTE does most of the work. Only the OPFPLG completes the 44-minute maximum race at maximum recommended takeoff weight; the F1-derived genset, despite flying 337 lb lighter, falls 3 minutes short, and the turboshafts reach about half the distance. Every 50 kg of genset mass costs about 2 minutes of race endurance, while every 5 points of BTE is worth about 4 to 6 minutes.
F1 power units exceed 50% BTE while still paying losses a free-piston engine does not: valvetrain and piston side load. Large marine two-strokes exceed 50% with a crankshaft and an exhaust valve. Removing those losses, plus two-stroke operation, is the first-principles basis for the higher figure. 67% is the continuous design point; about 55% applies at peak power.
| Item | Rule book (x1racing.com/rule-book) | Used here |
|---|---|---|
| Fuel | Propane (LPG) only for racing | Propane, 46.35 MJ/kg LHV |
| Fuel quantity | 200 lb maximum onboard | 200 lb = 1,168 kWh chemical energy |
| Maximum recommended takeoff weight | 3,500 lb, ready to race | 3,500 lb initial-vehicle case |
| Minimum dry weight | ~2,500 lb | 2,500 lb dry + 200 lb propane = 2,700 lb goal case |
| Accumulator | 10 kWh nominal maximum | Within the limit; not credited for propulsion energy |
| Power | Unlimited | 1,500 hp continuous OPFPLG |
| Race duration | 22 to 44 minutes plus one lap | 44 minutes plus one lap |
| Lift fans | 4 locations, 8 counter-rotating motors, 44 in max diameter | 8 × 44 in; disc area 7.85 m² (84.5 ft²) |
| Forward-flight fans | 2 ducted fans, 36 in max diameter | 2 × 36 in; 1.31 m² (14.1 ft²) |
| Thrust-to-weight | Greater than 2:1 encouraged | Race power = 2× hover power |
| Power-off landing | Mandatory (accumulator plus regeneration) | Landing reserve carried in the accumulator |
The minimum dry weight is a goal and a lower limit on vehicle mass; early vehicles are expected to be well above it, which is why the 3,500 lb case is the baseline. The rule book does not specify rotor tilt. This analysis assumes fixed rotors and a 15° maximum forward pitch, which is conservative: a team using tilting fans would gain forward thrust and top speed.
| Item | Initial vehicle | Goal vehicle |
|---|---|---|
| Airframe and systems (structure, fans, motors, SI Boost, seats, accumulator) | 2,025 lb | 1,225 lb |
| OPFPLG genset, 533 kg (OPFPLG companion paper) | 1,175 lb | 1,175 lb |
| Propane tank (0.5 lb of tank per lb of propane) | 100 lb | 100 lb |
| Propane (rule-book maximum) | 200 lb | 200 lb |
| Gross weight | 3,500 lb | 2,700 lb |
| Dry weight | 3,300 lb | 2,500 lb |
Engine BTE as stated per case → generator and power electronics 90% → motor and inverter 98.5% → ducted fan, hover figure of merit 0.78. The accumulator is treated as lossless in steady state. Useful lift work as a fraction of fuel energy is 34.6% at 50% BTE and 46.3% at 67% BTE; the duct, motor, and generator terms are near their practical limits, so engine BTE is the remaining lever.
Ideal ducted-fan induced power, P = T1.5 / √(4ρA), divided by figure of merit. Hover power scales as W1.5: going from the 2,700 lb goal to the 3,500 lb maximum (30% heavier) costs 48% more power.
| Gross weight | Disc loading (lb/ft²) | Rotor shaft hp | Motor input hp | Per motor hp (×8) |
|---|---|---|---|---|
| 2,700 lb (goal) | 32.0 | 365 | 370 | 46 |
| 3,000 lb | 35.5 | 427 | 434 | 54 |
| 3,250 lb | 38.5 | 482 | 489 | 61 |
| 3,500 lb (max recommended takeoff weight) | 41.4 | 539 | 547 | 68 |
| Gross weight | 50% BTE | 60% BTE | 67% BTE |
|---|---|---|---|
| 2,700 lb (goal) | 1 h 54 min | 2 h 17 min | 2 h 33 min |
| 3,000 lb | 1 h 37 min | 1 h 57 min | 2 h 11 min |
| 3,250 lb | 1 h 26 min | 1 h 44 min | 1 h 56 min |
| 3,500 lb (max recommended takeoff weight) | 1 h 17 min | 1 h 33 min | 1 h 44 min |
| Engine BTE | Fuel-to-rotor chain | Hover at 3,500 lb | Hover at 2,700 lb | Reference |
|---|---|---|---|---|
| 25% | 22.2% | 39 min | 57 min | Small piston, poor part load |
| 30% | 26.6% | 46 min | 1 h 08 min | Light turboshaft at part load |
| 35% | 31.0% | 54 min | 1 h 20 min | Good aviation piston |
| 40% | 35.5% | 1 h 02 min | 1 h 31 min | Modern hybrid-vehicle engine |
| 45% | 39.9% | 1 h 10 min | 1 h 43 min | Best heavy-duty diesel |
| 50% | 44.3% | 1 h 17 min | 1 h 54 min | F1 power unit / large marine two-stroke |
| 55% | 48.8% | 1 h 25 min | 2 h 06 min | OPFPLG at peak overboost |
| 60% | 53.2% | 1 h 33 min | 2 h 17 min | Program initial target |
| 67% | 59.4% | 1 h 44 min | 2 h 33 min | OPFPLG continuous design point |
Drag area CdA = 1.2 m² (estimate). Shaft power = lift-fan power + drag power ÷ propulsive efficiency 0.80. Translational lift is credited conservatively: only half of the ideal momentum-theory reduction in induced power, induced power never below 70% of its hover value, and no reduction in profile power. Net effect: lift-fan power falls by roughly 6% at 50 mph, 17% at 100 mph, and a maximum of 23% at 150 mph and above.
| Component | CdA (m²) | Share |
|---|---|---|
| Body and canopy | 0.45 | 38% |
| Eight lift-fan ducts | 0.40 | 33% |
| Cooling and ventilation | 0.15 | 13% |
| Landing gear | 0.10 | 8% |
| Antennas, sensors, gaps | 0.10 | 8% |
Duct closure doors or fairings for cruise could plausibly take CdA toward 0.85 m². Above about 800 hp of horizontal-fan power, tip Mach on the 36 in fans, not installed power, becomes the practical speed ceiling.
Lift motors: 8 × 110 hp continuous / 140 hp peak covers 3,500 lb at 2× hover with margin. Horizontal motors: 2 × 300 hp continuous / 400 hp peak. Total continuous demand of about 1,480 hp matches the 1,500 hp continuous OPFPLG rating; the rule book places no limit on power.
This edition supersedes the April 2026 edition. Figures follow the X1 Hover, Forward Flight, and Powertrain Analysis v7, aligned with the published X1 Cup rule book and the OPFPLG companion paper.
A maximum-weight vehicle completes the 44-minute maximum race at the 60% initial target. Racing is the most energy-intensive way to fly; point-to-point travel is more conservative and burns roughly 30 to 40% less fuel per hour.