SPRINT Hot Fire Test - December 15th and 16th, 2025
The December 16–17 hotfire campaign was run in roughly -5°C ambient conditions and was dominated by cold-soak and schedule delays that ultimately limited the team to a single hotfire attempt with reduced performance and a minor post-test fire during purge.
Timeline and delays
The team arrived around 6pm on Dec 15, unloaded equipment, and then left for dinner before returning the next morning to start setup at 7:45am. A noticeable amount of time was lost on basic setup logistics, including ethernet runs where two cables were too short and the connector to join cables was not on hand, forcing use of the remaining cable on the spool. Additional delays came from a missing required CGA fitting (borrowed on-site), resulting in the nitrous supply tank being left outside in the cold for about 2.5 hours in the cold, plus roughly a 2-hour delay tied to livestream coordination.
Tank heating and propellant conditions
Because the nitrous supply cylinder was too cold, a tank heater had to be used, and an initial attempt to insulate it with cotton blankets led to minor scorching before switching to a fiberglass blanket that worked effectively. Even with the fiberglass insulation, it took about 1.5 hours to bring supply pressure up from roughly 400 psi to roughly 800 psi, largely because the heater was not applied immediately in the cold conditions. Critically, the rocket-side nitrous/vehicle tank was not heated, allowing the oxidizer to chill significantly prior to firing.
Hotfire results and lessons learned
During the hotfire, the rocket tank nitrous pressure fell to about 350 psi, which produced substantially lower thrust than expected. The colder nitrous altered feed conditions such that flow through the injector increased, the run went oxidizer-rich, and fuel was not fully depleted; when the nitrogen purge was commanded, residual fuel was pushed out and ignited, causing a small fire. Key takeaways were that vehicle-side propellant temperature management is just as important as supply-cylinder heating, the heater needs to go on immediately in cold weather, and all small but mission-critical accessories (fittings, cable joiners, correct-length ethernet) must be verified before departure to avoid consuming test time. Furthermore, the nitrogen purge should be performed with the fuel valve closed.