The Long Gap Between SpaceShipTwo Powered Flights 3 & 4 Explained

Nitrous nylon engine test on Jan. 16, 2014. (Credit: Ken Brown)
Nitrous nylon engine test on Jan. 16, 2014. (Credit: Ken Brown)

One of the most interesting aspects of the National Transportation Safety Board (NTSB) investigation into the SpaceShipTwo accident is the ability to find out what was actually going on behind the scenes during the vehicle’s development and testing. Reality is at times at sharp variance from what was being said publicly at the time.

One of the more fascinating NTSB documents is this one: Operations Factual – Attachment 24 Flight and Test Reviews. It is a detailed response from Scaled Composites to NTSB questions about the modifications the company made to SpaceShipTwo between powered flight no. 3 on Jan. 10, 2014 and the failed powered flight 4 on Oct. 31 of last year.

The excerpt below has some information redacted for proprietary reasons, (Worry not, I’ll fill in the blanks below.)  But, it still provides a good description of the modifications Scaled made to the ship before and after Virgin Galactic announced a switch from a rubber to a nylon hybrid motor at the end of May 2014.

Preparation for PF04 began immediately following PF03 which occurred on January 10th 2013 approximately 8.5 months prior to PF04. The goals for PF04 morphed a couple of times driven by dependency on the rocket motor. PF04 was initially envisioned as a longer duration burn with the SNC HTPB hybrid. To this end, the vehicle was being configured with extra [REDACTED] tanks in the wings [REDACTED]. This work was wrapping up in the early spring of 2014. However, due to continued issues with that path, work was ceased on that effort by June1.

The parallel Scaled Nylon fueled hybrid was made the primary path in May‐June of 2014. This required repurposing the extra wing [REDACTED] tanks for [REDACTED] in addition to retrofitting, modifying and replacing portions of the [REDACTED]. Additional [REDACTED] were also integrated. This modification cycle along with associated rocket motor development and qualification testing paced the program from May through PF04 in October of 2014.

Integration of the tanks was done in concert with other major mods to the airframe. In an effort to maintain test team proficiency, and as an effort to efficiently utilize the schedule, a strategic decision was made to perform a functional check flight of SS2 with the new modifications. Though much of the new [REDACTED] was integrated, the system was flown inert [REDACTED]. An FRR was held specific to the structural other airframe mods. The GF029 FRR (Glide Flight 29 Flight Readiness Review) was held 7‐17‐2014.

1Tank integration was wrapping up and material was being gathered for an FRR focused on the addition of tanks [REDACTED]. This FRR also would have covered qualification testing of the full duration HTPB motor. These efforts started and stalled out in mid‐May.

OK, so what was redacted? Well, sources say the additional tanks were originally for helium to stabilize the rubber engine burn. After the switch to the nylon engine, the tanks were re-purposed for methane that was needed to make sure the new engine started properly. The new system also injected helium at the end of the burn to stabilize the shutdown.

SpaceShipTwo's propulsion system using the nylon engine. (Credit: Scaled Composites)
SpaceShipTwo’s propulsion system using the nylon engine. (Credit: Scaled Composites)

The work involved a whole lot of changes to the vehicle that added risk and failure modes to a hybrid engine that was supposed to be fairly simple to operate. It’s little wonder that three flight tests were required in July through October to test out all the modifications involved.

This document provides further proof that Virgin Galactic’s claims that the motor change was largely a switch of interchangeable fuel grain casings with some minor plumbing adjustments were false. They misled the media and their own ticket holders on the complexity of the changes and the risks involved.

The attachment also provides some details about the qualification of the new nylon motor, which officials claimed was thoroughly tested on the ground before it flew on powered flight no. 4. The actual full qualification tests are called Qual 1 through Qual 3 in the list below.

Tests critical to PF04 were the rocket qualification tests. Prior to every new series of flights with a rocket motor Scaled performs a “qualification” hot fire series on the ground. For the series of flight tests starting with PF04, the key variables for the rocket motor consumables (Nitrous, were enveloped and the repeatability of the system was demonstrated. It was Scaled policy to hold a Test Readiness Review (TRR – Ground test equivalent of an FRR) for every ground hot fire. The following is a quick summary for the TRR’s (Test readiness reviews) and tests that comprised the rocket motor test program in preparation for PF04.

Rocket Motor Qualification Firings & TRR’s4

  • Plastic Motor 21 TRR, 9‐18‐2014
  • Qual 1 – PM21, 9‐22‐2014
  • PM‐22 TRR, 9‐30‐2014
  • Qual 2 – PM22, 10‐2‐2014
  • PM‐23 TRR, 10‐8‐2014
  • Qual 3 ‐ PM23, 10‐10‐2014

Major topics:

  • Manufacturing/ structural consistency
  • Repeatability of motor performance
  • Adherence to qual pass/fail criteria

A final Delta PF04 FRR was held as a clean‐up review following the rocket motor qualification effort, and completion of the majority of the open action items from the PF04 FRR. The Delta FRR was held on October 27th, 2014.

4 Attendees to rocket TRR’s included mostly rocket team, T1b program and Scaled Management. Although not required, on occasion some of the flight crew members or other T1b participants would participate. Regardless, it is included here for completeness.

So, there were only three full qualification tests of the new nylon (plastic) motor before it was used in flight last Halloween. Whether that is sufficient ground testing on the ground to be considered thorough is an interesting question.

The tests appear to have involved the 21st through the 23rd plastic motors. This presumably means there were 20 plastic motors were fired before the qualification tests. The document does not contain any information about these hot fires.

Scaled Composites’ RocketMotorTwo Hot Fire Test Summaries web page does have information about tests. It’s interesting because the three qualification tests of the plastic motor are listed as the 55th, 56th and 57th “full scale flight design RM2 hot-fire” tests. That means there are 34 other tests listed in the summaries that in all likelihood used the rubber motor.

The summaries page does not distinguish between the types of motors being tested. So, it is impossible to tell for most of these hot fires what exactly is being tested. In a couple of cases, Sierra Nevada Corporation is mentioned; that company only worked on the rubber engine.

There was also a hot firing in May 2013 that sources confirmed was a test by Scaled Composites of its plastic motor.

Fire: 28
Date: 17 May 13


Perform hot-fire of off-nominal, non-flight configuration motor at Scaled’s test facility.

  • Safety systems evaluation
  • Test stand evaluation
  • Data Acquisition system evaluation
  • Rocket Motor Controller performance’
  • Pressurization System Controller performance
  • Rocket system performance
  • Valve / Injector / Igniter evaluation
  • Fuel formulation evaluation
  • CTN structural evaluation


Firing at Mojave test site by Scaled Composites of a non-flight experimental rocket motor in which flaws had been intentionally introduced to improve knowledge of different design components. Tested experimental grain to destruction. This unique test, which was necessary to perform during the test program, successfully collected data for several key safety systems.

The test blew the fuel casing and the attached nozzle some distance across the desert while damaging the nitrous oxide tank beyond repair and destroying the test stand. Although Scaled tried to put a big happy face on the test, nobody sets out to destroy an expensive test stand.