Boeing Discloses Cause Of Starliner Parachute Anomaly

Boeing’s CST-100 Starliner’s four launch abort engines and several orbital maneuvering and attitude control thrusters ignite in the company’s Pad Abort Test, Nov. 4, 2019. Photo: NASA

NASA’s head of Commercial Crew Kathy Lueders and Boeing’s John Mulholland, V.P. and Program Manager, Commercial Crew Programs, Space Exploration, held a press conference today, Nov. 7, at 1:30 PM ET to discuss the results of last Monday’s pad abort test of Boeing’s Starliner commercial spacecraft. While the test is nearly every respect was exactly as desired, or what is called “nominal” in aerospace circles, there was one anomaly–only two of Starliner’s three main parachutes deployed.

Deployment of drogue chutes during Boeing Starliner’s pad abort test. Image: NASA

One issue that Boeing discussed the discharge of hydrazine from the Starliner service module, as seen in the orange cloud surrounding Starliner after it rotated into an aft-forward flight position. Hydrazine is extremely corrosive and caustic. There had been some speculation on social media that the orange cloud had been the cause of one of Starliner’s three parachutes not deploying. It turns-out that the hydrazine cloud was expected and played no part in one of Starliner’s main parachutes failing to deploy.

Every liquid propulsion system, really every system transporting liquid, has what is called a dribble volume. In rocket propulsion, the dribble volume is that volume of propellant that remains between a shut isolation valve and an engine’s main reactor or manifold. Think of a hose used for watering that, subsequently to its bib, or faucet, valve shut-off still has water. Dribbled propellants will continue the combustion process at much reduced pressure and therefore efficiency after the cutoff by the isolation valve1.

Hydrazine cloud due to dribble volume from Starliner service module. Image: NASA

Both Boeing and NASA were clear that the hydrazine cloud around the Startliner service module after the module’s engines shutdown was not the cause of one of the spacecraft’s main chutes not deploying.

Boeing’s Mulholland went to lengths to praise the Starliner parachute rigging team for taking the initiative to find the root cause of the parachute anomaly and doing so within a day of the test. While the riser team were waiting to gain access to the test spacecraft after the test, they used photos and video to begin to zero-in on the possible cause. By the time they were able to reach the spacecraft, according to Boeing’s Mulholland, the team had a good idea where they needed to look to verify their guess. And the team turned-out to be correct.

So what then was the cause? The Boeing Starliner has three pilot parachutes, the job of each being to drag-out, or deploy, a main parachute. Each the pilot is connected to the spacecraft via a kevlar riser. On each pilot kevlar riser there is a loop to which the main chute riser should have been attached via a pin. The pin connecting one of the pilot riser and main parachutes was not properly attached. As the pilot chute extended, it did not have a solid enough connection to drag out the main. The pilot parachute riser and linkage are covered by a sheath to protect the pilot parachute from abrasion, making a vision inspection difficult after installation.

Both Boeing and NASA went to lengths to not classify this as a parachute failure. An issue like this has never occurred because Boeing has never had a parachute test issue. Boeing has already gone out to inspect all of the 18 linkages on the Starliner being prepared for the Orbital Test Flight (OTF) next month. Going forward, Boeing and NASA will be developing new procedures to inspect and test the pilot parachute riser connections to the main parachutes. In particular, Boeing will not just be doing inspections but also possible pull tests and others.

Starliner landing at end of pad abort test. Image: NASA

For crewed missions, NASA will also be inspecting the parachute installations to ensure Monday’s anomaly doesn’t reoccur. Monday’s parachute anomaly will inform how NASA will work with Boeing to put together a joint action program to catch this.

A bright spot in the pad abort test parachute anomaly is that Boeing now has data on how its spacecraft and its airbag landing system react to a two parachute landing. Boeing’s Mulholland stated that the airbags handled the landing loads well.

All of the other test items, such as separation planes and sequences, were down the middle.

Footnotes:


  1. Modern Engineering for Design of Liquid-Propellant Rocket Engines, pp. 106 ↩︎

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