Fwd: Aviation Week — SpaceShipTwo Goes Back To Improved Rubber Fuel

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From: "Gary Johnson" <gjohnson144@comcast.net>
Date: October 12, 2015 at 7:28:20 PM CDT
To: "Gary Johnson" <gjohnson144@comcast.net>
Subject: FW: Aviation Week — SpaceShipTwo Goes Back To Improved Rubber Fuel

 

SpaceShipTwo Goes Back To Improved Rubber Fuel

Oct 12, 2015 

Guy Norris | Aerospace Daily & Defense Report

 

 

Virgin Galactic

LAS CRUCES, NEW MEXICO and LOS ANGELES—Virgin Galactic confirms that following a series of successful hot-fire rocket tests, it has reverted to an improved form of the original rubber-based fuel for powering the company's suborbital SpaceShipTwo (SS2), the second version of which is nearing completion in Mojave, California.

Virgin Galactic—which plans to resume flights of SS2 in 2016—used a more recently developed and more energetic nylon-based fuel for last October's ill-fated fourth and final powered flight (PF4) of the prototype SS2, during which the vehicle broke up following the co-pilot's earlier-than-planned unlocking of the feathering tail mechanism.

But company officials recently hinted they were considering a change back to the rubber-based hydroxyl-terminated polybutadiene (HTPB) fuel used in the initial test flights. The switch was confirmed by Virgin Galactic CEO George Whitesides at the International Symposium for Personal and Commercial Spaceflight (ISPCS) here in Las Cruces. 

Perfecting the propulsion system has remained the most challenging aspect of the suborbital spaceplane project. Virgin Galactic based its original power plan on a scale-up of the basic HTPB-fueled hybrid motor used in the original X Prize-winning SpaceShipOne, but in 2014 announced it was opting to use a polyamide-based grain to achieve a step change in thrust and propulsion consistency. Meanwhile, parallel development of the HTPB fuel continued through 2014, and by the time PF4 occurred, so much improvement had been made that Virgin decided it would revert back to the original fuel.

According to Whitesides, the change not only provides adequate power but results in a lighter and simpler installation. The switch to a polyamide-based grain involved changes to the pressurization system that feeds liquid nitrous oxide into the solid fuel of the rocket motor. These included additional piping to improve initial combustion as well as adding helium to stabilize the motor toward the end of the burn.

Commenting on the continuing development of the hybrid rocket motor, Virgin Galactic chief pilot Dave MacKay says, "The extended downtime since the accident has given the Virgin Galactic rocket motor team time to take over development of the commercial hybrid motor. We are pleased to report the new design demonstrates superior thrust and beautiful, smooth, beyond-full-duration burns at a lighter system weight and with less system complexity."

MacKay—who was updating progress towards a return to flight at the Society of Experimental Test Pilots symposium in Anaheim, California, in late September—described several other vehicle changes that have been made as a result of lessons learned from the loss of the prototype. Virgin Galactic has completed an in-depth systems review of both the SS2 and WK2 vehicles "with a focus on the human factors using a different mindset," MacKay said. "Modifications are being made to improve human factors to the cockpit hardware and software. We are running a safety interlock to prevent both a premature unlock or premature locking of the feather."

Changes also include a safety catch that has been added to the landing gear handle to preclude inadvertent actuation during critical flight regimes. "We have also carried out a thorough review of crew resource management [CRM] procedures and standard callouts. Despite the higher workloads and split responsibility allocation there remains some irreversible processes that must be cross-checked.

"Prior to future flights, the flight crew will unfasten their emergency oxygen handle from its Velcro housing to allow for one-handed operation. Plans are in work to practice powered-flight CRM with representative displays in a centrifuge under boost accelerations, and prior to test flights a final integrated simulator session will consist of multiple nominal full-mission profiles for the benefit of both the crew and control room," he adds.

Production staff at Virgin Galactic affiliate The Spaceship Company (TSC) are "working three shifts" as they near completion of SS2 serial number two, Whitesides says. Although largely unchanged from the first vehicle, the new spacecraft will have slightly larger horizontal tails to counter the tail-stall condition experienced during tests in September 2011. "We had already planned to go to larger horizontal stabilizers on subsequent serial numbers and we have made that change with No. 2," MacKay says. Following the incident, which occurred on glide flight 16, SS2 was modified with larger, one-piece, horizontal strakes.  

Assembly of the second suborbital vehicle was brought forward following the loss of the prototype SS2 in last October's test accident. Since the new aircraft sat on its own wheels in May, "we have made a lot of progress, largely on systems installation. The airframe and propulsion system is sound and we require very few changes since our accident," Whitesides says. 

Recent accomplishments include fitting the main oxidizer tank. "We have not done the final bond of that yet but the tank fits well. We are now working to integrate all the systems," Whitesides says, noting "this is the first vehicle that TSC has built." The prototype SS2, as well as the WhiteKnightTwo (WK2) carrier aircraft, were built and tested for Virgin Galactic by Scaled Composites. 

"The next steps will be installing systems along the wings and finishing the avionics inside the cabin as well as putting in the seats," Whitesides says. "We've been working on developing better seats. We just now finished a couple of weeks ago new versions of our pilots seats which will be integrated into the vehicle. Another important milestone we just finished last week was our fatigue test of the cabin. Over the past year and a half we have done a 10,000-pressure-cycle test of our cabin. That's important because we are building a reusable spacecraft, and we were very happy to get through that."

"When we return to flight, we will do so with a vehicle that's not just ready to glide but will also be able to transition quickly to powered flight-tests," MacKay says. "The upcoming flight-test program will be a joint effort of the TSC and Virgin Galactic and will consist of a mix of regression testing and envelope expansion."    

Meanwhile, WK2 is also close to returning to flight following an annual inspection. The aircraft will be used to carry SS2 for initial captive-carriage flights early in 2016, leading to glide and powered flight-tests later in the year.

 

Copyright © 2015, Penton.  All rights reserved.

 

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