NASA’s SLS Booster gets a chill before QM-2 test
On June 28, 2016, the test area at Orbital ATK’s facility in Promontory, Utah, promises to get very hot… but not before the world’s largest human-rated solid rocket booster (SRB) takes a more than month-long chill-down. Engineers have begun cooling the five-segment solid rocket motor down to nearly 40 degrees Fahrenheit (4.44 degrees Celsius) in preparation for the second – and final – qualification ground test for the Space Launch System (SLS) booster.
As part of the qualification regime, NASA requires that the booster is tested at the extreme limits of its launch criteria. Last year’s successful Qualification Motor-1 (QM-1) test saw the booster tested at its upper operational temperature limit (90 °F / 32.22 °C). After this test, the motor will not be fired again until SLS’ first flight, currently slated for launch in 2018 from Kennedy Space Center.
These testing limits are not arbitrary guidelines; rather, they’re determined from examining years of accumulated data from human-rated flights. Temperature can have an impact on the characteristics of the propellant, and it’s important to understand how it may alter the performance of the booster. Due to the mass of the booster casing and the propellant, it will take more than a month for the internal temperature to reach the desired reading.
The structure housing the booster will be rolled away on the morning of the test, ensuring that the booster will retain as much of its cold temperature before the test as possible. Even so, engineers will need to chill the motor to a few degrees below the target temperature to account for the anticipated warm-up from the hot summer air.
Cooling the boosters requires something a bit more powerful than an average home air-conditioning unit. The test stand house utilizes three large air-conditioning units, each similar to what may be found in outdoor ice skating venues, and are capable of supplying a constant stream of air at 25 degrees Fahrenheit (–3.89 degrees Celsius) into the structure.
Temperature sensors in and around the booster will measure the propellant’s temperature, allowing engineers to predict how long it will take the booster and propellant to reach the desired temperature.
Matt Bevill, deputy chief engineer in the SLS Boosters Office at NASA’s Marshall Space Flight Center, discussed the booster’s temperature extremes: “Propellant temperature shouldn’t be mistaken for the temperature of the booster when it’s fired. It may be conditioned to 40 degrees Fahrenheit, but once it fires, it is extremely hot – about 6,000 degrees Fahrenheit (3,316 °C). That’s hot enough to boil steel.”
In fact, much of the sand around the aft-end of the booster will melt into glass from the intense heat of the booster’s exhaust plume, and the test area will need to cool for an extended period before it’s considered safe for personnel to approach.
As with last year’s QM-1 test, the QM-2 test will be a full-duration two-minute burn. Orbital ATK will measure more than 530 data channels in support of 82 design objectives. Beyond validating performance values at the lower end of the booster’s propellant temperature range, the motor will also incorporate SLS’ command and control system, mimicking flight-like ignition and nozzle actuation.
Shortly after conclusion of the test, the test data will be sent to Marshall Space Flight Center and integrated into flight simulation software, allowing engineers to perform further testing, albeit in a virtualized environment.
Though similar in appearance to the Space Shuttle’s solid rocket boosters (SRBs), the motors for SLS have an additional segment, which allows for more propellant. This extra propellant increases the rated thrust from 2.8 million pounds-force (12,000 kN) on the Shuttle to 3.6 million pounds-force (16,000 kN) on the SLS. Other design changes from the Shuttle’s SRBs are the elimination of an asbestos-based casing insulating liner, upgraded avionics, are 1,900 pounds (860 kilograms) lighter, and will not be recovered after splashdown.
Curt Godwin has been a fan of space exploration for as long as he can remember, keeping his eyes to the skies from an early age. Initially majoring in Nuclear Engineering, Curt later decided that computers would be a more interesting - and safer - career field. He's worked in education technology for more than 20 years, and has been published in industry and peer journals, and is a respected authority on wireless network engineering. Throughout this period of his life, he maintained his love for all things space and has written about his experiences at a variety of NASA events, both on his personal blog and as a freelance media representative.