In September the Airbus Perlan 2 glider achieved a new altitude record of 76,000ft. BILL READ, FRAeS reports on the on-going project which not only intends to fly higher than any other manned winged aircraft but also to learn new insights in the Earth’s atmosphere, the ozone layer and global warming.
Troposphere, stratosphere and mesosphere. (UCAR Center for Science Education)
A well-known weather phenomenon used by glider pilots to keep aloft is to surf on ‘mountain waves’. In the same way as a river forms waves when it flows over a rock, strong winds crossing a mountain range will make standing waves in the air. Such waves need particular conditions to be created – if the winds are blowing more than 15kt sideways over the mountain and the atmosphere is stable, then waves will form on the lee side of the mountains.
The aim of the Perlan 2 project is to ride the polar vortex mountain waves up into the stratosphere. (Perlan Project)
Using the upward moving part of this wave system to climb, gliders can ascend as high as 10km up to the top of the troposphere where the cold air of the mountain wave encounters warmer air at the boundary with the stratosphere and cannot rise further. However, during the 1990s, NASA test pilot, Einar Enevoldson and Dr Elizabeth Austin discovered that in sub-polar regions in winter, mountain waves could extend beyond the troposphere and well into the stratosphere. This is because of high altitude winds which exist at the outer boundary of the polar vortex in winter known as the stratospheric polar night jet which can exceed 260kt. In conditions where the stratospheric polar night jet aligns with the lower-level jet stream over mountains, it can create winds which increase with altitude through the tropopause and upward to 100,000ft creating ‘stratospheric mountain waves’ which ascend way beyond normal mountain waves. Given that such waves exist, would it be possible to use them to take a glider up to heights previously thought impossible? To discover the answer, the Perlan project was created.
World record goal height chart. (Perlan Project)
The origins of the project began in 1992 when Enevoldson saw new LIDAR images of standing mountain waves west of Kiruna, Sweden on an office wall during a visit to the DLR research establishment in Germany. Over the next six years Enevoldson collected evidence on the location, prevalence, and strength of stratospheric mountain waves. From 1998 the data analysis was expanded by Dr Austin and the significance was realised of the stratospheric polar night jet in propagating high altitude standing mountain waves. Meanwhile, researchers at the NASA Dryden Flight Research Center were looking at the flight dynamics and aerodynamics of sailplane flight up to 100,000ft.
In 1999 air adventurer Steve Fossett heard that Enevoldson was seeking funding to build a glider to test the concept of riding stratospheric mountain waves and asked to join the project. The Perlan 1 glider was created using a modified Glaser-Dirks DG-500 motorised glider with the engine equipment replaced liquid oxygen tanks and Li-SO2 batteries. The glider was also fitted with faceplate heat controllers, high altitude stabilised parachutes and full pressure suits loaned from the US Air Force. The name Perlan, which means ‘pearl’ in Icelandic, was chosen for the project after the pearlescent nacreous clouds that appear in the uncharted winter polar stratosphere.
In 2002 Enevoldson and Fossett began the first test flights of Perlan 1 over the Sierra Nevada mountains of California where then reaching heights of over 42,000ft. Flights were also carried out in New Zealand but failed to reach the stratosphere. In 2005, further flights were made over Argentina where, after some technical hitches with the pressure suits, on 29 August Enevoldson and Fossett climbed for four hours to reach a new world altitude record of 50,671ft – the first ever glider flight into the earth’s stratosphere. To commemorate this achievement, the Perlan I is now on display at the Seattle Museum of Flight.
Following the proof that high level mountain waves can be used to lift a glider into the stratosphere, Steve Fossett agreed to fund a follow-on mission: to build a sailplane with a pressurised cabin to fly up to 90,000ft. Work was carried out on the structural and aerodynamic design of the aircraft but, sadly, Steve Fossett died in 2007 and funding to continue the project was provided by individuals and partners in the US and Australia. After much preliminary design work was carried out by Greg Cole of Windward Performance, responsibility for the manufacture of the Perlan 2 was given to Oregon-based aviation research, design and development company RDD Enterprises. In 2010 Dennis Tito joined the mission as a pilot and major funder, as did world soaring record holder Jim Payne, who joined as chief pilot. In 2014 the Airbus Group agreed to become the title sponsor and to provide funding for completion of the aircraft, flight testing and the altitude flights. The mission was renamed the Airbus Perlan Mission II. In addition to the Airbus Group, the project is supported by a number of other sponsors, including Weather Extreme, United Technologies and BRS Aerospace.
The aircraft was completed in the summer of 2015 and was first flown on 23 September that year. This was followed by six months of flight testing in Minden, Nevada. In 2016 the team relocated to El Calafate in southern region of Patagonia, Argentina, a region where the weather in the Andes mountains in the two months between mid-August and mid-October can trigger the conditions needed to create stratospheric mountain waves. The aircraft was towed up to a height of 5,000ft for an hour over the Andes and positioned to ride stratospheric waves from behind the range.
Picture taken from Perlan 2 at 76,000ft. (Perlan Project)
In 2016, Perlan 2 achieved an altitude of 52,221ft but the atmospheric conditions were never quite right to achieve the higher altitudes desired. The team returned to Argentina in 2017 and achieved a height of 52,172ft on 3 September. However, this year the conditions were more favourable and in August and September 2018 the team began to set new records as the glider soared to higher and higher altitudes. On 26 August, Perlan 2 reached a record-breaking 62,473ft, followed by 65,000ft on 28 August and 76,124ft on 2 September – higher than the 73,000ft altitude achieved by the US Air Force’s U-2 spy plane.
Perlan 2 is not your average glider but is essentially a spacecraft with an 84-foot wingspan. Unlike some gliders, Perlan 2 is not built for speed but for climbing. The glider is optimised for high altitudes which means that the aircraft will not perform as well as a typical sailplane with a similar wingspan at low altitudes (see graph). One of the 2018 flights was to test the performance of the glider at the higher speeds that would be needed to keep it aloft at heights above 90,000ft
Despite having no engine, the glider’s true flight speed in the strong winds encountered in the stratosphere could exceed 400mph. Windward Performance designed the aircraft to be flutter-safe at very high air speeds and also to be strong enough to cope with potentially heavy turbulence that could be encountered at 90,000ft. To fly in the stratosphere, Perlan 2 must be able to fly in air less than 3% of normal density and at temperatures of –70°C conditions similar to those on Mars. Instead of using pressurised suits as in Perlan 1, the two pilots will be inside a pressurised cabin with much smaller windows than on a conventional glider. The carbon-fibre-sandwich construction of the aircraft will provide good insulation against the cold but there will be enough capacity in the battery for the pilots to plug in electric heated clothing. The cabin is fitted with air re-cycling systems and other life-support systems similar to those used on a spacecraft. The crew will breathe pure oxygen provided by a rebreather system similar to those used in scuba gear. Two parachutes will also be carried in case an emergency descent is needed.
The Perlan Project has the number of goals, summarised as: Exploration, Innovation and Inspiration. In addition to pushing back the frontiers of flight by soaring into near space, the Project is also taking advantage of its unique position in the sky to advance the boundaries of scientific knowledge. Currently, very little is known about the stratosphere, since no aircraft can remain at that altitude long enough to gather data. However, the Perlan 2 sailplane will be able to either traverse or remain relatively stationary in a particular portion of the stratosphere for several hours.
The Perlan 2 is fitted with a modular bay for scientific instrumentation which will enable precise recording of air mass motion, together with collection of air samples for analysis – which will not be contaminated by engine emissions. One area of research will focus on the dynamical and microphysical processes at work in the upper stratosphere, including the conditions needed for the formation of high-level aerosol- nucleated ice clouds. Atmospheric scientists are keen to learn more about the structure and intensity of vertical waves and their characteristics. Using this data, it is hoped to gain an understanding of the interaction between stratospheric mountain waves and the polar vortex and their effects on the energy balance of the atmosphere, as well as the effect of mountainous terrain on larger scale jet/front systems. This data is important, as current models used to predict climate change are based on the assumption that there is little interaction between the lower troposphere layer of the atmosphere and the stratosphere. However, it is now established that the two layers exchange heat, air masses and chemicals and it is hoped that data from Perlan 2 can be used to help create new, more accurate climate models.
Another area of scientific interest is ozone levels which are most concentrated between 80,000-100,000ft. Current research believes that the banning of the use of chlorofluorocarbons (CFCs) used in aerosols by the Montreal Protocol of 1987 has succeeded in stopping and possibly reversing ozone depletion. Whether this assumption is true can be proved by taking direct air samples from the stratosphere. Air samples can also show the concentration of chorine-based chemicals at high altitudes. The movement of ozone and other constituents in the stratosphere can also affect aircraft flying though them, as the constituents can damage or adversely affect engine performance.
In addition, aerodynamic experts are hoping to learn more about the performance of aircraft in the thin air of the upper atmosphere – knowledge that could also be applied to extra-planetary space missions.
Airbus is also keen to promote the Perlan mission to inspire future generations to pursue careers in aerospace by studying mathematics, science and technology. The project is working with Universidad Maimonides in Buenos Aires and US-based Teachers in Space initiative. Some of the experiments being carried out on the aircraft originated from school and university classrooms which were invited to submit entries to Perlan CubeSat contest in 2016 in which experiments were fitted inside a SpaceTec CubeSat kit. Winning entries include experiments to measure the presence of particles and fungal spores in the air, the effect of air from the stratosphere on plants (which could be applicable to terraforming on Mars),searching for rare elements, such as the noble gas Xenon, which could be harvested for use in space travel or other industries and the detachment of metal ‘tin whisker’ filaments that form on metal solders due to flight induced stresses.
The project is also seeking volunteers to develop classroom modules for pre-school up to graduate levels and to create videos and scientific data online which could be accessed by the public. It is hoped that a number of masters and PhD students will write meteorological, aviation and aeronautical engineering theses on the project.
Touching the void
The latest series of Perlan flights in Argentina ended on 12 September. However, the project is not over. If successful, a further stage is planned to build a glider capable of flying at heights of up to 100,000ft. When Perlan 2 was being designed, Einar Enevoldson wanted a glider capable of flying up to 100,000ft. However, Greg Cole of Windward Performance said that, while designing a sailplane for 90,000ft was relatively straightforward, building one for 100,000ft was more complicated and expensive. Phase 3 of the Perlan project involves building a glider fitted with transonic wings to cope with increased flight speeds. Flights of the new glider are provisionally planned for May 2019 and may also be extended to exploring the Polar Vortex in the northern hemisphere.
For more information on the Perlan mission, see: www.perlanproject.org/ https://twitter.com/perlanproject