Aircraft parabolic flights provide repeated periods of up to 20 seconds of reduced gravity during ballistic flight manoeuvres, preceded and followed by 20 seconds of 1.8 g. Such flights are used to conduct short microgravity investigations in physical and life sciences, to test instrumentation and to train astronauts before a spaceflight.
Since 1984, ESA's Microgravity Projects Division has organised 20 parabolic flight campaigns using three different types of aircraft. More than 1700 parabolas have been flown, representing nine and half hours of microgravity in slices of 20 seconds, or equivalently, six low Earth orbits. A total of 235 experiments have been performed using this unique microgravity tool.
Aircraft parabolic flights are a useful tool for performing short-duration scientific and technological experiments in reduced gravity. Their greatest value is that they allow verification tests of experiments to be conducted before they are flown in space, in order to improve the experiment's quality and success rate, and after a space mission to confirm or invalidate the sometimes conflicting results obtained from space experiments.
For these purposes, ESA's Microgravity Projects Division has organised 20 para-bolic flight campaigns since 1984, within the framework of the ESA Microgravity Programme.
The main advantages of parabolic flights for microgravity investigations are:
Microgravity is created in free-falling carriers when the sum of all forces acting on the carrier, other than gravity, is nil or strongly reduced. Scientific and technological experimentation in microgravity can be conducted in different carriers either on Earth or in orbit. Generating a weightless environment on Earth imposes practical constraints on the level and duration of the microgravity required for investigations and on the cost at which this environment is obtained.
Depending on the carrier's initial velocity, the free-fall trajectory is either linear vertical for drop facilities, parabolic for aircraft flights and sounding rockets, or circular and elliptic for orbital platforms. The characteristics of the different types of microgravity carriers are compared in Table 1.
Aircraft parabolic free-fall flights attain reduced g levels, in the range of 10 to - 2 to 10 to - 3 g, with the added advantage of allowing human intervention while the experiment is in progress in reduced gravity. The total available working volume in which reduced gravity is attained can be as large as the aircraft cabin.
Space missions with sounding rockets and orbiting platforms need relatively long preparation times and must be considered for long-duration experiments. In comparison, aircraft parabolic flights offer the possibility of performing microgravity experiments within 20 to 25 seconds, with a short turnaround time of a few months. Their use must be considered as complementary and preparatory to space missions.
Aircraft parabolic flights are also the only sub-orbital microgravity carrier that provides the opportunity to perform medical experiments on human subjects in real weightlessness, complementing studies conducted in simulated weightlessness, such as immersion and bedrest studies.
Table 1. Characteristics of microgravity-providing carriers
Parabolic flights provide investigators with a laboratory for scientific experimentation in which the gravity levels change repeatedly, with successive short periods of microgravity and high gravity. ESA and the scientists invited by ESA undertake such flights in pursuit of two objectives: scientific and technical.
From a scientific point of view, the objectives are as follows:
Scientists have identified this last aspect as being one of the major advantages of the experimental environment attainable during aircraft parabolic flights for investigations in several disciplines such as combustion and human physiology.
Furthermore, for scientific experiments that will be performed during space missions, the following goals can be pursued during parabolic flights:
With respect to the preparation of experiment hardware for manned or automatic space missions, the following objectives can also be achieved during parabolic flights:
Since 1984, ESA has used three different airplanes to conduct its parabolic flight campaigns:
The characteristics of the three types of aircraft and the locations used during ESA campaigns are compared in Table 2. The ground infrastructure includes a hangar room for equipment set-up and check-out and office rooms for experiment preparation.
The three airplanes have similar parabolic flight profiles and provide a microgravity environment with similar characteristics. The microgravity environment is created in an aircraft flying the following manoeuvres (illustrated in Figure 2 for the Caravelle):
These manoeuvres can be flown either consecutively in a roller-coaster fashion for the KC-135 and the Ilyushin, or separated by intervals of several minutes for the Caravelle, the KC-135 and the Ilyushin. The duration of the intervals between parabolas is determined prior to the flight to give the investigators enough time to change the experimental set-up. A typical flight lasts about two and half hours, allowing for 20 to 40 parabolas to be flown, depending on the requested interval between parabolas. During ESA campaigns, 30 parabolas are usually flown in sets of three with two-minute intervals between parabolas and with four to five minutes between sets of three parabolas.
Figure 3 shows typical acceleration levels for the Caravelle aircraft Z-axis (floor to ceiling direction), measured during a parabola with ESA's micro-accelerometers strapped to the cabin floor structure. During the reduced gravity period, a transitory phase of about 5 s appears first, with variations of about 10 to - 1 g in the Z direction, followed by a period of approximately 20 s with acceleration levels of a few10 2 g, while accelerations along the aircraft longitudinal X-axis (aft to front) and transversal Y-axis (right to left) are less than 10 to - 2 g. Similar values were measured for the KC-135 and the Ilyushin.
The residual accelerations sensed by experimental set-ups attached to the aircraft floor structure are typically in the order of 10 to - 2 g, while for an experiment left free floating in the cabin, the levels can be improved to typically 10 to - 3 g.
For the three airplanes, the piloting is done manually along the X-axis by adjusting the engines thrust, and along the Z-axis using visual references provided by a coarse (+2 to 2 g) and a fine (+0.1 to 0.1 g) accelerometer. With the Caravelle, the aircraft can be piloted using another method to provide dedicated free-floating experiments with levels in the order of 10 to - 3 g for up to 10 s. This method aims at reducing the relative displacement of the free-floating package by providing the pilot with visual information on the free-floating state through a closed TV circuit, allowing to pilot the aircraft 'around' the free-floating package.
Figure 1. The three aircraft used in ESA's 20 parabolic flight campaigns:
Fig. 1a. The NASA KC-135 during the pull-up manoeuvre at 45 degrees (Photo:NASA)
Fig. 1b. The Caravelle during the pull-up manoeuvre at 50 degrees
Fig. 1c. The Ilyushin IL-76 MDK
Table 2. Characteristics of the three airplanes used by ESA
Figure 2. The Caravelle parabolic flight manoeuvre
Figure 3. Acceleration levels during a parabola on board the Caravelle. The measuring equipment is attached to the aircraft's floor structure (sample rate of 10 Hz)
Since aircraft parabolic flights are considered to be test flights, specific precautions must be taken to ensure that all flight operations are performed safely and that flying participants are adequately prepared for the repeated high and low gravity environment.
Prior to a campaign, ESA provides support in the design of the test equipment and in related safety aspects. Several months before the campaign begins, experts review all experiments to be performed and all equipment to be installed on board the aircraft from the structural, mechanical, electrical, safety and operational points of view. Technical visits are made to the experimenters institutes to check the equipment. A safety review is then held one month before the campaign to assess the overall safety of the campaign. Finally, before the first flight, a safety visit of the aircraft is made to verify that all installed equipment complies with the safety standards.
All experimenters that ESA invites to participate in parabolic flights must pass a medical examination (FAA Class III) and a hypobaric chamber physiological test. All certifications are verified prior to the first flight of the campaign. For experiments to be conducted on human subjects, the ESA Medical Board reviews the medical protocols submitted by the investigators two months prior to the campaign to ensure that the proposed research is conducted according to the ethical and safety rules for spaceflights. In particular, all interfaces applied to human subjects are verified and procedures to be followed in nominal and non-nominal situations are reviewed. Human subjects are made aware of the purpose of the research and are informed that they can withdraw at any moment from the investigation, as long as their withdrawal does not present any risk.
For the Caravelle campaigns, the ground infrastructure at the CEV includes the Laboratoire de Médecine Aérospatiale (LAMAS) which provides medical facilities and on-site support for the preparation of experiments involving human subjects.
During the flights, specialised personnel supervise and support the in-flight experiment operations. In addition, a flight surgeon participates in all ESA flights to supervise the medical aspects of in-flight operations and to assist flying participants in case of motion sickness. Due to the sequence of flight phases of low and high gravity, motion sickness is quite common among participants in parabolic flights, sometimes hindering them from conducting their tasks. Anti-motion sickness medication is made available to flying participants, on request, before the flights.
In 10 years of parabolic flight campaigns organised by ESA, 235 experiments have been performed. Table 3 gives an overview of the number of experiments undertaken per campaign and in each scientific field. In general, the type of microgravity experimentation conducted during parabolic flights follows the trend of research conducted during space missions and prepares experiments for orbital microgravity laboratories, either manned (Spacelab, Mir space station) or unmanned (EURECA).
ESA's 10 years of utilisation of parabolic flights can be divided into five periods.
Table 3. ESA parabolic flight campaigns
1984-88, Campaigns No. 1 to 6
From December 1984 to August 1988, six ESA campaigns were organised. NASA's KC-135 aircraft was used, flying from Houston. The first two campaigns were devoted to preparing for the Spacelab-D1 (SL-D1) mission. ESA and DLR science astronauts tested the hardware and experiment procedures for the Fluid Physics Module (FPM) (Fig. 4). Investigators involved in the SL-D1 mission performed preliminary tests to prepare for their fluid physics (Fig. 5), biological and vestibular experiments respectively with the FPM, Biorack and SLED facilities developed by ESA.
Shortly after those two campaigns, the importance of preparing space experi-ments during parabolic flights was recognised and the scientific interest in performing short microgravity investigations (within 20 s) increased.
After SL-D1, during the third campaign, an experiment on surface forces between two colliding solid bodies (Fig. 6) was prepared for the Eureca mission. The first European microgravity combustion experiment was performed and other experiments were carried out, mainly in fluid physics and on the vestibular system (Fig. 7) to complement results obtained during SL-D1.
For the subsequent campaigns, ESA invited European scientists to perform their experiments on board the KC-135. Several theme campaigns were organised. The fourth campaign was dedicated to seven microgravity combustion experiments with other add-on experiments. The fifth campaign was devoted to life sciences with 11 human physiology investigations on the vestibular, respiratory and cardiopulmonary systems. For the first time, a multi-goal physiology experiment was conducted with several investigators measuring different physiological parameters on a single subject.
The sixth campaign saw the performance of 16 experiments in various fields, some of them being follow- ons to experiments conducted on previous campaigns by the same or other investigator teams.
Figure 4. ESA astronaut Ulf Merbold training on the engineering model of the Fluid Physics Module (FPM) (1st ESA campaign, Dec. 1984) (Photo: NASA)
Figure 5. J.C. Legros (University of Brussels, B) performing a test on stability of a fluid interface within a cell, in preparation for his SL-D1 experiment with the FPM (1st ESA campaign, Dec. 1984) (Photo: NASA)
Figure 6. G. Poletti (University of Milan, I) monitoring his experiment on surface forces between contacting solids within a prototype of the Surface Force Assembly (SFA), later flown on Eureca. The equipment is installed in a confining structure with a protective net. (3rd ESA campaign, Mar. 1986) (Photo: NASA)
Figure 7. ESA astronaut Wubbo Ockels repeating an experiment with a Video-Oculographic (VOG) mask from the University of Mainz (D) (3rd ESA campaign) (Photo: NASA)
1989-91, Campaigns No. 7 to 14
At the end of 1988, ESA accepted CNES's offer to use its newly refurbished Caravelle aircraft to conduct parabolic flights in Europe. This new opportunity made the flights geographically closer and easier to organise. Between 1989 and 1991, three campaigns were organised per year with mixed payloads of physical and life science experiments.
Microgravity experimentation during parabolic flights evolved significantly, demonstrating the importance of that tool for microgravity research and attesting to the growth of the microgravity parabolic flight community. Experimental equipment became more sophisticated, with precise optical diagnostic means being introduced. Other experiments of the 'look and see' type, however, were also still being performed. Large experimental set-ups were flown on several campaigns to allow 'repeat data' to be collected. Experiments were also repeated over several campaigns but with some parameters changed. The results were compared with those obtained using other microgravity carriers such as drop towers, sounding rockets, and Spacelab missions, as well as with results obtained in normal gravity on ground and in hypergravity with centrifuges.
Investigations continuing and extending those previously conducted were carried out in fluid physics, mainly on liquid/liquid and liquid/gas interfaces, on bubbles and drops, and on critical point phenomena, in combustion on pre-mixed flames and on burning droplets, and in human physiology, mainly on the vestibular (Fig. 8), respiratory and cardiopulmonary systems. In parallel, new experiments in material sciences were performed: attempts were made to grow small crystals and processing alloys. New and more daring experiments were conducted in human physiology, using invasive methods (catheters were inserted in the veins of subjects' arms to measure the central cardiac pressure, and esophageal pressure probes inserted through the nose were used for respiratory experiments) and a growing number of multi- goal physiological experiments with simultaneous measurements. For example, during one experiment, the following parameters were recorded on a subject pedalling on an ergometer: ECG, heart rate, transthoracic impedance, ergometer work rate, venous lactate concentration, blood pressure, cerebral artery flow velocity by Doppler method, respiratory flow and expired CO 2 concentration) (Fig. 9).
Preliminary experiments were still conducted to prepare for the Spacelab IML-1 and D2 missions for the Advanced Fluid Physics Module (Fig. 10), Anthrorack and Biorack facilities developed by ESA. Preliminary tests were performed for the Bubble Drop, Particle Unit (BDPU) facility in view of the IML-2 mission.
Microgravity technology and ergonomy investigations were also developed during this period. Starting in 1989, ESTEC's Columbus Utilisation Department began testing on board the Caravelle, crew support equipment to be used by astronauts on board the future International Space Station (Fig. 11). After having participated in several campaigns organised by the Microgravity Projects Division, the Columbus Utilisation Department organised two separate campaigns dedicated to tests of Hermes and Columbus systems in addition to those reported in Table 3.
Figure 8. W. Oosterveld (University of Amsterdam, NL) monitoring a subject after injection of hot water (at 44 C) in his inner ear, for an experiment on the vestibular caloric stimulations and nystagmus. Eye rotatory motion is recorded by electrodes around the eye. (11th ESA campaign, Oct. 1990)
Figure 9. P. di Prampero (University of Udine, I) pedalling an ergometer while breathing through a mouthpiece for an experiment on the cardiopulmonary function during exercise (10th ESA campaign, July 1990)
Figure 10. A prototype of the Wet Satellite Model (WSM), the experiment of J. Vreeburg (NLR Amsterdam, NL), being tested in free float. The engineering model of the Advanced Fluid Physics Module (AFPM) in the background is used to rehearse crew handling procedures in preparation for the Spacelab D2 mission. (13th ESA campaign, June 1991)
Figure 11. ESA astronaut J-F Clervoy assessing the two-person procedure for removing a 1/1 scale mock-up of a Columbus rack from its stand (11th ESA campaign, Oct. 1990)
1992-Early 1994, Campaigns No. 15 to 18
In 1992, ESA's policy on manned spaceflight policy changed, resulting in reduced parabolic flight activities. One to two campaigns were organised per year.
Investigations continued in fields previously explored in fluid physics, mainly on Marangoni convection and interface driven phenomena, and on bubbles and drops; in combustion on flames and combustion of dust; and in human physiology on vestibular, respiratory and cardiac systems (Fig. 12, Fig. 13). For the first time, a fluid physics experiment on heat pipe performance in microgravity was conducted on board the Caravelle for the RADIUS (Research Associations for the Development of Industrial Utilisation of Space) programme.
New biology experiments were initiated on animals and on osteoblast cells. The BDPU facility was actively prepared for the Spacelab IML-2 mission with several tests of the bubble/droplet injection and release systems.
Following the change in policy, no further technological investigations to prepare for the Columbus orbital facility were conducted.
Figure 12. W. Oosterveld (University of Amsterdam, NL), sitting on a rotating chair, is the subject of his own experiment on vestibular rotatory stimulations nystagmus. Eye rotatory motion is recorded by electrodes around the eye. (15th ESA campaign, March 1992)
Figure 13. For the Transmural Central Venous Pressure experiment of R. Videbaek (Damec, Copenhagen, DK), a subject, lying and lightly restrained, has a 45 cm catheter with a pressure probe inserted through an arm vein up to the heart to measure left atrial cardiac pressure, and an esophageal pressure probe inserted through the nose to measure pressure variations in the esophagus. Both pressure signals are recorded on two medilogs in the two white belt pouches. Simultaneous ultrasound echocardiography is conducted on the subject to measure, during parabolas, the changing dimensions of the heart during a normal heartbeat cycle. The subject is also partaking in the experiment of V. Demaria-Pesce (Coll ge de France, Paris) in which activity, light exposure and body temperature are recorded on the joblog in the black belt pouch, in preparation of an experiment for the EuroMir-94 mission. (17th ESA campaign, Nov. 1993)
Mid-94, Campaign No. 19 with Ilyushin
In May 1994, ESA was invited to participate in demonstration parabolic flights with the Russian Ilyushin IL-76 MDK aircraft in Berlin. During those demonstration flights, microgravity levels were measured with an ESA-developed micro-accelerometer system. The measurements showed that the levels attained during Ilyushin parabolas are approximately similar in duration and quality to those achieved by the Caravelle and the KC-135.
The next campaign, ESA's 19th, was scheduled to be conducted with the Caravelle, to prepare human physiology experiments foreseen for the EuroMir-94 and -95 missions and to test equipment under development for the EuroMir-95 mission. Because the Caravelle was not available until September 1994, this campaign could not be conducted in time for the EuroMir-94 mission of October 1994. It was decided to use the Ilyushin aircraft in Berlin again for this campaign, as a replacement. Within two weeks, the campaign was rearranged and performed in July 1994 with the participation of ESA EuroMir astronauts, to the satisfaction of the investigators (Fig. 14, Fig. 15).
Figure 14. ESA Astronaut Christer Fuglesang, free floating, attempting to sit on the Munich Space Chair designed by E. Pfeiffer (Technical University of Munich, D). The chair allows an astronaut to anchor himself in front of a workstation using feet and thighs, leaving both hands free. (19th ESA campaign, July 1994)
Figure 15. ESA Astronaut Thomas Reiter rehearsing the donning procedures of the Analog Biomechanical Recorder (ANBRE) suit in preparation for a EuroMir-95 experiment to measure body limb positions in weightlessness. On the left, the seated subject wearing a mask with two CCD cameras is performing the experiment of C. Markham and S. Diamond (University of California, Los Angeles) on torsional nystagmus (rotatory eye movements), in preparation for experiments for the EuroMir-94 and EuroMir-95 missions. (19th ESA campaign, July 1994)
End of 1994, Campaign No. 20 with Caravelle
Since 1993, new orientations in microgravity research paved the way for the emergence of new research themes, which are now reflected in experiments of the last campaigns and of those planned for the future.
With the Caravelle again available for parabolic flights, ESA conducted its 20th campaign, with experiments exploring new areas of fluid physics in microgravity, i.e. investigations of ferromagnetic fluids (Fig. 16) and aggregates. A second RADIUS experiment was performed on capillarity in porous media. A first preliminary experiment was carried out for the Fluid Physics Facility (FPF), a third generation ESA instrument for fluid physics research, which is now under development and is foreseen to fly on an unmanned Russian Foton satellite.
In addition to these microgravity science and technology campaigns, ESA recently undertook a new initiative proposed by students of the Delft University of Technology in The Netherlands, as part of the European Union's Week for Scientific Culture (see 'The First Parabolic Flight Campaign for Students' in this issue). Following a Europe-wide competition, 49 students from 11 European countries, including Poland, took part in a series of parabolic flights and performed their own experiments while in microgravity. The experiments were of a high scientific level and involved innovative ideas pertaining to several fields: general physics, fluid physics, combustion, material processing, crystal sciences, astrophysics, geophysics, biology and technology.
Figure 16. During the experiment of S. Odenbach (University of Wuppertal, D), a non-Newtonian fluid (solution of oppanol B200 in heptane) climbing on a rotating shaft under microgravity. The climbing is caused by the non-Newtonian properties of the fluid. The surface tension of the liquid forces the free surface to form a sphere at the shaft. (20th ESA campaign, Oct. 1994) (Photo: University of Wuppertal)
The investigators invited by ESA to perform experiments during ESA parabolic flights are asked to present their results several months after the campaign. ESA has regularly organised scientific workshops in the past 10 years where the results are publicly discussed, and the presented papers are subsequently published. The results presented during these workshops have also reflected the trend observed in the experimentation, spanning from reports of simple visual tests to presentation and discussion of scientific results acquired with laboratory instrumentation and compared to those obtained by other methods.
In November 1994, ESA and CNES organised a joint workshop in Toulouse where results of experiments conducted during the last seven ESA and CNES campaigns with the Caravelle were presented. The discussions among the 100 investigators involved in CNES and ESA campaigns prompted many new ideas for future experiments to emerge. The tenth anniversary of the first ESA campaign, held in December 1984, was also celebrated on that occasion.
During those 20 campaigns, ESA staff members have contributed greatly to the success of many experiments. ESTEC's Design Office, Mechanical and Electrical Workshop, and Instrument Technology Division, in particular, have supported efficiently the preparation and the performance of several experiments.
ESA will continue to organise parabolic flight campaigns for the European scientific and technical microgravity communities. The Agency regularly invites scientists to submit proposals for microgravity experiments for the parabolic flight programme.
Three campaigns are scheduled for 1995. The 21st campaign took place in March and was devoted to the preparation of experiments to be conducted during the EuroMir-95 mission. The 22nd and 23rd campaigns, respectively foreseen for the end of April and the autumn of 1995, will be dedicated to microgravity experiments with mixed payloads of physical and life sciences. Further campaigns are foreseen for the following years at a rate of two to three per year.
Furthermore, in view of the educational and media success of the recent student campaign, a second campaign is now being organised.
As far as the aircraft used for ESA campaigns is concerned, the Caravelle is presently certified for parabolic flights until July 1995. Discussions about a replacement aircraft are presently underway in France. To bridge the gap until a new aircraft is fully operational, the flight certification of the present Caravelle could be renewed for several months.
Within specific cooperative frameworks with NASA, CNES and the Russian CTC, ESA has had the opportunity to organise campaigns for microgravity experiments with the three main airplanes in the world that are used for parabolic flights. The unique experience acquired on board the NASA KC- 135, the CNES Caravelle and the CTC Ilyushin is reflected in the number of experiments successfully conducted over the last 10 years.
The quality and duration of microgravity obtained, the flexibility and variety of possibilities for experiments and tests, and the ease of flight preparation make aircraft parabolic flights a unique and versatile tool for European scientists to perform experiments in microgravity and at different g levels.
In particular, aircraft parabolic flights are highly recommended for the conducting of gravity-related physical investigations and physiological experiments on human subjects, to complement research conducted with other Earth-based carriers and to prepare for the future space station missions.
As the space agencies of the world embark on the ambitious building of the International Space Station, one can forecast that aircraft parabolic flights will be used intensively throughout the various phases of the programme. Over the next 10 years, the programme will include firstly, precursor flights on board Spacelab and Mir missions; secondly, the so-called Phase II of the International Space Station during which Europeans will participate in American and Russian experiments and European microgravity payloads will be developed; and thirdly, the period during which the ESA Columbus Orbital Facility will be available for continuous microgravity research. During all of these three phases, the preparation of experiments, the microgravity testing of payloads and the training of astronauts will still be conducted during aircraft parabolic flights.