A full scale atmospheric flight experimental research environment for the Mars helicopter
AA full scale atmospheric flight experimental researchenvironment for the Mars helicopter
J. Pablo Afman ∗ Eric Feron † Mitchell Walker ‡ October 22, 2019
We propose to develop a full-accuracy flight test environment for the Mars helicopterand related Mars-atmospheric vehicles. The experiment would use reduced- g atmosphericflights with an aircraft that houses a properly sized vacuum chamber. Introduction
Reduced atmospheric density flight has been the object of much interest throughout thehistory of aviation. Indeed, aviation is about reduced air density flight: A jet liner takesoff from Boston Logan Airport at sea level. There air density is 1.225 kg/m . As it crossesthe Atlantic ocean, it reaches an altitude up to 38,000 feet above sea level, where airdensity is approximately reduced to 1/5th of what it is an the ground. It takes flying upto 100,000 feet above sea level for the atmospheric density to reduce to 1/100th of its sealevel density, which corresponds to the atmospheric conditions on Mars’ surface. On Earth,several aircraft are capable of flying at 100,000ft. They include the X15 and the Heliossolar-powered aircraft.NASA and the Jet Propulsion Laboratory (JPL) are in the process of developing a”Mars Helicopter Scout” (MHS) capable of sustained flight over the surface of Mars. Thishelicopter is the latest of a long sequence of atmosphere-borne candidate Mars vehicles thatinclude Aurora Flight Sciences’ ARES, a ”Mars airplane” that would be directly droppedfrom a re-entry vehicle. A complete table of Mars airplane concepts can be found onWikipedia,see https://en.wikipedia.org/wiki/Mars aircraft . Earth-based testing of atmosphericMars vehicles offers good potential to mitigate the possibility for financial and time lossesassociated with typical Mars missions. So far, the experimental tests performed by JPLinclude a static flight of the MHS in JPL’s 25 ft vacuum chamber with reduced atmospheric ∗ Yamaha Motor Corporation, juan-pablo [email protected] † Georgia Institute of Technology, [email protected] ‡ Georgia Institute of Technology, [email protected] a r X i v : . [ phy s i c s . pop - ph ] S e p igure 1: Left: JPL Mars Helicopter Scout (MHS) [?]. Right: JPL Mars Helicopter inJPL’s 25ft vacuum test chamber.density,see . Other tests include vibration testsand operation at very low temperatures. A core issue is reproducing the Mars gravity con-ditions. The Jet propulsion Laboratory has addressed this problem by ”emulating” the 0.39 g on Mars surface by ”assisting” the Mars aircraft with a cable-based gravity reductionmechanism. In the following, we propose an alternative flight testing arrangement thatcombines the lower Mars gravity with lower density atmospheric effects using a standardcargo aircraft with an embedded vacuum chamber. Reduced gravity atmospheric flight
General considerations
Reduced-gravity atmospheric flight consists of using an atmospheric vehicle (typically anaircraft, but not necessarily,see https://ieeexplore.ieee.org/document/8431251 and https://ieeexplore.ieee.org/document/8618690 for example) that flies along trajec-tories where a given level of gravity is ”felt” in the reference frame of the vehicle. The mostpopular form of reduced-gravity flight is the zero- g flight, whereby the aircraft follows ex-actly the part of an Earth orbit to reproduce weightlessness conditions. In practice actualtrajectories, inexactly called parabolic trajectories, last on the order of 18 seconds. A vari-ant on zero- g flight is micro - g or µg flights, whereby micro-gravity conditions are createdto reproduce those encountered in low-gravity environments, such as asteroids. Accurate µg flights are considerably more difficult to create than zero- g flights: the latter can beeasily regulated by a skilled pilot by ”controlling” the test aircraft against the referencetrajectory provided by a proof mass (reportedly a plastic duck initially sitting on the pilot’sknees sometimes), and deviations from the nominal trajectories do not matter as long asthe proof mass does not deviate exaggeratedly from its free-floating position. Coarselyspeaking, that means that human and material subjects in floating conditions will also not2igure 2: Various contemporary uses of atmospheric zero- g flights. Left: Experimen-tal research. Middle: OK Go, ”OK Go - Upside Down & Inside Out” video. Right:Kate Upton posing in the Zero- g corporation Boeing 727 zero- g g is about producing very precise gravity conditions for objects thatare fixed relative to the aircraft. The proof-mass concept then does not work anymore andvery precise regulation needs taking place using other sensors than proof masses.Creating a Mars gravity environment is similar in nature to the foregoing activities.The point is to create an environment where local gravity is approximately Mars’, thatis, 0.39 g (or 3.711 m/s ). Flying 0.39 g trajectories is nearly the same as flying a zero- g trajectory, only that the near parabolic trajectory of the aircraft must mimic being pulledto the ground in the vacuum with a constant gravitation of 0.61 g instead of 1 g . Suchdemand on the aircraft is less aggressive than performing a 0-g maneuver, especially duringmaneuver recovery.According to , the ”useful part” of theMHS flight test is approximately 25-30 seconds, which places the flight within the timewindow offered by a mars g flight, described below.3
50 -40 -30 -20 -10 0 10 20 30 40 50
Time [s] Z [ m ] -50 -40 -30 -20 -10 0 10 20 30 40 50 Time [s] V [ m / s ] -50 -40 -30 -20 -10 0 10 20 30 40 50 Time [s] -40-30-20-10010203040 [ deg ] Figure 3: Mars- g trajectories for standard cargo transport aircraft. The Red dashed linesindicate the boundaries of the Mars- g maneuver. Left: aircraft altitude and speed as afunction of time (units are second, meters, and meters per second, respectively). Right:Flight path angle. Flight characteristics
The characteristics of an atmospheric, Mars- g parabolic flight are similar to that of astandard zero- g flight: after a horizontal, rectilinear acceleration phase, the test aircraftinitiates a pull-up maneuver so as to reach the proper initial attitude and speed to performthe Mars- g phase of the flight, which resembles an inverted parabola. Once the Mars- g maneuver is complete, the aircraft pulls up to resume straight and level flight. Othermaneuvers may follow along the same principle. The challenges that come with the designof these maneuvers include the necessity to avoid stall at all times on the one hand, andto keep Mach number below transonic regime, on the other hand. In addition, we haveadded the constraint that the maneuver be performed by a standard cargo airliner, ratherthan a specifically modified aircraft, such as the Zero-g corporation’s Boeing 727 or theEuropean Space agency’s Airbus A310. The latter aircraft are instrumented with pumpsand special equipment that allows the aircraft to operate in zero-g conditions without anyissues. For our Martian maneuver, this type of special equipment is likely unnecessary, anda maximum aircraft load limit has been placed at at 1.3 g .The simulation shown in Fig. 3 indicates that a 26 sec. Mars- g maneuver is achievablewithout inducing excessive stress on the aircraft (1.3 g max). Moreover, the flight pathangle does not exceed 35 degrees in magnitude. On top of the maneuver, true airspeed is105 m/sec, resulting in a maximum angle of attack < irplane handbook/media/06 afh ch4.pdf . Reduced atmospheric density chamber
While this report does not intend to enter into the details of the vacuum chamber design,several remarks can be made about some of the constraints that must be met before thesystem can be constructed.
Geometric constraints
First, there are the geometric constraints posed by the dimensions of the test aircraft. Inshort, the bigger the aircraft, the better. As a benchmark, the cargo bay of today’s reduced g research aircraft in the US, a 727-200 operated by the Zero- g corporation, is about threemeters, thus providing a comfortable fit for the ”naked” mars helicopter as shown in Fig. 4if it were flying within a standard container with empty side space at around 1 meter onboth sides. Vertical clearance is about two meters. Container length is limited by cargodoor width (about 3.2 meters), as shown in Fig. 4.Figure 4: Left: Mars helicopter in B727 aircraft. Relative dimensions are approximate.Right: B727 ”vomit comet” zero- g aircraft with large cargo doorThe European zero- g flight test aircraft, a converted Airbus A310, offers an alternativeto the 727. According to the web site describing the characteristics of the payload bay, see https://m.esa.int/Our Activities/Human and Robotic Exploration/Research/Airbus A310 Zero-G , the dimensions of thetesting volume are 20 x 5 x 2.3 metres (L x W x H), thus offering superior space available forexperiments. However, according to the same web site, the door for equipment loading has5igure 5: Left: Mars helicopter in A310 aircraft. Relative dimensions are approximate.Right: ESA A310 zero- g aircraft. Doors are those of a standard commercial jet. Bottom:A310 cargo aircraft with large cargo door.a height limit of 1.80 metres and a width limit of 1.06 metres. Thus the vacuum chamberwould almost certainly have to be build from several sections and assembled inside theaircraft. Perhaps preferably, a standard A310 cargo aircraft might be used because of thebenign flight conditions and the presence of a much larger cargo door, as shown in Fig. 5.With a relatively low stress on the aircraft, the relatively benign nature of the completemaneuver, and the closed nature of the proposed experiment, it can be surmised thatan even larger cargo aircraft can be used to perform the experiment with no additionalconcern for safety. This opens up the possibility of using considerably larger cargo aircraft,eg one among several available Boeing 747 freighters, whose cargo doors are multiple andvery large, as seen on Fig. 6. That possibility opens the perspective for using a muchlarger vacuum chamber whose horizontal dimensions will be close to those used by theJet Propulsion Laboratory’s 25-ft solar thermal vacuum chamber to flight test the currentMars helicopter prototype. Aerodynamic constraints
There are clear challenges of operating a flying machine in a confined space such as avacuum chamber: managing the gas flows and limiting boundary effects come first. It is6igure 6: Left: Mars helicopter in B747 cargo aircraft. Relative dimensions are approxi-mate. Right: 747 front cargo door.well-known that wind-tunnel boundary effects can be deleterious to experimental results.Some of these effects can be mitigated by adding well-located louvers and to recirculatethe air appropriately. The larger the vacuum chamber, the more accurate the flight testrelative to actual Mars conditions. Preliminary computational fluid dynamics can help liftthese uncertainties, just as they can lift those associated with the flight test that took placein JPL’s 25-ft vacuum chamber.
Navigation issues
A proper, full-scale flight test environment should also be capable of replicating the condi-tions necessary for the unmanned vehicle to navigate its environment. The MHS navigationsystem includes a combined INS-vision navigation system. Algorithms used to extract sys-tem position and orientation rely on the necessary relations that link GPS readings andoptical information in fixed environments. There is a risk, however, that such algorithmsmight be fooled if the airplane goes through perturbations, such as turbulence. In thatcase, the relation between optical and inertial readings could be temporarily de-correlated.The question as to whether such perturbations are observable and rejected requires morework than that envisioned to prepare the present report.
Complementarity with other tests
The core benefit of the proposed test over ground-based tests is the possibility of exactlyreproducing the gravity conditions encountered on Mars using an atmospheric device. Inaddition, it is also possible to effect large attitude changes on the coaxial helicopter, some-thing strictly impossible to do if the machine is suspended to a cable to emulate low7ravitational conditions. Moreover, it becomes possible to obtain a better idea of thehelicopter behavior as it takes-off from the ground , including if it takes off not exactlyhorizontal, a distinct possibility when landing in a largely unknown area, and despite localleveling opportunities offered by robotic ground platforms. Last, it becomes possible tostudy the very real possibility of ”brownout” that may occur when dust gets blown awayby the airflow created by the helicopter. Much is known about Mars dust and could bereproduced for the specific purposes sought for the proposed experiment.
Other proposed uses
A high-fidelity Mars environment may be used in several ways. For example, there mightbe value testing Mars probe landing mechanisms, and Mars rovers, at least those whosedimensions are acceptable. There might also be the possibility of testing human responseto Mars’ gravity environment as part of a human deployment on the planet. Coming backto flying vehicles, the proposed environment may also be used to test smaller systemswhose capabilities could eventually match those of the current Mars Helicopter thanks tothe rapid evolution and miniaturization of computer, sensor, and actuator hardware, andaerodynamics that scale in favor of smaller systems.