PETER LA FRANCHI / BRUSSELS
Flying in the Martian atmosphere has become a focus for unmanned air vehicle designers at the US Naval Research Laboratory
For the past 15 years, the US Naval Research Laboratory (NRL) has been playing a quiet role in the development of future unmanned Mars exploration aircraft. At various times in collaboration with NASA and at other times in response to open solicitations by that agency, the studies have included examination of various approaches to achieving Martian atmospheric flight.
The most recent effort is a design study conducted on behalf of NASA's Ames Research Center, resulting in proposals for a blended wing-body design that would fly for 55min across the Martian landscape.
The aircraft, designated Ares, is the hybrid outcome of studies of "classical" approaches to Martian aircraft design as well as NRL researchers' own interest in flying hypersonic aircraft through the skies of the red planet. The NRL design is not to be confused with the existing NASA Langley-Aurora Flight Sciences Aerial Regional-scale Environmental Survey (ARES) Mars aircraft concept which had competed for NASA's Mars Scout 1 requirement but was turned down in favour of the Phoenix lander proposal from the University of Arizona.
The NRL Ares proposal was unveiled at the European Unmanned Vehicle Society's Unmanned Vehicle Systems 2003 conference in Brussels on 3 December. According to Joe Mackrell, a senior UAV researcher at NRL: "We've looked at several different configurations. Do you want to fly a very lightweight, slow-flying, low-flying aircraft on Mars? Or would you like to fly with something like the Space Shuttle that burns in and proceeds across a whole lot more of the Martian surface at fairly high Mach? Or do you have to compromise between the two? The compromise aircraft came out first in our study."
The classical approach to Mars aircraft design, Mackrell says, has been based on a slow-flying airframe with moderate endurance levels, moderate flight speeds, and a small turn radius to revisit targets of interest detected by on-board sensors.
"The scientists like the slow-flying aeroplane because it gives their sensors the longest dwell time for a particular spot of land. However, that aeroplane has got limited range and basically half the mass that you deliver to the top of the Martian atmosphere goes away because that is the aero shell and your parachute and your heatshield," says Mackrell.
Hypersonic options, Mackrell says, offer a highly efficient way around that weight problem because the air vehicle outer skin would be its own heatshield. "You build a spaceship that flies from Earth to Mars and enters the Martian atmosphere and flies. It covers a lot of territory, it covers thousands of kilometres. However, it does it at a very high speed and from fairly high altitude. The scientists don't like it for that reason."
The hypersonic aircraft option would also tend towards complexity in its design. Mackrell says the most likely candidate would be "a hypersonic wave-riding type vehicle", necessitating a sophisticated airframe. Such a vehicle would also present problems in developing sensors that could collect and process data at very high speeds, particularly any imaging system.
The NRL Ares concept, Mackrell says, retains sufficient speed to provide it with a wide-area capability, but flies slowly enough to meet scientists' needs and carry a substantial sensor suite. The central wing/body of the aircraft incorporates a crash-proof housing for the sensor payload, avionics, data storage unit and communications link.
He adds that one of the basic problems facing any Mars aircraft is that "you can gather a whole lot more information with a vehicle like this than you can transmit. One of the ways around that is to build a crash-proof aeroplane that lands and continues to transmit to an overhead satellite over the next couple of weeks and months."
The NRL Ares aircraft would have a folded wingspan of 1.9m (6.3ft), increasing to 4.1m when fully unfolded, and a length of 1.9m. The total wing area would be 3.38m2 (36.4ft2). Proposed height is 0.95m, with the tail raked backwards to enable it to fit within the aeroshell cone. Unlike the NASA Langley-Aurora ARES concept, the tail is integrated into the main body rather than fitted to a separate fold-out boom.
The NRL Ares aircraft main engine would be a hydrazine and nitrogen-tetroxide fuelled rocket. Three split control surfaces have been incorporated into the design, but Mackrell says the aircraft would be fitted with a reaction control system (RCS) comprising four venturi and these may be sufficient to meet all in-flight manoeuvring requirements.
The aircraft would cruise at a speed of 29,500ft/min (150m/s) with a Mach limit of 34,000ft/min. Stall speed is 19,900ft/min at gross weight and 12,400ft/min empty. Cruising altitude would be 13,600ft (4,150m), but the aircraft could travel as low as 3,300ft. The maximum range is forecast at 600km, with a reduction of flight speed in later phases to 17,700ft/min, but at cruising speed is expected to reach 500km.
In comparison, the Langley-Aurora ARES aircraft was designed to fly 850km at 5,000ft over the Martian southern highlands, conducting near-surface measurements of water vapour and atmospheric gas compositions.
In September 2002, a half-scale ARES prototype, packaged similarly to that required to fit into a Mars re-entry vehicle, was carried to 103,500ft above Oregon and released. It successfully unfolded its wings and tail to achieve stable flight, ending 90min later in a pre-programmed skid landing.
In May 2003, Langley commissioned Aurora to build a full-scale prototype of the vehicle to explore actual flight dynamics in a proposed series of trials in Earth's upper atmosphere by late 2003. But following the Phoenix selection, the status of those trials and the next steps remains unclear.
Mackrell says the compromise between hypersonic and classical design approaches is best reflected in the proposed deployment methodology from the re-entry vehicle. Classical design, he says, calls for a parachute to stabilise the aeroshell in the Martian atmosphere, with the heatshield then dropping away. The aircraft would remain attached to the upper section of the descending aero shell while unfolding its wings. Once that manoeuvre was completed, it would drop with its wing generating sufficient lift in the dive phase to allow a transition to horizontal flight.
This method was first successfully flight tested by Aurora in May 1999 using its JASON electrically powered Mars concept aircraft. JASON was folded into an aeroshell and carried aloft by a hot air balloon near Bealton, Virginia. The package was dropped with several seconds of freefall before the parachute deployed with JASON transitioning into horizontal flight shortly thereafter. This approach is also understood to have been proposed by NASA Langley and Aurora for their Scout 1 ARES aircraft.
However, Mackrell says: "When you drop an aeroplane out of an aeroshell at very low speed and very high altitude, it has to do a pull out and you run into the same problem the [Lockheed] U-2 does at high altitude. If you go too fast you run into your Mach limit and if you got too slow you stall and those two points meet somewhere at the top of your flight envelope. So it is a difficult thing to fly the airplane out of a 90¡ nose-down, zero-airspeed position at 5km above the surface of Mars."
The NRL Ares concept has the re-entry vehicle hitting the Martian atmosphere, which reaches 125km above the surface, at a speed of 5.4km/s. At an altitude of 40,000ft the parachute would deploy to slow the re-entry vehicle sufficiently to allow the heatshield to be jettisoned at 38,000ft. ARES would be ejected from the upper aeroshell at an altitude of 36,000ft.
Mackrell says the most critical moments of the sequence come immediately after ejection. "One of the problems is that you don't know the attitude when it comes in. With a lifting-surface aeroshell you can position it because you have to have a reaction control system on the re-entry vehicle. These do not have that, so you have no idea whether you are coming nose-up or nose-down. So the aeroplane has to figure this out and orient itself over the next several seconds after it pops out of its entry vehicle."
The orientation process will be physically achieved by using the reaction control system to turn the aircraft into a nose- up profile. By this time the aircraft would be at 33,000ft, but still falling rapidly.
At 30,000ft the wings would unfold and lock into place and the main engine would fire to power the aircraft out of its high descent rate, then transition into horizontal flight at cruise altitudes.