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Advancements in materials are needed to allow development of diesel engines to approach
the power to weight ratios of gasoline engines. The high cylinder pressures associated with
the diesel cycle will require advanced materials not presently found in reciprocating engines.
Concurrently, dynamic components such as crankshafts, connecting rods and bearings also
need improved weight to strength/wear for suitable use in aviation engines.
Weight reductions in the area of diesel fuel systems and ancillary components will also be
required. This includes the fuel injection system, turbochargers, intercoolers, scavenge
pumps, cooling systems. Increasing efficiency requires advanced fuel system components
such as lightweight high-pressure pumps/fuel injectors and advanced fuel control techniques
such as rate shaping. These systems are required for diesel cycle engines operating on JP
fuels.
Shortcomings of Current Approaches. The ongoing development of the OPOC engine shows
significant promise for meeting the need for a low cost heavy fuel engine. Other proposed solutions,
such as low pressure diesels and modified two cycle gas engines have been without merit to date. To
ensure the provision of reliable, efficient, lightweight JP burning engines for aviation use, additional
in depth technology programs must be pursued. The resulting influence on UA designs (and their
inherent capability) of the different design approaches is depicted in Figure D-3. Without an in-depth
UAS ROADMAP 2005
APPENDIX D – TECHNOLOGIES
Page D-5
technology program the best that can be hoped for are mediocre solutions that meet some of our
requirements, but fall significantly short in providing the true solution needed.
Propulsion – Electric and Alternative Technologies
Many of the smaller UA (mini- and micro-UA) use battery power instead of two-cycle engines. Low
noise signature makes these electric drives attractive in many situations, despite the low efficiency and
low power-to-weight ratios compared to reciprocating engines. Recent improvements in the ability to recharge
lithium based batteries have resulted in significant logistics improvements for users in the field.
Further improvements are needed in power-to-weight ratios for the next generation of batteries to improve
the performance and endurance of these small platforms on a single charge. Currently, most batteryoperated
MAV have a fraction of an hour of endurance, while mini-UA fair only slightly better, only
because they can carry larger numbers of the same lithium-based batteries.
Future-looking efforts for UA propulsion include the use of fuel cell- or nuclear-based power schemes.
NASA has pushed fuel cell development for use in UA and by the Army's Natick Laboratory for soldier
systems (i.e., small scale uses), and specific energy performance is approaching that of gasoline engines.
The gaseous hydrogen fuel cells being used on NASA's Helios UA in 2003 have over 80 percent of the
specific energy of a two-cycle gasoline engine (500 vice 600 Watt hours/kilogram) and 250 percent that
of the best batteries (220 W hr/kg); further improvement is anticipated when liquid hydrogen fuel cells are
introduced. Still in development by NASA are regenerative power systems combining solar and fuel cells
in a day/night cycle to possibly permit flight durations of weeks or longer. Additionally, several
commercial aviation initiatives are exploring fuel cells for both primary propulsion and auxiliary power
units (APUs), see Figure D-4. In the nuclear arena, the Air Force Research Laboratory has studied the
feasibility of using a quantum nucleonic reactor (i.e., non-fission) to power long endurance UA. However
this remains a concept study, no prototypes or flight worthy hardware are currently planned.
FIGURE D-3. ENGINE EFFECTS ON TAKE-OFF GROSS WEIGHT FOR A DESIRED MISSION
ENDURANCE.
UAS ROADMAP 2005
APPENDIX D – TECHNOLOGIES
Page D-6
Battery Propulsion
Electrolyte Wgt
+ Cathode & Anode Wgt
+ Case Wgt
+ Power Conditioning & Wiring Wgt
+ Motor Wgt
Total Wgt
Specific Energy = W*hr/Total Wgt
Fuel Cell Propulsion
Fuel Density * Fuel Volume
+ Fuel Cell Wgt
+ Reformer System Wgt
+ Power Conditioning & Wiring Wgt
+ Motor Wgt
Total Wgt
Specific Energy = W*hr/Total Wgt
Gasoline Engines
Fuel Density * Fuel Volume
+ Engine Wgt/HP *HPreq
+ Accessories Wgt
Total Wgt
Specific Energy = HP*hr/Total Wgt
FIGURE D-4. SPECIFIC ENERGY CALCULATION.
Propulsion - Hovering
The ability to take-off and land vertically can provide added operational benefits, such as being able to
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