Instrumentation and applications relating to an airborne large aperture telescope which is envisioned to reside on a multiple function lighter than air platform, as presented at the JPL Advanced Concepts Program Mini conference for "Innovative Space Mission Applications for Thin Films and Fabrics" on May 8, 1995 and published in the proceedings. ALAT was first presented as part of my ARDOC (Airborne Relay for Deep-Space Optical Communication) presentation at JPL's "Seminar 331" on December 8, 1993. Some data from the earlier presentation has been added to this file. Even though this material was presented at JPL and subsequently proposed for JPL/NASA Project(s), it was all conceived and developed with my own time and resources.
A multiple function lighter than air robotic platform which is capable of residing in the lower stratosphere for an extended period of time would be an elegant solution for a variety of long standing research and communications problems. Such a platform would be invaluable to the continued pursuit of many disciplines, including astrophysics,
high resolution imaging and ranging of satellites and orbital debris, air and space communications utilizing weather dependent frequencies (including optical), atmospheric research, commercial broadcasting, cellular communications, regional position information (including "smart maps" for automobiles), and surveillance. The platform
can house inflatable structures which would serve both as ballonets and as reflectors for power collection, communications, and radio astronomy. Additionally, the platform could be used for educational payloads and as a high altitude test bed for materials and instrumentation.
The Airborne Large Aperture Telescope (ALAT) is envisioned to be a multiple function optical telescope (or series of telescopes) which will reside on a high altitude lighter than air robotic platform. The large aperture (~2-3 m) of the telescope and the altitudes at which it operates (~16-22 km) will allow it to perform nearly as well as a space based
telescope, yet have many additional advantages, including lower cost, accessibility for maintenance & upgrading, the ability to perform long integrations over a large portion of the sky, and mobility to facilitate strategic positioning for specific events such as eclipses, occultations, or communications. ALAT is the cost effective way to provide the high performance telescopes required for expanded research on a global
scale.
Copyright 1993, 1994, 1995, 1996, Jeffrey R. Charles, All Rights Reserved
Regional position reference (smart maps, etc.) benefiting motorists, police.
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-4-
PERFORMANCE FEATURES
-4a-
Performance Features;
Multiple Function Lighter Than Air Platform:
Why Use a Lighter Than Air Platform?
-
Deployable at 0~23 km altitude, but usually deployed at 16~21 km altitude (depending on latitude and other conditions).
-
Above most clouds and turbulence.
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Less power (~75 kW SHP in lowest 98.5% winds) required to maintain station at ~21 km due to lower average wind speed.
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Does not require continuous motive power to remain airborne; can stay up for weeks; longer than an airplane.
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Less vibration than an airplane.
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Low air speed:
-
Reduces boundary layer effects on payload.
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Eliminates need to fully enclose telescope.
-
Facilitates larger aperture and greater sky coverage
-
Airship hull can serve as or house an inflatable antenna and/or solar/beamed power reflector = large reflector size at less cost.
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Platform potentially has larger payload capacity than an airplane, allowing it and its instrumentation to be used for multiple applications: Atmospheric observation, environmental study, mapping, surveillance, deep space communications, domestic communications, broadcasting, etc. = less cost per function and a wider range of customers.
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Large size and potentially continuous presence = public visibility.
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-4b-
Performance features;
Why an Airship Instead of a Conventional Balloon?
-
Reliable recovery of payload and lifting gas = less risk and lower long-term cost.
- Ability to maintain constant position in spite of prevailing wind simplifies command and data transmission, facilitates arraying, and allows use for commercial broadcasting & communications, atmospheric study, surveillance, and other applications.
- Ability to maintain constant attitude facilitates extended observations and simplifies tracking routines.
- Transportable and controllable - can be deployed at a specific time and place for observations of rare events.
- Storage and rapid deployment from existing airship hangars - payloads can remain relatively undisturbed between missions.
- Not subject to limitations of a tether. Platform is free to drift during observations or to maintain a specified station.
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-4c-
Performance Features;
Airborne Large Aperture Telescope
- Unaffected by most cloud cover - one airborne telescope can have the same availability as multiple ground stations.
- Facilitates serious astronomical observation from nearly anywhere in the world regardless of weather conditions.
- Perpetual good "seeing" at altitude = high resolution
- Reduced effects from atmospheric scattering, absorption and emission, particulates, moonlight, and man made light sources.
- Potential for night and day observation = better utilization
- Provides many of the observational advantages of a space telescope, yet is less expensive and is accessible for servicing and upgrading.
- Facilitates airborne optical communication research and development.
- Large aperture facilitates use as an airborne relay for deep space optical communication (ARDOC).
-
Fewer real and perceived environmental issues and related legal delays than ground based stations.
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-4d-
Airborne Large Aperture Telescope &
LTA Platform;
Combined Performance Features & Specifications
- Platform dimensions for existing hangars & reasonable drag coefficient: Diameter = 40 meters; length = 182 meters; total volume = 170,000 cubic meters; available lifting gas volume = 165,000 cubic meters; total lift @ sea level = 154 metric tons.
- Preliminary mass properties estimates (metric tons):
Composite structure & rigging=7; envelope > 4; ballonets=1; primary propulsion=3; secondary propulsion=0.5; power generation=3; flight management & avionics=0.3; two telescopes=3; telescope and T&C instrumentation with environmental control=1.1.
Total = 22.9
The probable ceiling for the specified platform and proposed payload if built with existing technology will probably be at ~0.15 density, or ~16 km altitude. This altitude in the upper tropopause is below most of the equatorial ozone concentration, but it is also below some cirrus clouds and the turbulence ceiling of powerful storms. This will impose certain restrictions:
- The mid-latitude jet stream (i.e. polar front jet stream) will have to be avoided, limiting long term platform deployment to polar regions and/or latitudes within ~35 deg. of the equator.
- Large equatorial storm systems will have to be avoided.
- It is preferrable to deploy ALAT in the lower stratosphere (20-23 km altitude) anywhere in the world. This is above convection and nearly all types of clouds. A ceiling of ~23 km (for reliable use at 20~22 km) is the goal which should ultimately be achieved. This presents additional challenges:
- Rarefied air and corrosive chemicals at high altitude increase cost of achieving a higher ceiling.
- Lifting gas volume / platform ratio will have to be increased; for the same payload mass, total platform volume may have to be increased ~250% over that shown above. Payload and support structure mass is critical.
- Noctilucent clouds can still influence observations.
- Plasma events above storms may be observed from the platform, but their influence on the platform and its payloads is largely unknown.
- Preliminary mass properties estimates for additional functions when integrated into ALAT and its platform:
Airbirne Relay for Deep-Space Optical Communication (ARDOC)=0.3; High Precision Imaging & Ranging of Sattelite & Orbital Debris Tracking (HOST)=0.1; high altitude atmospheric & environmental observation=0.3; optical and other surveillance (0.75m aperture)=0.5; forestry & mapping=0.1; secure air-ground optical communications=0.2; public cellular communications(STRATOCOM (TM))=0.4; public television and other broadcasting=0.3; deep-space RF communications with inflatable reflectors > 0.5; beamed power development & use=0.6; regional position reference (smart maps, etc.)=0.2.
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-5-
Thin Film / Fabric Structural Material Application Concept Summary, continued:
Design Characteristics
-
Manufacture and storage in existing airship hangars will substantially reduce cost but will limit platform size and payload capacity.
-
Non rigid or semi rigid airship with composite payload support structure which may include a partial keel.
-
Remote control & programmable robotic operation at high altitude, pilotable at low altitude.
-
Multiple internal gas bags and ballonets for trim and altitude control.
-
Lift from helium. In-flight renewal of helium from other vehicles is being considered, as is supplemental lift from hydrogen in external balloons or isolated internal gas bags. Beamed power can be used to electrolyze condensed or stored water.
-
Inflatable structures inside hull can serve as antenna reflectors, solar and beamed power concentrators. Concepts such as using rigging and an outer protective envelope to prevent surface deformation and provide tension and stability are applicable to "stand alone" inflatable structures.
-
Telescopes probably mounted on platform structure under protective covers; deployable telescope array structures and arrayable platforms have been considered.
-
Primary propulsion at rear to preserve laminar flow; some vectored thrust available near instrumentation gondola below envelope. Primary propulsion will almost certainly be from electric motors. Solar and / or beamed power may be used if missions are long. Limited propulsion provided by a "conventional" high altitude engine. Steam power is being considered due to low boiling point of water at high altitude.
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-6-
Thin Film / Fabric Structural Material Application
Concept Summary, continued:
Environment Description:
-
Launch
-
Ground level winds pose risk, especially near hangar.
-
Air pollution can potentially damage platform, etc, while in storage.
- Ascent
-
Jet stream will carry the platform downwind during ascent, it must then fly to its intended station
- Temperature extremes during ascent can range from ~45 deg. C to -85 deg. C.
- Use conditions on station
-
Ozone, volcanic material, and other corrosive or eroding elements @ 21 km (or at lower altitudes if platform deployed near the poles)
-
Direct and reflected UV
- Low ambient temperatures and daily thermal cycling
- Station keeping against wind requires power - site selection is important, particularly if platform is deployed at lower altitudes.
- Typical mission duration: ~1-9 weeks. Operational lifetime: ~20 years.
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-7-
CHALLENGES
-7a-
Major Structural / Material / System Challenges
Design:
-
Development of a suitable LTA platform that will withstand the ~20:1 change in atmospheric pressure (~12:1 if deployed from the South American altiplano) that would be encountered during ascent to ~21 km.
- Optimization of telescope and platform to ameliorate boundary layer and thermal effects on "seeing" and platform performance.
- Optimization of platform for long endurance flight; integration of beamed power systems; provision for adequate shielding of other systems.
- Minimizing altitude change due to day/night temperature change.
- Minimizing mass of platform, telescope and mirror.
- Integration of platform control, payload tracking routines, GPS, and DGPS.
- Determination of appropriate emphasis on LTA platforms and payload capability (i.e. quantify feasibility; identify and prioritize "customers" and their payload requirements; determine the optimum quantity and location of platforms and control stations required to better satisfy current and future commercial and research needs).
- Determination of optimum deployment altitudes and latitudes.
- Ground launch and recovery site selection based on existing infrastructure, fiscal and mission requirements, and political stability.
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-7b-
Major Structural / Material / System Challenges
Materials:
- Minimizing mass of platform (optimizing material mass/strength ratio).
- Selection of materials and techniques to minimize long term effects of high altitude atmospheric chemistry (ozone, etc.) and erosion on various platform and payload materials; particularly the telescope mirror coatings. Possible mirror coatings include Rhodium or Gold.
- Long term resistance to direct and reflected UV.
- Retention or renewal of lifting gas (minimizing permeability to Helium).
- Minimizing deformation of inflatable reflectors.
- Resistance to effects of low temperatures and thermal cycling
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-7c-
Major Structural / Material / System Challenges
Demonstration, phase 1:
Ground based demonstration & materials testing
-
Research activities:
Feasibility study, material & design studies, weather studies, thorough documentation, definition of phase 2, phase 2 implementation plan.
- Demonstration activities:
Acquisition of two or more small (~20 cm aperture) telescopes, testing and demonstration of telescope imaging, tracking, & communication routines, concurrent with materials testing and the development of flight management, telemetry & command, & other subsystems.
- Ground based demonstration:
20 cm aperture demomstration system testing and imaging of astronomical and terrestrial objects and satellites, followed by reflecting laser pulses off of a suitable satellite and detecting & verifying the returns; then repeating the above experiments with the telescope and its mounting positioned on actuators that will simulate the motion of an aircraft.
- Physical testing of design and materials:
Design, construction, and testing of a full scale lightweight composite 2.5~3 meter telescope and mounting (sans optics) with all pointing controls and actuators. Testing assisted by small mirrors and lasers. Operating this mounting on a proper test bed in windy conditions should identify many problems and facilitate their solution. (Construction of full size mounting could be deferred to a later phase).
- Long term chemical testing:
Includes testing of mirror coatings and other materials for erosion and oxidation in a quasi-wind tunnel with an ozone environment.
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-7d-
Major Structural / Material / System Challenges
Demonstration, phase 2:
Limited airborne demonstration
- Research activities:
Advanced feasibility studies, detailed weather studies and materials analysis, identification and analysis of major challenges, identification and prioritization of customers and their requirements, mission definition, frequency allocation, conceptual system design, site selection, identification of probable suppliers & industry partners, cost estimates, phased prototype implementation plan.
- Demonstration activities:
Use an existing well maintained blimp at ~2 km altitude. A blimp better simulates many anticipated high altitude LTA platform conditions and
offers many of the same advantages, such as long flight duration and a low air speed which will facilitate "outside" deployment of the telescopes. The demonstration will ultimately involve imaging of astronomical and terrestrial objects from an airborne platform. Satellite tracking with both telescopes will follow, one goal being to facilitate uninterrupted tracking (i.e. a smooth transition between telescopes) when possible. A simplified air-space optical communication demonstration may be possible by repeating the laser demo.
- Airborne demonstration instrumentation:
2 small (20 cm 50%) aperture tracking telescopes equipped for imaging and collimated ~532 nm to 2000 nm laser projection will be mounted below and to either side of the blimp envelope.
- Comments:
The blimp demonstration will allow the development of mission and control hardware, software, and procedures; but will not subject the hardware to the degree of ozone, etc., and low temperature that will be encountered at 16~21 km. Extrapolating from small scale low altitude demonstration data will not address many issues including platform resonance and maintenance of an accurate mirror figure. An additional phase could include the building and testing of a small high altitude
LTA platform for two or more ~20 cm +/- 50% aperture telescopes.
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-7e-
High Resolution Imaging and
Air to Retroreflector Satellite Demonstration Hardware:
- 20 cm +/- 50% aperture tracking telescope with a steering mirror or an equivalent means of high frequency precision pointing. Telescope also equipped for imaging and collimated ~532 nm to 2000 nm optical communication laser projection & reception. (Twin synchronized telescopes if budget permits).
-
Primary mirror = F/1.5; Cassegrain focus = F/13; Secondary obstruction = 18% of diameter.
-
Focusing telecompressor = 0.8x to 0.95x
-
Wide field imaging telecompressor = 0.25x
-
Fields of view = 0.04 deg. & 0.3 deg; 5 deg. & 50 deg. with attached video finders.
-
Mounting = 3 Axis. Azimuth = 1020 deg. travel; Elevation = 240 deg. travel; Cross axis = 60 deg. travel. This mounting configuration will allow the telescopes to maintain a satellite track regardless of the airborne platform orientation.
- Telemetry and command links.
- On board memory and dedicated air to ground RF links for payload.
- Possible low altitude platforms:
- Kuiper airborne observatory - includes a telescope.
- Existing blimps - telescope(s) probably below and to side of envelope. This is the most likely demonstration platform because it better simulates anticipated LTA platform conditions and offers many of the same advantages, such as long flight duration and a low air speed which allows "outside" deployment of the telescopes.
- High Altitude Platforms:
- SR-71 Aircraft - Reliable recovery but limited sky coverage & greater expense.
- Balloons - Good sky coverage but higher risk to payload and subject to spinning.
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-7f-
Simplified Experimental Imaging, Tracking, & Optical Communication Demonstration:
The demonstration will ultimately involve imaging of astronomical and terrestrial objects with small telescopes from an airborne platform. To demonstrate feasibility of an airborne relay for deep-space optical communication, or ARDOC, satellite tracking and a simplified air-space optical communication demonstration will follow. Active space borne optical communication systems are not readily available; therefore, a passive space borne retroreflector (such as that on Lageos) will be utilized for the near term. As necessary studies are completed and a platform has been selected that is compatible with fiscal and mission requirements, it is envisioned that a small scale system test would progress as follows:
-
Acquisition of telescope, instrumentation, and software.
-
Ground testing of telescope imaging, tracking, and communication routines involving imaging of astronomical and terrestrial objects; concurrent with platform flight management and RF link development.
-
Ground based satellite tracking and system testing, followed by laser pulses off of a suitable satellite and detecting and verifying the returns.
-
Repeat the above experiments with the telescope and its mounting positioned on actuators that simulate the motion of an aircraft.
-
Repeat the above experiments with the telescope and its mounting temporarily installed in a suitable aircraft. Additional activities could include imaging of planets, satellites, and terrestrial objects from the platform for public relations, possibly during network coverage of sporting events from the same LTA platform if lifting capacity permits.
-
Repeat the above experiments with the telescope interfaced to an RF relay, memory, etc., in the aircraft. Originate RF transmissions or pulses from the ground, optically re-transmit the information to an orbiting retroreflector, optically receive the reflected information, then re-transmit it to the ground with the RF link & verify accuracy.
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-8-
Proposed Payloads & their Locations on the Actual Multiple Function ALAT Platform:
- 3 TO 3.5 METER TELESCOPE(S) (On top, lower sides, or on a controllable array assembly which can be rotated to protect telescopes).
- Primary Mirror = F/1.0; Cassegrain Focus = F/15.
- Focusing Telecompressor = 0.6x to 0.8x.
- Wide Field Imaging Telecompressor = 0.2x.
- Fields of View = 0.016 deg. & 0.08 deg. with 12 mm CCDs.
- Field Acquisition Fields of View = 50 deg., 4 deg., & 0.3 deg.
- Mounting = Altitude / Azimuth with Optional 3rd Cross Axis.
- Used for observation, surveillance, tracking, ranging, & deep-space optical communication.
- 20 CM CORONAGRAPH
- Enhanced by nearby deployment of an optional small opaque robotic LTA device to create an artificial "eclipse" (should be effective at ~21 km altitude).
- 0.5 to 0.75 meter air to air & air to ground OPTICAL COMMUNICATION & SURVEILLANCE TELESCOPE (Below middle front).
-
Can theoretically resolve license plate numbers from up to 40 km.
-
Primary Mirror = f/1.0 to f/2.0; Cassegrain Focus = f/6 to f/15.
-
Mounting = Inverted Alt /Az with Optional 3rd Cross Axis.
-
RF RELAY TRANSCEIVER (Below center or inside).
-
DEEP SPACE COMMUNICATION AND OTHER ANTENNA(S)
In addition to utilizing conventional antennas, sections of the airship hull can serve as inflatable antenna reflectors or the hull can enclose inflatable antennas nearly as large as its diameter. This will shield them from deformation by the wind and other forces. Steerable feed allows
antenna pointing without changing platform orientation. Uses include communications, radio astronomy, and orbital object imaging/ranging. Concept can also be applied as a means to concentrate beamed power in order to permit use of a smaller and lighter beamed power collector (rectenna).
-
BEAMED POWER (Below or inside middle front or rear of platform).
-
CELLULAR TELEPHONE, BROADCASTING
-
REGIONAL POSITION REFERENCE and automobile, etc. tracking sensors.
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-9-
Disadvantages of a large lighter than air "airship" platform:
- Requires large structure for fabrication, maintenance & storage
- Vulnerability to surface winds and jet stream related turbulence
- Platform development and fabrication cost.
- Lightweight payload development cost.
- Power or tether required to maintain platform altitude & location.
- Requires FAA and FCC clearances.
- Platform motion complicates pointing and tracking routines.
- Atmospheric effects at 16~21 km can influence some aspects of payload performance.
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-10-
What Else is Being Done or Proposed Now?
-
Ground based optical telescopes
- subject to cloud cover.
-
Proposed ground based optical communication network
- subject to cloud cover.
-
Kuiper airborne observatory
- medium aperture, lower altitude, short time aloft.
-
Proposed SOFIA 2.5 meter telescope in a 747
- less time aloft than an airship.
-
Proposed microwave powered airplanes (JPL, Canada)
- low payload capacity.
-
Air force air to satellite O.C.D.
- medium aperture, limited range.
-
Proposed Polar Stratospheric Telescope (POST)
- tethered @ ~30K', limited to deployment near polar regions.
-
HALROP (proposed Japanese solar powered airship)
- a platform that may be adaptable to ALAT.
-
Balloon and rocket borne telescopes
- short time aloft, stabilization difficult.
-
Hubble space telescope
- expensive, difficult to service.
-
Proposed deep space optical communication relay satellite(s)
- expensive.
-
Various other proposed inflatable antennas and structures.
(Most are for space missions).
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-11-
Additional "Spin-off" Technology and Applications:
- Long endurance lighter than air platform research.
- Test bed for inflatable structure research and development.
- Test bed for high altitude instruments, power plants, propulsion.
- Platform and telescope mounting can be used to facilitate other temporary or permanent scientific and commercial payloads.
- Inflatable structure concepts applicable to ground, air, and space based structures. Small to very large (>100 meter) precision inflatable antennas with lifting gas assisted adaptive surfaces can be deployed at various altitudes in the atmosphere or under inflatable covers at terrestrial sites. The antenna weight is supported mostly by lifting gas. Under some conditions, liquid (such as water) can be used to assist in positioning the antenna. In the case of terrestrial sites, liquid can also be used for partial support, particularly if the antenna is surrounded by a nearly spherical outer envelope.
- Steerable antenna feed concepts can be applied to rigid fixed dish antennas of either wide or limited scan range.
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-12-
Illustrations:
The following illustrations show the development of the concept and related "spin-off" technology. To allow a long mission duration and minimize boundary layer effects on the payload, all airborne embodiments utilize lighter than air platforms.
Airplanes with adequate lift require substantial power and forward air speed in order to fly. This causes undesirable boundary layer effects and makes the aircraft more difficult to track; and tracking can be important for optical and some other communications, or in beaming power. Flying rotors (variants of the helicopter) can remain in a fixed location but can require a lot of power. The necessity to "de-spin" the payload on a flying rotor also adds complexity.
Since the stability of the telescope is very important, it is either attached to the airship structure or suspended below widely separated support points. The concept has evolved considerably since conception, resulting in "spin-off" applications and technology that will benefit other fields. The drawing and illustration list is shown below.
All illustrations are under construction, so the drawing files are not posted. When posted, each drawing file will be about 120K.
- Illustration of the ALAT Platform***
- Illustration of ARDOC (Airborne Relay for Deep-Space Optical Communication)***
- Original ALAT Conceptual Drawing; Arrayable LTA (Lighter Than Air) Platform Unit with Telescope***
- Second Conceptual Drawing; LTA Platform with Multiple Payloads***
- Third Conceptual Drawing; LTA Platform with Internal Inflatable Reflector***
- Detail of Steerable Feed Inflatable Antenna Concept***
- Ground Based Steerable Feed Antenna with 110 deg. Fully Illuminated Scan Range (based on inflatable version)***
- Ground Based Steerable Feed Antenna with Limited Scan Range***
- Airborne or Spaceborne Inflatable Antenna with Inflatable Spherical Enclosure***
- Large Ground Based Inflatable Antenna with Inflatable Spherical Enclosure under an Inflatable Dome Housing***
Do you need creative solutions for your engineering, motion picture, or other project? Jeffrey R. Charles performs systems engineering, conceptual design, and technical writing for a variety of applications. In addition, Jeff can provide science consulting in regard to total solar eclipse phenomena, and engineering consulting for optical instrumentation. Please direct inquiries to Jeffrey R. Charles jcharles@versacorp.com or click here
for more information.
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