National Aeronautics and Space Administration

Figure 2 Altitude Requirements Summary

Figure 3 Weight Requirements Summary

Figure 4 Power Requirements Summary

Figure 5 Pointing Requirements Summary

Figure 6 Data RatesRequirements Summary

Preliminary requirements based on 32 strawman experiments in the spreadsheet (the gamma-ray group consensus was not included in the statistics).

Minimum Science Requirements:

1. Need to achieve an altitude of >115 kft to satisfy 75 % of the strawman experiments.
2. Need to achieve an altitude of >125 kft to satisfy 100 % of the strawman experiments.
3. Need to achieve >400 Watts power to satisfy 78 % of the strawman experiments.
4. Need to float >2000 pounds science weight to satisfy 75 % of the strawman experiments.
5. Recovery is desired by most missions, but not required.
6. Latitudes higher than 70° are required by 7 experiments.
7. A thermal control system maintaining a temperature in the range from 0°C to 20°C appears to satisfy all the experiments.
8. Pointing accuracy/knowledge requirements range from none to spin scan, to a few degrees, down to sub-arcseconds. There are 3 to 4 distinct clusters to satisfy. Pointing under 1 arcminute will be a challenge.
9. Data collection occurs continuously for 11 experiments, daytime only for 6 experiments, night only for 4 experiments and 11 experiments provided no requirement.
10. 50 % of the experiment Principal Investigators wished to operate as a PI mission.
11. Per day data volumes up to 80 Gigabits.

Design Options Needed Based On Science Requirements

(Mission operations profiles or concepts based on the strawman science payload requirements from the October 96 workshop)

There are two concepts with different requirements based on latitude.

1. Inside The Arctic Or Antarctic Circles
   • Night time operations require nonsolar power source
   • Reliable communications for polar zones of exclusion
     • Geostationary communications satellites can not see poles
     • Many of the LEO, Little LEO & MEO communications ventures tend to have
       • 60° inclined orbits, again excluding the poles
     • Need to find those with coverage at the poles. There are at least 3 known candidates --
       • IRIDIUM, low data rate
       • GLOBALSTAR, 19 kbps
       • ICONET, rates unknown
     • Need to investigate Military Communication Satellites ( U.S., Russian, etc.)
     • Need to investigate amateur radio operators communication satellites.
   • 100 day missions will experience extreme day/night cycles
     • need to design for 60 day long days and nights
       • 4 experiments want day time observations
         • this limits mission to ² 60 days
       • 2 experiments want continuous dark
         • this allows 100 day mission with substantial power needs
       • 1 experiment wants continuous observations
         • this allows 100 day mission (30 to 60 days sun then 40 to 70 days dark)
     • this impacts the power design and
     • the thermal design
   • equipment needs to survive radiation at magnetic poles
   • risk of cutbacks in NSF program (cost would become great)

2. Low Latitude

• Need to investigate power systems sufficient for the science instrument and the support system
  • a sun tracking solar array or an omni directional array
  • battery or other storage for 12 hours
  • non-traditional power source, e.g., wind/electrical generator a few 1000 feet below balloon
• requires thermal control for 12 hour day/night cycle
• may require pointing control for communications antenna
• may require new international agreements
• inadvertent technology transfer considerations

Design concepts common to both latitude options described above are as follows.

1. The payload will be "tracked" continuously from a central ground station.

2. Trajectory forecasts will be maintained and continuously updated.

• forecasts will include wind predictions.

3. Real-time data and commanding will be available at the launch site, central ground station, and PI institution.

• Need to design for a line of sight in flight checkout period after launch (~ 5-6 hours duration)

• 50% of PIs want PI mode of operation.

1. Science data will be recovered at a frequency that insures mission success and no more than 25% of accumulated data is lost.

5. Science instrument pointing requirements show need for four different systems. Appropriate modular design and interface needed.

• No pointing required
• Spin scan system required
• Pointing to one arcmin required
• Pointing to arcsec and sub-arcsec required

Current Balloon Program Capabilities

• Power - The SIP provides 300 Watts, as an upper limit 600 Watts has been supported.
• Commanding & Data Return - Omni/TDRS supports 2 kbps, up to 6 kbps maximum supported.
• Thermal design - The thermal environment is much more severe than the typical spacecraft environment when looking at the cyclic thermal loads. The thermal analysis techniques and control methods employed for ballooning are fairly well established and have been proven on many flights. Most of the control methods are passive and do not require thermal blankets or complicated active systems. The tools currently used are TRASYS, SINDA, and TSS. Due to the long days and nights a totally passive system may not be possible. The required power allocation for thermal control may be higher than for a typical spacecraft which is around 5%.
• Automated operations - These include 1) an aneroid flight termination switch in the event a balloon descends below a minimum acceptable altitude for flight safety; 2) a burst detector which will terminate the flight in the event of a balloon structural failure; 3) an automated balloon differential pressure control/valve system for pressurized balloon systems; and 4) an automated ballast control system for the dropping of ballast for maintenance of altitude.
• Location of Balloon Craft - Balloon/ballooncraft position is determined on-board by redundant GPS receivers with the information transmitted to the ground station through the FM/PCM line of sight link, the INMARSAT Standard C over-the-horizon (OTH) link, the HF/ARGOS OTH link, or the TDRSS MA or SA link. In addition, position is also obtained vis ARGOS PTT's (Platform Transmitter Terminal) received at the Wallops Remote Operations Control Center or the Palestine Operations Control Center.

Areas Requiring Further Technical Definition

The information received from the science community has some requirements that appear technically challenging. This section attempts to describe some of the areas that require more technical definition.

Weight: Some of the strawman missions have requirements of up to 3300 lbs., many require 2000 lbs. For the demonstration mission only a 2000 lbs hang weight is advertised. This is a challenge for the ULDB program.

The system weight must be viewed from a system standpoint. There are many areas where the structural system can be "designed" instead of "built" for significant weight savings. This requires a weight analysis for each mission.

Power: Some of the strawman missions require over 800 Watts of power. The challenge is to meet higher power needs with manageable impact to weight and stability. An engineering trade study is needed to identify which power source might best meet the needs of the ULDB program. Some potential candidates for power sources are provided in the Summary List Of Technologies Under Consideration section beginning on page 40.

A combination of the different types of power systems may be the solution.

Location: Redundant GPS will provide location information. ARGOS is currently used as a backup position source.

Pointing Control: Several of the strawman missions require pointing control and knowledge. The challenge will be to achieve the desired accuracy in a craft acting like a pendulum with some elasticity in the load train. Also, for those missions requiring a lot of power the size of the solar arrays could introduce jitter into the system.

Candidate pointing systems for study are provided in the Summary List Of Technologies Under Consideration section beginning on page 40.

Terminate and Recovery Systems

Payload recovery is not a requirement.

• It is desired by the majority of PIs.
• High data rate line-of-sight telemetry and preemptive cut down plans for recovery and re-flight in case of payload failure may be feasible for some missions.

A study on feasibility and systems needed should be performed.

• An alternative concept could be developed for payloads that require recovery with defined tradeoffs.
• An aircraft could be made available at potential termination areas for cut down and recovery operations.
• Recovery systems (parachute) could be deployed on flights.
• Is a self-destruct system needed?
• Other terminate and recovery options for evaluation are:
  • steerable parachute systems to improve recovery operations,
  • ground transmitter to ease finding a lost payload,
    • look at animal collar systems,
    • emergency transmitter systems,
  • inflatable flotation devices,
  • "smart" auto cut down systems that use GPS + wind predictions, and
  • improved wind prediction.

Autonomous Operations: This is desired on many of the strawman missions and will likely be needed on most missions to handle functions like thermal stability, battery discharge and charge cycles, other day/night cycle activities, and to execute safety procedures under various scenarios such as failure of balloon location communications with the operations team. These systems need to be designed and tested to provide high probability of survival for 100 days at altitude.

Thermal: Thermal control needs to be maintained to within the required operations temperature range for both the science instrument and the support package. The tools currently use for thermal analysis are TRASYS, SINDA, and TSS. They are proven for the existing balloon programs; they have yet to be proven for the ULDB Program. The following tasks need to be undertaken to provide models to help evaluate different ideas for maintaining thermal control of the balloon craft packages.

1) Characterize the range of wind speeds likely to be encountered by a balloon payload at float. An initial estimate can be obtained from existing measurements of wind speed vs. altitude which NSBF takes with their routine soundings, by taking the derivative of this curve and multiplying by the length of the flight train.

2) Develop a model for wind cooling at float conditions. There is likely already such a model at least for pressure ~1 atm; if not it is fairly clear how to develop the framework, since the airflow is likely to be nearly laminar. If necessary, develop a plan to validate this model for float conditions.

3) Develop a model for convective cooling, or establish design rules under which convection can be safely ignored.

These are only models to help determine thermal design. Since a consistent, half degree increase or decrease in the temperature could put the balloon craft into a non-operating state, a challenge will be to devise a test strategy that can ensure high probability of thermal survival of the balloon craft for ³100 days. Additional systems for cooling or heating will be needed for some of the experiments. An area of concern is that these additional systems will impact power requirements for the balloon.

Communications: There appear to be low rate options that can accomodate both commanding and return of the balloon craft and science instrument engineering and housekeeping telemetry. Costs of various options needs to be studied. Options for the return of high rate science data are limited and need further study. Some of the strawman missions require real time response. Some of the experiments call for daily data return, daily commanding or even hourly commanding. This raises the question of what is an acceptable level of cost. TDRSS and Military satellites cannot be relied on to give balloons top priority, other options should be explored. Table 4 outlines initial information on some of the communications options that need further study. Figure 7 provides a high level concept for communications requirements and packaging schemes.

Table 4 Rough Comparison of Possible Satellite Communications Service Options

Figure 7 High Level Communications Concept

Continuous Coverage

Continuous coverage may not be possible, practically or due to cost. There are two ways to go: non-continuous communications, or meeting the PI's requirements. It is not clear if much thought has been given to continuous coverage issues such as personnel to monitor communications around the clock for 100 days or the cost to support such operations.

Zones of Exclusion

An initial calculation of the zones of exclusion is presented in Figure 8. The ZOE is described for a 3 TDRSS system, expected in the future, not currently (2 TDRSS system). If the balloon drifts into these zones there needs to be an alternate way to contact the balloon for safety reasons. These issues are under study.

A hybrid solution of a 2 kbps communications connection for command and housekeeping and a higher rate line for data return may be appropriate and will be studied.

Figure 8 Plot of a Balloon Ground Trace and the ZOE at the Poles

System Engineering Information and Concerns

How might the physical environment affect the balloon craft? This relates to passage to altitude and in retrieval as well as operation at the limits of the atmosphere in the range of 100,000 ft to 130,000 ft. The balloons may fly mainly around the poles or in equatorial areas. Conditions that may affect the balloons performance and integrity are of concern. A summary of the known balloon environment follows.

Balloon Environment Summary

Balloon Behavior:

Balloon Ascent Rate : typical 800-1000 fpm. It takes around three hours to attain altitude.

Balloon Rotation Rates : typical < 60 deg/min at float

have seen during ascent/descent ~ 180 deg/min

Balloon Dynamics : (Vertical Oscillations & Frequency Forthcoming)

Loads:

Launch : typical < 1.5 g's

Ascent : typical < 1.1 g's due to wind shears, ballast drops, etc.

Terminate : typical < 10 g's

Impact Velocity : typical < 20 fps

Wind or wind shear effects TBD. This is particularly important for those experiments that require pointing accuracy and have large power demands.

Release acceleration - 10 g pulse when parasail opens.

The termination loading can be around 10 g's. We have typical curves for the acceleration and velocity at termination for balloon payloads. The implementation of a flight termination load reduction technique is now being explored using a rip stitch attenuator. This promises to reduce the 10 g loading by half or more. The technique used could also be tailored to the specific payload to reduce the shock loading even more. The method and procedure to do this has been determined and it is a matter of implementation and testing.

Landing acceleration (use airbags like the Mars lander)?

This is a known quantity, and much less severe than the release accelerations. Crush pads are not as elegant as an airbag system, but can be easily designed to do the job for minimal weight, minimal complexity and minimal cost.

A related issue to all of the accelerations that a payload may see concerns what constitutes a fully recovered payload. It is obviously not acceptable to have pieces fall off the payload at termination and then fall to the ground. Depending on the parts, it may be acceptable for them to become non-functional at termination or upon ground impact. This is an area for a trade study or cost benefit analysis. The core of the instrument which accounts for most of the cost of the payload may be able to handle the imposed acceleration loads. The associated costs and increased weight to ensure survivability of the other parts may not be worth it. Some items could be considered as "throw away" if the effort to ensure survivability costs more than replacement/refurbishment.

This should all be put in the context that the main acceleration event is after the operating portion of its life. This is exactly opposite of a launched spacecraft which sees its worst accelerations at launch before being put into operation. One could envision, for example, a detector system that is built to handle the launch acceleration, but not the termination event. To build the same detector that can survive the termination would be a significantly heavier and more expensive.

Atmospheric :

Tropics : -90C @ ~ 50-60 k-ft altitude

Polar : -45C @ ~ 30-35 k-ft altitude

mid-latitude : -55C @ ~45-60 k-ft --> -80C in summer

(seasonal & latitudinal fluctuations)

Temperature profile - Troposphere can reach -90C and balloon can take 20 to 30 minutes to travel through the troposphere during launch. Launch temperature range can be from -10_C to +40_C

Chemical components and vapor levels TBD.

Radiation:

Solar Constant (seasonal) : 1358 W/m2 (nominal)

1312 W/m2 (minimum)

1404 W/m2 (maximum)

Albedo : 0.1 (minimum)

0.9 (maximum) polar

Earth Flux: 90.7 W/m2 (minimum, Tropospheric cloud top temperatures of -90°C)

594. W/m2 (maximum, Desert @ 320K planet temperature)

Electro-static gradients, Electro-magnetic fields TBD.

Lightning Strike:

A concern at high altitudes is lightning strikes at float altitude coming up from clouds below (which has happened catastrophically on one mission) when flying across severe storm boundaries and the type of payload. Special hardening of instrumentation or procedures may need to be developed.

Programmatic System Engineering Concerns

In studying the Balloon craft subsystems the Balloon Program needs to be tied into NASA objectives. We need to identify NASA needs that correspond to the Balloon Program needs. Examples are given below.

Data Collection - Which methods is more useful to future NASA missions? Satellite cellular command/TDRS telemetry link hybrid would be of interest to some small missions, what are other options?

Thermal - What new thermal control systems being designed for use in space might have application on balloons?

Power - Are there new solar cells, storage batteries, or fuel cells not yet tested that could be used on a balloon craft to provide test data useful for future space missions.

Pointing Control and Autonomous Operations - Can the balloon program perform pathfinder flights that test new technology that could be used on small satellites or proposed balloon exploration of other planets?

Risk Mitigation - Can any of the new technologies identified be flown on current LDB programs to reduce to the 100 day programs?

Differences between balloon and space mission or ground experiment

• Launch
  • Large static charges can be generated during balloon inflation
  • Minimal vibration
  • Three hours needed to attain altitude
  • Restrictions based on launch vehicle
• In Flight Check of Balloon craft
• Operation
  • Long day/night cycles
  • Long periods in the ZOE
• The Environment
  • Different radiation environment
  • Residual atmosphere

• State Department Concerns on Technical Transfer.
• Risk Of Cutbacks In NSF Program (Cost Would Become Great).
• Adequacy of Launch support services.
  • Does the launch site need upgrading?
• General International Agreements.
• International involvement in development.

(This is a partial list given that all study team members are not yet onboard.)

(This will be a summary list of items identified in the report, it is not yet complete.)

Communications

Use of various communications satellite options.

• TDRSS
• Commercial (Little LEO, LEO, MEO, Geosync.)
• Military (USA, Russian)
• Amateur Radio Operator Satellites

Use of new Antenna Technologies.

Also various options on storage media drops over recoverable site.

• Ruggedized Mass Data Storage Device

We are seeking a very low cost, compact, rugged mass data storage system with >1 terabit capacity. We are seeking a reusable system in the <$100K range. Several of these systems will be used for on-board recording in the Advanced Long Duration Ballooning Program. Data will be recovered by parachuting to the ground (must withstand 10 g shocks). The drop package could be the whole system or just storage components. Multiple drops are required for reliable recovery of the data. The system needs to be able to operate at altitudes between 100000 and 130000 feet (pressures between 11 and 3 millibar). It needs to operate from a 28 V unregulated battery input. The operating temperature range without thermal control may be extreme so that extended operating temperature range is desirable.

Balloon Craft Location

• GPS
• Weather Based Predictions

Power Systems

Some potential candidates for power sources are:

• Solar Arrays
  • The system must be designed, weight and size, for the worst case operating conditions for each, the polar flights and the low latitude flights. The size of the system is not as much a concern as the size of the stowed system for launch. Deployable arrays, which can be either unrolled or inflated, may be a very desirable option. A sterling engine may also be an approach for using solar energy.
• New battery technology (rechargeable Lithium batteries?)
  • we want deep discharge but only 100 cycles + some TBD margin.
  • A fuel cell system can be attractive for high power "short" flights (1 kW, 20 days) or for moderate power for longer flights (200 W, 100 days). Fuel cells also offer the advantages of "waste heat" for thermal control, water drops for ballasting, and the possibility of using the waste water for thermal storage (solar heated during the day and acting as a supplemental heat at night).
• Flywheel energy storage systems
• Wind power generators suspended a few hundred or thousand feet below the balloon craft.

Thermal Systems

• Thermal Blankets that will work at balloon flight altitudes.
• High Efficiency Heat Pump System

We are seeking a high efficiency active thermal control system for a long duration balloon experiment. Total thermal loads will be in the 300 to 1600 W range. Altitude of operation will be 100 to 130 kft (residual pressure between 11 and 3 millibars). The output thermal load must be radiated to the Earth or to space under all possible conditions (clouds, over water, over land, etc.) We require thermal control on the input side to +/- 10 degrees C with a goal of +/- 1 degree C. Thermal control must be maintained in daytime and nighttime conditions (12 hours daylight and 12 hours darkness at low latitudes). Expected mission lifetime is ~100 days and we require a mean time to failure >200 days.

Pointing and Control Systems

Candidates for study involving the pointing system are as follows:

• Improved sensors are the primary requirement for better pointing
  • Fiber optic gyros
  • Phase comparison GPS orientation measurement
  • Daytime star cameras (special cooling and baffles required)
• Drive mechanical systems
  • Improved decoupler - three axis floating ball suspension torque sensing decoupler.
  • Load train improvements
    • Better mechanical model
    • Increase stiffness
    • Non-magnetic and lighter using composites
  • Magnetic torque's or cold gas jets
  • Active damping for pendulum motion
  • Composite structure for less multipathing error in GPS and less magnetometer error
  • Active balancing systems

Superpressure Balloon Materials and Technology

Code 741 response

This represents sub-SMEX space mission technologies that are applicable to and interested in the ULDB.

Economical Approach

• A modular payload buss based on a commercial (industrial) PCA Pentium processor with a 1553 buss and R-442 I/O (Spartan type design).
• 1553 based GPS, sensor, and motor controls are available.

Power requirements especially with respect to the flight paths reveals a wide range of power options.

• At the low end a solar array rechargeable lead gel-cell will produce the power required at the lowest cost with a weight penalty.
• A more weight efficient and costlier approach would be a lithium primary system or a solar array silver-cell secondary system with individual cell charge control.

• Mission Operations Concept Document

• Identify Different Design And Technology Options
  • Communications Options
  • Power And Thermal Options

• Develop Cost Estimates For These Options