AEOLUS – WIND WEIGHTING PROGRAM AND CONCEPT FOR

UNGUIDED SUBORBITAL LAUNCH VEHICLES

Wolfgang Jung(1), Raul de Magalhães Gomes(2), João Emile Louis(3), Olavo de Souza Neto(4)

(1) Deutsches Zentrum für Luft und Raumfahrt (DLR) e.V., RB-MR, D-82234 Wessling, Germany (2),(3),(4) Flight Safety Activities (FSAct), São José dos Campos, Brazil

(1) Phone: (+49) 8153 28 2724, E-mail: wolfgang.jung@dlr.de (2) Phone: (+55) 35 3343 3191, E-mail: raulm.gomes@flightsafetyactivities.com.br (3) Phone: (+55) 12 3941 6164, E-mail: joao.louis@flightsafetyactivities.com.br

(4) Phone: (+55) 84 4008 7348, E-mail: olavo.souza@flightsafetyactivities.com.br ABSTRACT Mobile Rocket Base (MORABA) of the DLR and Flight Safety Activities Ltda. Of Brazil are currently developing a new real-time wind-weighting program for Unguided Suborbital Launch Vehicles. The program, named after the Greek god of winds Aeolus, is embedded in the launch safety concept consisting of a real-time 6-DOF program, a remote launcher control, the use of an unmanned aerial vehicle, and a GPS-based instantaneous impact point prediction. Aeolus provides all Go/No-Go criteria necessary for the final launcher adjustment as they are:

o Release sequence and altitude levels for high, medium and low altitude radiosondes

o Ground, ballistic, absolute, and shear wind limit verification

o Booster and sustainer impact prediction o Elevation and azimuth variation verification

after final launcher adjustment o Wind stability analysis based on

meteorological and actual wind measurements The presented wind-weighting setup supports flight safety personnel with a state-of-the-art system, ensuring safe sounding rocket launches, minimizing countdown attempts, and optimizing countdown duration. 1. PRESENT WIND WEIGHTING SET-UP The present wind weighting technique comprises various 6-DOF, no-wind, pre-flight trajectory computations, to calculate the wind sensitivity per altitude layer and for different launch elevation angles. Unit wind and tower tilt computation complete the wind weighting process. A RSS (square root of the sum of the squares) or Monte-Carlo analysis provides the 3-sigma dispersion ellipse. During the countdown, ground winds are derived from a wind tower with fixed anemometer levels to determine

wind direction and velocity data with a frequency of 1 Hz. The alternative use of RADAR, SODAR or LIDAR profilers was investigated by Andøya Rocket Range in Norway and Esrange Space Center in Sweden, but they were not implemented, because of inadequate accuracy (e.g. acoustic profiler such as SODAR near the sea shore), or by their considerable cost impact. GPS radio sondes are launched frequently, providing position, wind velocity and direction with a resolution of around 5-6m/s, depending on the ascent velocity of the balloon. The launcher adjustment is performed with the so called “Lewis” method, meaning the winds at dedicated altitudes are multiplied with the corresponding wind sensitivity of the vehicle. Finally the computation is verified with a 6-DOF trajectory program. The launcher adjustment settings are then communicated to the launch crew, which manually sets the launcher to the desired values, several minutes before lift-off. Once the final launcher settings are applied, the wind anemometer readings and their effects are monitored by flight safety to verify that any changes, without additional launcher correction, are within allowable tolerances. 1.1 Dispersion of Unguided Suborbital Launch

Vehicles The flight of Unguided Suborbital Launch Vehicles (USLV) is often perturbed by effects, which can cause angular dispersion before rocket spin starts to build up, as follows:

o Thrust misalignment o Fin misalignment o Variation in launch velocity causing variation

in wind effect o Nozzle erosion and distortion o Separation dispersion (boost carriage)

___________________________________________________________________________________ Proc. ‘19th ESA Symposium on European Rocket and Balloon Programmes and Related Research, Bad Reichenhall, Germany, 7–11 June 2009 (ESA SP-671, September 2009)

To minimize the dispersion, the VSB-30 and other launch vehicles are equipped with an additional spin-up system. Further effects which can cause impact dispersion in the range are:

o Launcher setting errors o Tip-off effect o Variation in burning time o Ignition delay of upper stages o Variation in ambient temperature at altitude

causing variation in speed of sound and density, affecting the apogee altitude

o Variation in total impulse o For a given payload, variations in vehicle

structure weight o Determination of aerodynamic coefficients

with half-empirical software o Wind

The most significant dispersion factors are the launcher setting error and the influence of wind variations. The launcher setting error can be easily determined prior to launch, so that the real impact is minor. The wind variation should not be underestimated, if one reflects on the wind measurement process. The placement of the wind tower is often a compromise. The tower has to be located close to the launcher, but the launcher itself might be “hidden” in a valley, or behind a sand dune, so that gusty or shear winds often are not detected adequately. Radio sondes are not launched as close as possible to lift-off, because most ranges lack an automatic release system. 1.2 VSB-30 Wind Sensitivity To provide a practical example of the wind related impact, the data for the VSB-30 sounding rocket vehicle are presented. The VSB-30 sounding rocket vehicle was developed in cooperation between the Brazilian Aerospace Institute (IAE) and MORABA. It reaches with a 400 kg payload an apogee of 250 km. The head, tail and lateral unit wind effect is calculated as 14 km/(m/s). This means that a 1-sigma ballistic wind uncertainty of 1 m/s causes an impact point deviation of around 14 km! The tower tilt effect at high elevation angles is 35 km per degree. A typical 1-sigma launcher setting tolerance of 0.1 degree would result in a impact point shift of 1.8 km. Taking all of the possible dispersion factors into account, the 1-sigma downrange dispersion radius is computed as 17 km.

Figure 1. VSB-30 Sounding Rocket

The fin of the VSB-30 was designed such that the structural integrity is guaranteed to withstand Angles of Attack (AOA) up to 4 degrees. The AOA is THE most important design parameter. For a launch at Esrange Launch Center the following boundary conditions apply:

o Nom. impact range: 70-75 km o Nom. no-wind launch elevation: 88.0 deg o Max. actual launch elevation: 89.0 deg o Max. 1-sigma dispersion radius: 20 km

Looking at the constraints, it can be easily seen that the wind weighting is not trivial, demanding high accuracy of the measured data. 1.3 Boost Guidance System A method to reduce launch delays caused by wind is the implementation of a guidance system, e.g. the S-19 family (DS19, S19D, S19L) from RUAG Aerospace Sweden AB. The systems are well known, flight proven, and qualified for single rail launchers. A negative aspect is certainly the additional mass, nevertheless even if it can be compensated with light weight structures such as carbon fibre. A guidance system requests a fully redundant flight termination system (receiver, charge) and related ground infrastructure (telemetry, telecommand, radar). The cost impact should not be underestimated. The break even point for such an investment depends on the project and the selected launch range. Of course, an improved wind weighting system will not change the meteorological conditions at the launch or substitute a guidance system, but it widens the limits such, that the launch probability increases. 2. PROPOSED WIND WEIGHTING CONCEPT A new wind weighting concept shall provide accurate wind measurements as close as possible to lift-off, using actual atmospheric profiles, and allow “last minute” corrections of the launcher settings. The realisation of the new wind weighting concept can be achieved with

o a 6-DOF real-time wind weighting program, called Aeolus,

o Unmanned Aerial Vehicle (UAV), o Remote Launcher Control (RLC)

The set-up is completed with the in-flight, GPS based o Instantaneous Impact Point Prediction (IIP)

The following sections describe the proposed concept in detail. 2.1 Wind Weighting Program Aeolus The 6-DOF real-time wind weighting program Aeolus complies with the Electronic Code of Federal Regulation [1]. The software offers a clear Go/No-Go

decision for launch flight safety. The software is based on MORABA’s Rocket Simulation (ROSI) mission proven program. The program automatically guides the Flight Safety Officer through the countdown operations, e.g. it provides him with the release sequences for the different types of radio sondes (high, medium and low altitude).

Figure 2. Go/No-Go Criteria

The program controls the limits for launch corridor, ground, ballistic, shear winds and/or AOA. The GUI further comprises a timeline of each ground wind level, and ballistic wind variations. The impact prediction for all stages is shown as well as the verification of the elevation and azimuth deviation, once the final launcher adjustment is made. The wind stability analysis is computed by comparing the results of the different sondes with respect to their influence on impact dispersion.

Figure 3. Wind Stability Analysis and

Impact Point Variation The program offers a high grade of mobility and flexibility, meaning that the software can be connected to a LAN and/or Serial I/O for anemometers and radio sondes independently. The number and position of the fixed anemometer levels can be adjusted manually.

All range and mission related set-ups can be recorded and simulations with archived or weather forecast profiles can be performed. Aeolus can be changed to the present wind weighting set-up for comparison. The software is a significantly improvement as the AOA can be computed accurately assuming that the wind profiles are determined frequently with an adequate resolution of 5-6 m/s. Note that the presented figures represent only an extract of the entire graphical windows available in Aeolus. 2.2 Unmanned Aerial Vehicle Currently GPS equipped radio sondes or corner reflector targets, are released to determine wind speed and direction as close as possible to lift-off. [1] requires two measurements within 30 minutes, up to an altitude corresponding to 80% of the entire wind sensitivity level, which means for a VSB-30 around 2 km above ground. The use of UAV as a measurement platform is a new idea introduced by Astrium Space Transportation and MORABA. Hand-launched, battery driven UAV’s offer real-time wireless transmission of:

o Position, wind direction and velocity from GPS o Air pressure, temperature, humidity o Video

The UAV has an autonomous navigation system which can be programmed to follow the intended ascent flight path of the launch vehicle. A second UAV could be used to determine the wind profile during descent. In-situ measurements of the meteorological data can be used to improve the atmospheric model of the 6-DOF program and to verify the stability of the weather information. A video camera offers the possibility of surveying the weather conditions in the impact area (fog or sufficient snow). Furthermore the parachute descent and the drift can be surveyed. Finally the UAV can be used to assist counting the population in the impact area for risk hazard analysis which is currently performed by helicopters. The maximum altitude is in the order of 5 km with an airborne availability of several hours. A UAV can replace the medium and low altitude radio sondes. Last but not least, the system is reusable and the operational costs are cheaper than using “one way” GPS sondes. 2.3 Remote Launcher Control MORABA’s newly refurbished MAN 2 launcher can be controlled remotely via internet or hard-line. If the Remote Launcher Control (RLC) is combined with the wind weighting program, this set-up allows

operating the launcher automatically via Flight Safety strictly within fixed limits, but nearer to lift-off. The last minute correction of the launcher settings minimizes the deviation of the impact point [2].

Figure 4. MAN 2 Launcher Control Display

2.4 Instantaneous Impact Point Prediction The on-board computed Instantaneous Impact Point (IIP) prediction, developed and in use by DLR [3], provides real-time data of the vehicle as well as impact prediction for both, guided and unguided vehicles. Figure 5 shows the graphical interface, delivering instantaneous information of vehicle position, heading and velocity, as well as IIP position and Estimated Time to Impact (ETI). The validity of the information and the satellites in view are given The program was initiated for the first launch of the VSB-30 in 2004.

Figure 5. IIP GUI Layout

3. CONCLUSION The improvement of the wind weighting set-up as presented, allows the flight safety officer to enlarge the limits within agreed standards, and consequently reduce launch delays due to unfavourable wind conditions. The suggested modifications have a minor cost impact and can be implemented successively.

Figure 6. New Wind Weighting Concept

It should be noted that, independent of the proposed system, the determination of the wind data as close as possible to lift-off, either by radio sondes, profilers, UAV or a combination of such, are the crucial criteria for an accurate prediction. A last minute correction by RLC is convenient, but can also be achieved with a revision of the countdown activities between flight safety and blockhouse, acknowledging that the launch crew will not support remote control of their launcher by flight safety. The ballistic wind limits of the VSB-30, measured with a conventional system are limited to around 6 m/s, while the proposed changes, 6-DOF inclusive AOA determination, would permit raising the limits to around 10 m/s. Comparing the flights in the past, this means more than 25% of the countdown delays due to wind could have been avoided! The 6-DOF wind weighting program Aeolus will be tested during the TEXUS 46/47 campaign in November 2007. The test trials for UAV and RLC are foreseen in 2010. 4. REFERENCES 1. Electronic Code of Federal Regulation, Title 14:

Aeronautics and Space, Part 14: Launch Safety, http://ecfr.gpoaccess.gov

2. Garcia A., Automatic Launcher Control, Master Thesis, ITA, TBC

3. Montenbruck O. et al., GPS Based Prediction of the Instantaneous Impact Point for Sounding Rockets, Aerospace Science and Technology 6, 283-294, 2002