Launch Vehicle Environmental Effects: The Stratosphere

Launch Vehicle Environmental Effects: The Stratosphere April 1997

Environmental Programs
The Aerospace Corporation
edited by Dr. Valerie Lang

ABSTRACTS

1. Stratospheric Ozone Reactive Chemicals Generated by Space Launches Worldwide, B.B. Brady, E. W. Fournier, L.R. Martin, and R. B. Cohen, TR-94(4231)-6, The Aerospace Corporation, El Segundo, CA, 1 November 1994.

We report quantities of inorganic chlorine compounds and aluminum oxide particles (A12O3) deposited in the stratosphere and troposphere by solid rocket propelled launch vehicles. Totals are presented by launch vehicle type, summarized on an annual basis, and projected to the year 2010 using standard mission models. Data are given for Air Force, NASA (shuttle and expendable vehicles), the European Space Agency (ESA) (Ariane 5) , and the Japanese Space Agency (H-1 and H-2).

Whereas inorganic chlorine compounds released by solid rockets are directly related to stratospheric ozone depletion, much uncertainty surrounds reactivity of aluminum oxide particles.

We also compare current and future effects of space launch on stratospheric ozone depletion with those of Ozone Depleting Chemicals (ODCs). As a baseline, we use projections of future ODC use by SMC, Air Force Materiel Command (AFMC), and the world. Relevant stratospheric chemistry is considered to make a legitimate comparison of ODC and solid rocket exhaust.

2. Ozone Decomposition on Alumina: Implications for Solid Rocket Motor Exhaust, M.A. Hanning-Lee, B.B. Brady, L.R. Martin, and J.A. Syage, Geophysical Research Letters,Vol. 23, pg. 1961 (1996).

Rates of ozone decomposition on aluminum oxide (alumina) particles were measured in a flow tube reactor equipped with molecular beam sampling mass spectrometry and ultraviolet absorption spectroscopy, and in a static reaction cell equipped with ultraviolet absorption spectroscopy. Reaction probabilities h are reported for ozone on a- alumina, g - alumina, and chromatographic alumina (hydroxylated alumina), respectively, over the temperature range -60 to 200oC. This work addresses the potential for stratospheric ozone depletion by launch vehicle solid rocket exhaust. Considering best estimates of plume particle size distributions and dispersion rates, we calculate ozone depletion profiles, for direct decomposition on alumina only. The calculated ozone holes are rather narrow. In the worst case, ozone levels are within 5 x 10-5 of ambient in the center of the plume. A simple analysis of the global impact of alumina particles on ozone decomposition indicates a potential steady-state daytime depletion of < 2.6 x 10-8 at present launch rates.

3. Systems Requirements Review for the High-Resolution Ozone Imager (HIROIG), D.L. McKenzie, D.J. Gutierrez, J.H. Hecht, D.J. Mabry, M.N. Ross, G.S. Rossano, M.G. Sivjee, and J.A. Stein, The Aerospace Corporation, El Segundo, CA, 15 September 1993, TR-93(3231)-2, SMC-TR-93-62.

The HIROIG System Requirements Review was held on February 17, 1993 at The Aerospace Corporation. The purpose of the review was to demonstrate the way in which the requirements for a spaceborne ozone imaging experiment are derived from the fundamental goal of the program, which is to measure how space-vehicle launches affect the ozone layer of the atmosphere. The review included, towards the end, short presentations on the experiment team's approaches to meeting the program requirements. This report is a compilation of the briefing charts that were presented at the review.

4. An Assessment of the Total Ozone Mapping Spectrometer for Measuring Ozone Levels in a Solid Rocket Plume, Jack A. Syage and Marty N. Ross , Geophysical Research Letters, Vol. 23, No. 22, Pages 3227 - 3230, (1996).

The question whether the total ozone mapping spectrometer (TOMS) is capable of measuring ozone levels in a solid rocket plume is examined. Simulated measurements were computed for a chemical kinetics and dispersion model of a Titan IV plume. The principal disadvantage of TOMS for measuring local plume ozone levels is that the detection field-of-view is typically much larger than the column area for ozone loss. A secondary problem is attenuation of backscattered light by plume species and particles that can distort the ozone measurement.

5. Modeling Solid Rocket Booster Exhaust Plumes in the Stratosphere with SURFACE CHEMKIN, B. B. Brady and L. R. Martin, The Aerospace Corporation, El Segundo, CA, 1 September 1995, TR-95(5231)-9, SMC-TR-96-19.

The results of a detailed chemical model of the transient stratospheric chemistry following passage of a large solid rocket booster is described. The model is based on SURFACE CHEMKIN, which is a newly developed multiphase chemical kinetic model. The model incorporates 34 chemical species and over 100 gas phase, heterogeneous, and photochemical reactions. The results show that passage of a Titan IV-sized rocket should produce an ozone "hole" 10 km in diameter at 20 km altitude, and 28 km in diameter at 30 km altitude, lasting from a few hours to a day. The size and persistence of the hole are very sensitive to the rate of dissipation of the rocket plume, which is poorly understood at present.

6. Computer Model Predictions of the Local Effects of Large, Solid-Fuel Rocket Motors on Stratospheric Ozone, P.F. Zittel, The Aerospace Corporation, El Segundo, CA, 10 September 1994, TR-94(4231)-9, SMC-TR-94-36.

The solid-fuel rocket motors of large space launch vehicles release gases and particles that may significantly affect stratospheric ozone densities along the vehicle's path. In this study, standard rocket nozzle and flowfield computer codes have been used to characterize the exhaust gases and particles through the afterburning region of the solid-fuel motors of the Titan IV launch vehicle. The models predict that a large fraction of the HCl gas exhausted by the motors is converted to Cl and Cl2 in the plume afterburning region. Estimates of the subsequent chemistry suggest that on expansion into the ambient daytime stratosphere, the highly reactive chlorine may significantly deplete ozone in a cylinder around the vehicle tract that ranges from 1 to 5 km in diameter over the altitude range of 15 to 40 km. The initial ozone depletion is estimated to occur on a time scale of less than 1 hour. After the initial effects, the dominant chemistry of the problem changes, and new models are needed to follow the further expansion, or closure, of the ozone hole on a longer time scale.

7. Local Effects of Solid Rocket Motor Exhaust on Stratospheric Ozone, Martin Ross , Journal of Spacecraft and Rockets, Volume 33, Number 1, Pages 144-153, (1996).

Solid rocket motor (SRM) exhaust contains chlorine, an important stratospheric constituent that plays a crucial role in the chemistry of ozone. Models of SRM plume combustion and chemistry suggest that a significant fraction of SRM exhaust chlorine might be in an active form as a result of afterburning reactions and be available for immediate ozone destruction as the plume mixes with the ambient stratosphere. If afterburning does produce free chlorine, the SRMs used by the Titan IV and Space Shuttle have the potential to cause nearly complete ozone loss in regions up to several tens of kilometers in radius, depending on altitude. We present a simulation of SRM exhaust mixing and chemistry that predicts the three-dimensional structure of the prompt local ozone loss during the 8 h following a daytime Titan IV launch. Maps of the loss of the total column ozone abundance show that the maximum loss of about 30% occurs about one hour after launch. Four hours after launch the area of column ozone loss exceeding 8% covers up to 2000 km2. The shape and severity of the ozone-depleted region depends only slightly on the interaction between launch trajectory and stratospheric winds. Solar ultraviolet flux on the ground beneath the plume changes in response to decreased ozone absorption, alumina scattering, and absorption by chlorine oxide compounds. The ultraviolet flux increases by 100% at 295 nm, shows no change near 300 nm, and decreases by 30% longward of 310 nm. Existing space instrumentation does not possess the spatial resolution required to measure ozone depletion of the size and magnitude predicted following launches involving large SRMs.

8. Effects of Launch Vehicle Emissions in the Stratosphere, B.B. Brady, L.R. Martin, and V.I. Lang , 35th Aerospace Sciences Meeting and Exhibit, January 6 - 10, 1997, Reno, NV, AIAA-97-0531.

A plume dispersion and chemical kinetic model based on SURFACE CHEMKIN has been used to estimate the total impact of motors of different propellant types on stratospheric ozone. In previous studies by other authors industry standard rocket motor performance and plume flowfield computer programs were used to model the chemistry in the rocket combustion chamber and expansion nozzle, and also in the downstream afterburning region of the plume. Our model, based on SURFACE CHEMKIN and the results of previous studied, was used to follow the plume chemistry for up to a day as the plume dispersed into the ambient stratosphere. Several large motor types were analyzed: two different solid-fueled motors without chlorine and one with chlorine, and amine/N204 fueled first stage, a kerosene/LOX fueled first stage, and a H2/LOX fueled engine with two nozzle variants. The modeled motors are based loosely on existing vehicles, but we varied several parameters to create hypothetical vehicles that may be viewed as prototypes of next generation launchers. Two dispersion rates were used, a worst case and a "best guess" based on published models. In the worst case, ozone depletion due NOx or other exhaust species was several orders of magnitude smaller than depletion due to chlorine in the exhaust. Depletion due to motors using LOX was minimal within five minutes of vehicle passage in all cases.

9. Laboratory Generation of Free Chlorine from HCl Under Stratospheric Afterburning Conditions, Burke and P.F. Zittel, Combustion and Flame, in press 1997. (also The Aerospace Corporation, El Segundo, CA, 1 March 1996, TR-96 (1306)-3, SMC-TR-96-10).

Experiments have been conducted using a low pressure laboratory flame apparatus to examine the chemistry of solid rocket motor (SRM) afterburning relevant for stratospheric altitudes. It has been found that a significant fraction of the HCl injected into H2-O2 and H2-CO-O2 flames can be consumed, with observed losses of up to 40%. The extent of conversion of HCl was found to increase with increasing oxygen:fuel (O/F) ratio and decreasing pressure; the loss at a given O/F was also higher for flames with equal flows of H2 and CO compared to flames with no CO in the fuel. The major product of HCl reaction was found to be Cl2, with no other chlorine-containing products observed via mass spectrometry. Distinct Cl2 B*X emission bands were observed along with very weak ClO A*X bands and a bright, white continuum emission which apparently arises from one or more chlorine-containing compounds. The general findings concerning the magnitude of HCl conversion and the formation of Cl2 are consistent with published modeling results for SRM stratospheric afterburning. This formation of "free" chlorine could lead to catalytic destruction of ozone in regions near the path the launch vehicle follows during boost through the stratosphere.

10. Solid Rocket Motor Exhaust Model for Alumina Particles in the Stratosphere, Edward J. Beiting, Journal of Spacecraft and Rockets, Vol. 20, No. 3, May - June 1997. (also The Aerospace Corporation, TR-95(5231)-8 and SMC-TR-95-44)

Based on available and new data, a unified model is presented for the particle size distribution, particle density, and geometrical dispersion for the alumina particles in the exhaust of solid rocket motor plumes in the stratosphere. The particle size distribution is trimodal with Sauter mean diameters of 0.056, 1.0, and 3.6 µm. Nearly all of the particles lie within the small-size mode but nearly all of the mass lies in the large-size mode. Approximately two-thirds of the particle surface area available for heterogeneous chemical reactions is due to the large particle mode while most of the remaining surface area is due to the small particle mode. The early horizontal dispersion rate of the plume is found to be about an order of magnitude greater than the dispersion rates used in several recent models of stratospheric ozone-plume chemistry.

11. Predicted Optical Characteristics of Solid Rocket Motor Exhaust in the Stratosphere, Edward J. Beiting , Journal of Spacecraft and Rockets, Vol. 20, No. 3, May - June 1997. (also The Aerospace Corporation, TR-95(5231)-8 and SMC-TR-95-44)

Optical characteristics of large solid rocket motor exhaust in the stratosphere are predicted for portions of the near-ultraviolet, visible, and near-infrared spectral regions. Mie scattering calculations show that attenuation by the alumina particles is caused principally by the large-particle mode of the trimodal particle size distribution. The local attenuation in the plume as a function of wavelength and the transplume total attenuation due to particles as a function of time are predicted for several wavelengths. An expression for the total surface area of the particles is derived in terms of known parameters and the Sauter mean diameter. Mie theory is then used to show that this Sauter mean diameter can be measured using a two-color transmissometer. The absorption spectra due to the chemical constituents are predicted for the 200--400-nm wavelength interval for several times after vehicle passage. It is predicted that these absorptions are comparable to the attenuations due to particle scattering.

12. K-2 Titan IV Stratospheric Plume Dispersion, E.J. Beiting and R.A. Klingberg, 10 January 1997, The Aerospace Corporation, El Segundo, CA, TR-97(1306)-1, SMC-TR-97-01.

Video images were recorded of the plume from the K-2 Titan IV launched 2 July 1996 from Cape Canaveral Air Station. These images were used to infer plume motion and expansion near an altitude of 30 km in the stratosphere. The plume was observed to move across the sky in a generally east-to-west direction with a speed of 19 + km/s. The plume diameter at an altitude of 30 km was measured for 12 min and found to increase as a linear function of time with a rate of 0.48+0.03 km/min. The diameter of a bulge that appeared in the plume at an altitude of 29.5 km was measured for 7 min and also was found to increase linearly with a rate of 0.60 + 0.07 km/min. The angular width of the plume increased to a value greater than the field-of-view of the cameras, restricting the observation times to those listed. The plume was visible at both visible and near-infrared wavelengths with good contrast until sunset at altitude, which occurred 15 min after vehicle passage.

13. Stratospheric Effects of Rocket Plumes, An overview and update of a program to understand the local effects of rocket plumes in the stratosphere, T. Spiglanin, and J. Edwards, 32nd AIAA/ASME/SAE/ASEE Joint Propulsion Conference, July 1 - 3, 1996, Lake Buena Vista, FL.

A program for understanding of rocket exhaust in the stratosphere is described. The program consists of two discrete activities: modeling and simulation, to predict effects based on the best possible understanding of the interaction between launch vehicle plumes and the atmosphere; and measurements to validate the predictions of the models and provide data for use in environmental analyses of launch systems. Modeling and simulation have produced detailed understanding of the nature of the chlorine and the aluminum oxide particles exhausted by solid rocket motors. Laboratory investigations provide important information used in the modeling studies, and verify the prediction of HCl conversion to Cl2 under stratospheric afterburning conditions. Efforts are now underway to validate these predictions by in-situ aircraft measurement and by space-based observation.

14. Overview of the Space Debris Environment, M.J. Meshishnek, The Aerospace Corporation, El Segundo, CA, March 1995, TR-95(5231)-3, SMC-TR-95-9.

There is a component of the space environment that is man-made pollution, termed "space debris;" it exists at all inclinations and, primarily, at altitudes of rough 350 km to 2000 km. The size of this debris ranges from several meters to a fraction of a micrometer in diameter, and the particle distribution follows and inverse power law, with the smaller size component far exceeding that of the larger. Debris is composed primarily of alumina from solid rocket motor exhausts, aluminum from spacecraft structures, and zinc and titanium oxides from thermal control coatings. The accepted model of the space debris environment is that of Kessler et al., a complex model that predicts the number of particles that will impact a surface as a function of altitude, inclination, solar cycle, and particle diameter, as well as their collision velocities.

Recent data from LDEF has demonstrated both the accuracy and shortcomings of the Kessler model. Measured debris impactor fluxes are in good agreement with the model for ram surfaces. However, predictions of the model for other surfaces of a spacecraft are less accurate, most notably for the wake or trailing side. While the Kessler model is appropriate for long-term, average flux predictions, spatial-temporal impact fluxes measured on LDEF dramatically illustrated the presence of strong debris clouds that do not dissipate quickly in space and will encounter an orbiting spacecraft cyclically and repeatedly over its lifetime. LDEF data has also indicated the presence of debris in elliptical orbits, a fact not predicted by the Kessler model. This fact is responsible for the discrepancy between measured impact fluxes and predictions on trailing-edge surfaces.

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