# Presentation: 587268

## Briefing to COMSTAC RLV Working Group

**DOT Volpe Center Contracts DTRS57-99-D-00062, Task 10,**
**and ****DTRT57-05-D-30103****, Task 7**

**Briefing to COMSTAC RLV Working Group**
**May 17, 2007**

**Bob Seibold**
**The Aerospace Corporation, El Segundo, CA**
**(310) 336-1326**
**robert.w.seibold@aero.org**

## Outline and Acknowledgements

_**Outline**_
**Backgroud**
**Approach**
**Electron Generation Process**
**Representative Vehicle Classes Analyzed**
**Plasma Flow Fields**
**Blackout Trajectories**
**Mitigation Methodologies**
**Conclusions**
_**Acknowledgements**_
_**FAA/AST Technical Lead**_**:  Pradipta Shome**
_**Volpe Center COTR**_**:  Ann DiMare**
_**Aerospace Principal Investigators**_**:  Dr. Richard Hartunian, Dr. Gordon Stewart**
_**Other Aerospace Contributors**_**:  Dr. Thomas Curtiss, Dr. Stephen Fergason**
_**Aerospace Program Manager**_**:  Bob Seibold**

**Notes:**
This briefing consists of six sections:  background, approach, a discussion of the electron generation process, a discussion of plasma flow fields and blackout trajectories for representative vehicle classes, methodologies for mitigating blackout, and conclusions.
This study was sponsored by AST, with Pradipta Shome providing technical direction and guidance.  Ann DiMare at the Volpe National Transportation Systems Center served as the Contracting Officer’s Technical Representative for the Department of Transportation.  At Aerospace, Dr. Richard Hartunian and Dr. Gordon Stewart served as the co-Principal Investigators, with significant contributions from Dr. Tom Curtiss and Dr. Steve Fergason.  I served as the Program Manager.

## Scope

**RF blackout is caused by high electron concentrations in plasma sheaths covering reentering vehicles.  These sheaths reflect or attenuate electromagnetic waves.**
**Vehicles likely to come before AST in the future, that could be affected by blackout, will include orbital RLVs and some suborbital vehicles traveling at high speeds.**
**Future RLVs could remain in attenuated communication mode during the most critical phase of flight, due to long glide phases for energy management.**
**Related operation and safety challenges.**
**Examined causes of blackout for various vehicle classes and potential alleviation methodologies, e.g.:**
**Aerodynamic shaping**
**Injection of electrophilic quenchants into plasma layer**
**Generation of local magnetic fields to create spectral windows through which RF waves can propagate**
**Study bounded by the following considerations:**
**Focused on direct RF transmission from vehicles to ground, without use of satellites for indirect transmission**
**Evaluated both manned and unmanned space vehicles**

**Notes:**
The scope of the study is summarized here.
RF blackout is caused by high electron concentrations in plasma sheaths covering reentering vehicles.  These sheaths reflect or attenuate electromagnetic waves.  The study addressed the types of vehicles likely to come before AST in the future, that could be affected by blackout, and included orbital RLVs and some suborbital vehicles traveling at high speeds.  Some future RLVs could remain in an attenuated communication mode during the most critical phase of flight, due to long glide phases for energy management.  Concomitant operation and safety challenges were considered.
We examined causes of blackout for various vehicle classes and potential alleviation methodologies, such as aerodynamic shaping, injection of electrophilic quenchants into the plasma layer, and generation of local magnetic fields to create spectral windows through which RF waves can propagate.
The study was bounded by two considerations:  First, we focused on direct RF transmission from vehicles to ground, without use of satellites for indirect transmission, and secondly, we evaluated both manned and unmanned space vehicles.

## Approach

**1.  Defined vehicle classes to be evaluated**
**Reusable vehicles for future space transportation**
**Other classes with published flight data relevant to problem**
**2.  Defined ionized flow fields for each vehicle class**
**Published flight data used where available**
**Analytic predictions using best state-of-the-art codes, in absence of published flight data**
**3.  Analyzed RF interactions with ionized flow field**
**Comparisons with analytic predictions and flight data where available**
**4.  Studied mitigation methods**
**Recommended promising mitigation methods for more detailed assessment**
**5.  Reviewed frequency bands for RF transmission**

**Notes:**
The approach taken in this study consisted of 5 subtasks:
First, vehicle classes to be evaluated were defined.  These included RLVs most likely to be used for space transportation in the future and other classes selected because of published flight data relevant to the reentry blackout problem.
Secondly, ionized flow fields for each vehicle class were defined over a characteristic trajectory.  In the absence of published flight data, the best state-of-the-art flow field codes were used to predict the ionized flow field.  In each case, the altitudes of onset and subsequent recovery from blackout were highlighted.
Thirdly, RF interactions with the ionized flow fields were analyzed, including comparisons with analytic predictions and flight data where available.
Fourth, mitigation methodologies were studied and promising concepts recommended for more detailed assessment in actual application to full-scale RLVs.
Finally, frequency bands best suited for RF transmission through plasma were reviewed. 

## Representative Vehicle Classes

**Blunted conical ballistic reentry vehicles**
**Sharp-tipped slender conical ballistic reentry vehicles**
**Unpowered lifting glide vehicle**
**Space Shuttle concept**
**Fully reusable 2-stage cargo vehicle for future commercial space transportation**
**Powered air-breathing lifting vehicle**
**Blackout unlikely**

**Notes:**
Five representative vehicle classes were analyzed.  These included two types of conical ballistic reentry vehicles, with blunt tips and sharp tips, respectively, unpowered lifting glide vehicles, of which the Space Shuttle is a primary representative, the Rocketplane-Kistler reusable 2-stage cargo vehicle configuration, and powered air-breathing lifting vehicles, such as the X43.  The RpK 2-stage cargo vehicle, i.e., the K-1 configuration, was included in response to a request from AST, so that this highly blunted “slightly lifting” configuration could be analyzed.  
Powered air-breathing lifting vehicles such as the X-43 demonstrators were designed to achieve an altitude of 100 kft and a Mach number range of 7 – 10.  Given those parameters, combined with the fact that the cruise vehicle design must attain minimum drag and, therefore, will likely have sharp leading edges and cruise at a near-zero angle of attack, the chance of having a blackout problem for this configuration appears remote.

## Ballistic Vehicles and Fully Reusable 2-stage Cargo Vehicle 

** ****Sharp-tipped Ballistic Reentry Vehicle**
**and Reentry Flow Characteristics **

**Blunt-tipped RAM C Vehicle and **
**Reentry Flow Characteristics.**
**Reprinted with permission of IEEE**
**[Rybak and Churchill, 1971] **

**RpK K-1 Vehicle.**
**Reprinted with permission**
** ****of Rocketplane-Kistler**

**Notes:**
Three classes of ballistic vehicles are represented here.  Blunt conical ballistic reentry vehicles are represented by the NASA Radio Attenuation Measurements, or RAM, flight experiments from the 1960s and 1970s.  The RAM experiments addressed the blackout problem in a comprehensive manner for blunted vehicle configurations.  Sharp-tipped RVs are represented by the schematic depiction at the lower left and tend to display two regions of reentry flow, laminar and turbulent, as depicted in the sketch.
The RpK K-1 Orbital Vehicle is shown on the right.

## Unpowered Glide and Powered Cruise Vehicles

**Drawing of Space Shuttle Orbiter **
**During Reentry Conditions**

**X-43A Air-breathing Hypersonic **
**Vehicle During Ground Testing **

**Reprinted courtesy of NASA**

**Notes:**
Unpowered lifting glide vehicles are represented here by the Space Shuttle Orbiter, and powered air-breathing hypersonic cruise vehicles by the X-43A.

## Sources of Electrons in Plasma Sheaths

**Plasma sheath created from conversion of vehicle’s kinetic energy, **
**compressing and heating air flow surrounding vehicle:**
**Temperature of air raised by:**
**Shock waves at all leading edges and some embedded structures.    ****	*****Conservation of energy equation for direction normal (90******o) to vehicle’s velocity ******	******vector:***
**	****(*****C******p******T)******atm****** + U******2******/2 = constant for normal flow across a 1-D shockwave***
***     ******where:  C******p****** = heat capacity, T = temperature, U = velocity of flow***
**Conversion of viscous stresses due to velocity gradients in flow**
**As air temperature increases, the following sequence occurs:**
**Excitement of molecule translation, rotation, and vibration degrees of freedom**
**Dissociation and ionization**
**Numerous complex reactions contribute to generation of electrons, beginning with 20% O****2**** and 80% N****2**

**Notes:**
Vehicles flying in the atmosphere at hypersonic velocities create shock waves at all leading edges.  Air flowing across shock waves experiences high temperatures much more for blunt leading edges than those that are sharp.  The other cause of air heating is viscous shear stresses of the air caused by velocity gradients, mostly in the layer contiguous with the vehicle body.  The equation in the first bullet quantifies the conversion of velocity to temperature.  As the velocity decreases, heat capacity times temperature increases as indicated.
As the temperature increases, a two-step sequence occurs.  The first is excitement of molecule translation, rotation, and vibration degrees-of-freedom.  This is followed by dissociation and ionization.  These processes take thermal energy from the flow.  The resulting endothermic reactions reduce the static temperature.

## Electron Generation Process

**High reentry temperatures dissociate the oxygen and nitrogen molecules of the air, which triggers complex chemical reactions producing ionization.  **
**All of these reactions need to be accounted for.  **
**26 example reactions are listed in a backup chart.  A representative example is:**
**	****	****	****O****2****  +  N****2****  ****	****    NO  +  NO****+****  +  e****-**
**In an exothermic reaction, molecules collide with enough energy to overcome the activation energy barrier and form an activated complex at a lower energy level.**
**The released electrons and charged molecules form the plasma sheath, inhibiting communications.**
**At lower altitudes, the forward and reverse rates become equal, producing chemical equilibrium.  At high altitudes, non-equilibrium chemistry prevails.**

**Notes:**
High reentry temperatures dissociate the oxygen and nitrogen molecules of the air, which triggers complex chemical reactions, producing ionization.  All of these reactions need to be accounted for.  Twenty-six example reactions are listed on a backup chart; a representative example is shown here.  In an exothermic reaction, molecules collide with enough energy to overcome the activation energy barrier and form an activated complex at a lower energy level.  The released electrons and charged molecules form the plasma sheath, inhibiting communications.  At lower altitudes, the forward and reverse rates become equal, producing chemical equilibrium.  At high altitudes, non-equilibrium chemistry prevails.

## Definition of Plasma Flow Fields

_**Blunt, Slender Reentry Vehicles**_
**NASA Radio Attenuation Measurements (RAM) Program**

**Flow Regions About a Conic Shaped **
**Vehicle at 40 km. Reprinted with permission**
** ****of IEEE [Rybak and Churchill, 1971] **

**Rake of Ion Probes**

**Notes:**
The NASA Radio Attenuation Measurements (RAM) Program addressed the blackout problem in a comprehensive manner.  Nosetips with various degrees of bluntness on an aft body with a 9 degree half-angle were instrumented heavily and reentered at velocities of 18 kft/sec in RAM Series A and B and 25 kft/sec in Series C. 
For blunt vehicles like these, the electrons form mostly in the nose region.  As they flow downstream, they fill the area between the vehicle surface and the shock wave.
In the NASA RAM-C flight series, an electrostatic rake was used to measure ion densities in the shock layer.  The rake employed a blade-like design with an extremely sharp nosetip of 0.01-in. radius, to hold small metallic ion collectors.  The rake was mounted on the rear end of the conical heatshield and survived heating down to 180 kft altitude, at which point it was retracted because of concerns that components would melt. 

## Definition of Plasma Flow Fields, Con’t.

**RAM C-II Reentry Trajectory with**
**Onset and End of RF Blackout at**
**3 Frequencies.  Reprinted courtesy of NASA [Grantham, **
**W. L., NASA SP-252, 1970]**

**Notes:**
This chart shows the trajectory for the RAM C-II vehicle, that is, velocity as a function of altitude.
The RAM-C vehicles had a 6-inch nose radius, and the vehicle was 49 inches long.  Four microwave reflectometers measured electron number densities at four locations.  As depicted on the previous chart, a rake of 14 probes measured ions from the body to the shock wave.  The objective was to provide accurate data for comparisons with the best available predictive codes.
S-band recovery from blackout, not shown on the chart, occurred at 80 kft altitude.
As seen in this chart, blackout of this blunt-nose configuration occurred over a wide range of transmission frequencies.  As expected, higher frequencies were associated with later onset of blackout.

## Definition of Plasma Flow Fields, Con’t.

_**Sharp, Slender Reentry Vehicles**_
**Sharp-tipped ballistic RV with shock wave**

**Orange layer (thickness exaggerated) represents **
**location of electrons generated at nose.**

**Notes:**
The sharp tip cone configuration consists of an axisymmetric cone body with a small half-angle and small nose radius.  The flow field for this configuration consists of an oblique shock wave that forms along the length of the body and a thin boundary layer.  This chart shows a turbulent wake along the aft portion of the vehicle.  In this case, the flow of molecules through the normal shock wave at the small nose generates electrons, but not enough to ionize a large portion of the flow.  Instead, the boundary layer on the surface provides the energy to increase the air temperature by viscous dissipation and cause ionization. 

## Electron Density Decay Along VehicleLength, Sharp-tipped RVs

**R****N**** = nose radius**

**Clean air peak electron**
**density decay.  Reprinted with **
**permission of AIAA [Steiger et al., **
**AIAA 70-220, 1970]**

**Notes:**
Steiger and co-workers at The Aerospace Corporation reported in 1970 that high-altitude telemetry blackout was experienced on a fairly sharp, slender reentry vehicle.  The vehicle was a sphere-cone with a 1/4-inch nose radius, an 8-degree cone half-angle, with a clean nosetip and heatshield, and an X-band circumferential ring antenna at about the five-inch station.  Telemetry plasma attenuation appeared around 190 kft and increased until blackout at about 145 kft.  An analysis indicated that the signal degradation was due to plasma attenuation with pattern distortion, resulting from a small look angle, as the main contributor. 
Their analysis indicated that a substantial reduction in plasma attenuation can be achieved by sharpening the nose radius or by moving the antenna to a more aft location.  The first method of alleviation was recommended. 

## Definition of Plasma Flow Fields, Con’t.

_**Fully Reusable 2-stage Cargo Vehicle for Future Commercial Space Transportation**_
**RpK K-1**

**Reprinted with permission**
**of Rocketplane-Kistler**

**Notes:**
RpK has been developing a two-stage launch vehicle, the K-1, designed to be fully recoverable.  The first stage would fly to an altitude of 142 kft, separate from the second stage with its engines off, relight the center of three engines, and fly back to the launch site following a ballistic trajectory.  Two drogue parachutes and two clusters of three main chutes would be used to decelerate the first stage for a soft landing on four airbags. 

## Definition of Plasma Flow Fields, Con’t.

_**Space Shuttle**_
**No direct experimental measurement of the ionized flow field on a reentering orbiter**
**Only known analysis of the ionized flow field was by Dunn and Kang in April 1973 – about 8 years before STS-1**
**They modeled the Orbiter as a 20****o cone with a 4-ft. radius blunt cone reentering at 2****0****o angle of attack.  Despite the fact that the nose radius on the as-built Shuttle is smaller and the entry angle of attack is actually 40o, they concluded that blackout duration would be 15 minutes, close to the actual 16 minutes experienced.**

**Notes:**
The Space Shuttle Orbiter is an obvious well-known example of an unpowered lifting glide vehicle.  There are no reported direct experimental measurements of the ionized flow field on a reentering Orbiter, for obvious reasons of not wishing to jeopardize safety by introducing instrumentation on the heat shield. 
However, Dunn and Kang analyzed the ionized flow field in April 1973, about 8 years before the first Shuttle flight.  They modeled the Orbiter as a 20o cone with a 4-ft. radius blunt cone reentering at 20o angle of attack.  Despite the fact that the nose radius on the actual as-built Shuttle is smaller and the entry angle of attack is actually 40o, they concluded that blackout duration would be 15 minutes, close to the actual 16 minutes experienced.

## Plasma Flow Profiles for Shuttle Configuration

**Electron Number Density Distributions in Shock Layer**

**20****o Cone**

**Blunted Delta at 20****o**

**ξ = S/R****N**** = ratio of downstream position to hemisphere radius**
**y =  distance normal to body surface (abscissa)**
**S = distance along surface of body from nosetip**

**Reprinted courtesy of NASA**
**[Dunn and Kang, **
**NASA CR-2232, April 1973]**

**Notes:**
Dunn and Kang’s results are shown here.  The abscissa on each plot is the distance across the plasma from the body to the shock at various locations along the body (S), divided by the nose radius.  
The left plot shows those electron density profiles at an altitude of 280 kft.  The critical plasma density for S-band RF is 6.5 x 1010 electrons/cm3.  Similar values are seen further aft on the vehicle, and as the vehicle descends another 15 kft to 265 kft, the entire vehicle will be covered with critical plasma.  When the plasma sheath over the antenna is covered with critical plasma, the RF will be totally reflected (blacked out) at high altitudes.  The Shuttle flights eight years later experienced blackout onset at 265 kft.
The right plot depicts the plasma shape at various locations along the body length for a delta-shaped wedge model of the Shuttle at an altitude of 180 kft.  The peak electron density is seen to be below the critical plasma density, indicating recovery from blackout, which is 15 kft higher than where the Shuttle emerges from blackout.
From these results, Dunn and Kang concluded that the duration of blackout would be approximately 15 minutes.

## Blackout Trajectories for Four Vehicle Classes

**Sharp RV**

**Ram C**

**Shuttle**

**RpK OV**

**30**

**Relative Velocity – km/sec**

**300**

**250**

**200**

**150**

**100**

**50**

**0**

**0**

**5**

**10**

**15**

**20**

**25**

**Relative Velocity – kft/sec**

**0**

**2**

**4**

**6**

**8**

**0**

**20**

**40**

**60**

**80**

**Altitude – km**

**Altitude – kft**

**Onset of**
**blackout**

**Recovery**
**From blackout**

**Notes:**
This chart compares blackout trajectories and times for reentry of the Shuttle, the RpK OV, the RAM C, and sharp-tipped RVs.  The absence of blackout for sharp RVs is of key importance.
Note that the RpK OV retains considerably higher velocity than the Shuttle at each equivalent altitude. The significantly longer time blackout time for the Shuttle (16 minutes) relative to that for the RpK OV (1 minute) results from the fact that the Shuttle is a lifting body that slows down at high altitude while remaining heated, while the K-1 essentially follows a fast ballistic trajectory, taking only one minute to descend from a very high altitude to a low altitude where recovery from blackout can occur.
The Shuttle experiences blackout onset at 265 kft and recovers at 162 kft.  We would expect the OV to have the same or higher onset altitude (both at 25 kft/sec velocity) and to recover from blackout when it slows down to a velocity of 9 kft/sec, which occurs at an altitude of 110 kft.  Velocity is the major determinant of the maximum temperature in the flow field, and temperature is the major determinant of degree of ionization in the flow.

## Mitigation Methodologies

_**Aerodynamic Shaping**_
**Sharp, slender probes/cones containing antenna protruding ahead of shock waves**
_**Issue**_**:  Likely to require active cooling of sharp nose region**

**Reprinted with **
**permission of AIAA**
**[Belov, et al., AIAA 99-3739]**

**Notes:**
The goal of final subtask of this study was to identify proposed techniques for mitigating blackout and make recommendations regarding which concepts are most promising for application to future commercial space transportation systems.  Eight major concepts in the literature were reviewed.  These were aerodynamic shaping, injection of liquid alleviants into the plasma sheath, application of a magnetic field, and use of high power, high frequencies, low frequencies, lasers, and indirect RF communication.
This chart addresses the first of these concepts, aerodynamic shaping.  Use of a sharp, slender probe, containing an antenna and made to project ahead of the bow shock of a blunt-nose vehicle, appears to be a promising approach.  This concept is reported in a paper by Russian authors, referenced in this chart, who have done considerable analysis and experimental evaluation of the aerodynamics, heat transfer, and nose radius required to assure sufficiently low electron density flowing aft, where the antenna is located.  The sharp nose-cone of the probe is a porous sintered metal, selected to accommodate cold gas cooling for survival. 

## Mitigation Methodologies, Con’t.

_**Injection of Liquids**_
**Injection of electrophilics or other liquids that form droplets that attach electrons and contribute to cooling ahead of antenna on LV**
**Successfully demonstrated on Gemini 3 Reentry capsule in 1956**
**Significant levels of signal strength increase during early portion of water injection sequence over altitude range of 272 to 246 kft noted by ground stations on VHF telemetry and VHF voice**
**Enhancement of C-band beacon signal observed during latter portion of water injection sequence from 200- to 160-kft altitude**
**Demonstrated alleviation of RF attenuation on blunt body by water injection in flow field**
_**Issue**_**:  How does the required mass flow of alleviant scale up to a full-size commercial vehicle?**
**Unpowered lifting-glide vehicle**
**Powered air-breathing vehicle**

**Notes:**
Regarding the injection of liquids into the flow to alleviate blackout, NASA has conducted successful flight demonstrations.  The first demonstration employing injection of quenchants to alleviate blackout was on the Gemini 3 Reentry capsule manned by two astronauts on March 23, 1965.  Significant levels of signal strength increase during the early portion of the water injection sequence over an altitude range of 272 to 246 kft were noted by ground stations on VHF telemetry and VHF voice.  Enhancement of C-band beacon signal was observed during the latter portion of the water injection sequence from 200- to 160-kft altitude.
The conclusion is that quenchants injected in an ionized flow field can significantly reduce RF attenuation on a blunt body.  However, the amount quenchant mass needed for scale-up to large vehicles remains a significant issue.

## Mitigation Methodologies, Con’t.

_**Application of Magnetic Field**_
**Application of a steady magnetic field in the direction of intended RF propagation**
**Theory supports concept in principle, e.g., *****Starkey, et al., 34******th****** AIAA Plasmadynamics and Lasers Conf., June 2003.***
**NASA experiment reported 20 db reduction with 750 Gauss field applied to laboratory plasma**
**Japanese team analytical experiment reports 1,500 Gauss penetrates plasma if magnetic field cyclotron frequency equals RF frequency (Whistler mode).  *****Usai, et al., 6******th****** Spacecraft Charging Technology Conf., Sept. 2000, p. 107.***
**Others estimated 10****4**** Gauss would penetrate air-breathing reentry blacked-out plasma layer**
_**Issue**_**:  Whereas current magnetic field strength generation technology may support up to 1,500 Gauss at acceptable weight and volume, the consensus is that achieving 10****4**** Gauss will require advances in superconducting technologies **

**Notes:**
Theory supports the concept of applying a magnetic field, and laboratory experiments at NASA-Langley concluded that a 750 Gauss magnetic field resulted in a 20 dB reduction in VHF attenuation, from 60 dB to 40 dB.
Starkey and coworkers analyzed a hypothetical air-breathing vehicle with very blunt leading edges (6” radius), and showed that this configuration generates thick plasma layers with very high electron density.  They calculated a magnetic field strength of 104 Gauss to penetrate this plasma.  However, no reasonable air-breathing vehicle will have leading-edge radii that large.
A Japanese team concluded from an analytical experiment that use of the Whistler mode (where the magnetic field cyclotron frequency equals the RF frequency) could provide a window for RF transmission with a magnetic field strength of 1500 Gauss.
Whereas current magnetic field strength generation technology may support up to 1500 Gauss at acceptable weight and volume, the consensus is that achieving 104 Gauss will require significant advances in superconducting technologies.

## Other Mitigation Approaches Considered

**High Power**
**Breakdown**
**Workarounds:  pulsed power, bandwidth, adaptive matching, electrophilics**
**High Frequencies**
**Atmospheric attenuation**
**Incompatibility with ground stations/satellites**
**Frequencies compatible with ****Automatic Dependent Surveillance - Broadcast Mode (ADS-B) Architecture.  500 MHz lower bound for this task.**
**Low Frequencies**
**Antenna size**
**Incompatibility with ground stations/satellites**
**Workarounds:  trailing wires?**
**Lasers**
**Atmospheric attenuation**
**Not all-weather**
**Incompatibility with ground stations/satellites**
**Indirect Communication**
**Transmit up to satellites**
**Bent pipe to ground stations**

**Notes:**
The RF blackout mitigation strategies described in this chart fall into two general classes:  passive approaches and active approaches.  Passive approaches focus on design approaches for minimizing the plasma effects on communications signals.  
Example passive approaches to mitigating the RF blackout problem are: transmitting at higher frequencies and radiating higher power from the vehicle.  Although increasing the frequency generally reduces attenuation, higher frequencies are more subject to atmospheric and rain attenuation.  As a practical limit, 10 GHz is often suggested as an upper limit.  Transmitting higher power from the vehicle is limited by aperture breakdown.  Typical breakdown powers are in the range of hundreds of Watts.  Increasing the transmit power to cover, for instance, even 30 dB of attenuation is clearly impractical.  
Implementing one or a combination of passive approaches will minimize and possibly eliminate the blackout phenomenon.  In our view, aerodynamic shaping is the most practical and influential approach for mitigating blackout.  However, there are limitations in implementing such passive approaches and competing design requirements.  For example, the Russians had to use cooling to protect their protruding tip from the environment, making it an active system. 

## Conclusions

**Sharp-tipped RVs experience little or no blackout**
**Very thin sheath also facilitates other mitigation approaches, e.g., quenchants or magnetic fields, should they be necessary**
**Blunted conical and lifting-glide vehicles can experience significant blackout**
**Aerodynamic shaping is most promising approach for mitigating blackout**
**Investigate sharp, slender probes/cones containing antenna protruding ahead of shock wave**
**Higher frequencies correlate with decreased blackout**
**Atmospheric attenuation > ~10 GHz**

**Notes:**
Several key conclusions were drawn from this study:
First, sharp-tipped RVs experience little or no blackout if the nose radius and the angle of attack are small.  The resulting very thin plasma sheath also facilitates other complementary mitigation approaches, e.g., use of quenchants or magnetic fields, should they be necessary.
In contrast to sharp-tip vehicles, blunted conical and lifting-glide vehicles do experience significant blackout.
Aerodynamic shaping is the most promising approach for mitigating blackout and warrants further investigation.  Designers of one vehicle class, that is, powered hypersonic air-breathing vehicles, have great incentive to sharpen leading edges to reduce aerodynamic drag, which has the complementary benefit of making continuous RF communication more achievable.
For blunt RLVs, the application of sharp, slender probes or cones containing antennas protruding ahead of the shock wave should be evaluated.
It was also found that higher frequencies correlate with decreased blackout.  However, atmospheric attenuation becomes an issue at frequencies greater than approximately 10 GHz.
A detailed report on this study was prepared by The Aerospace Corporation and is posted on the FAA/AST website.

## Backup

**Backup**

## Chemical

**Chemical**
**Reactions and**
**Rate **
**Coefficients**
**Used in**
**Nonequilibrium**
**Calculations**

**Reprinted courtesy of NASA**
**[Dunn and Kang, **
**NASA CR-2232, April 1973]**

**Notes:**
As shown in this chart, the high reentry temperatures dissociate the oxygen and nitrogen molecules of the air, which triggers complex chemical reactions producing ionization. All of these reactions need to be accounted for.
The second column shows the dissociation and ionization reaction speeds.  The third column shows the reverse-direction recombination reactions, which are much slower under many conditions at altitudes above 120 kft.  At lower altitudes, the forward and reverse rates become equal, producing chemical equilibrium.  At high altitudes, non-equilibrium chemistry prevails.