High Altitude Venus Operational Concept (HAVOC): An Exploration Strategy for Venus

Full text

1
American Institute of Aeronautics and Astronautics
High Altitude Venus Operational Concept (HAVOC):
An Exploration Strategy for Venus
Dr. Dale C. Arney
1
and Christopher A. Jones
2
NASA Langley Research Center, Hampton, VA 23681
Humans are on their way to becoming a spacefaring civilization, and Venus presents an
intriguing destination for expanding humanity’s journey beyond Earth. The atmosphere of
Venus is a suitable environment for both further scientific study and future human
exploration. Fifty kilometers above the Venusian surface is one of the most hospitable, Earth-
like locations in the Solar System; the pressure, density, gravity, and radiation protection are
all similar to Earth surface conditions. A recent internal NASA study of a High Altitude Venus
Operational Concept (HAVOC) led to the development of an evolutionary program for the
exploration of Venus, with a focus on the mission architecture and vehicle concepts for robotic
missions and 30-day crewed missions into the Venusian atmosphere. Initial analysis has shown
that both robotic and human exploration of the Venusian atmosphere is feasible contingent
on the development of key capabilities: human-scale aeroentry vehicles, high dynamic
pressure supersonic decelerators, long-duration cryogenic storage, Venus and Earth
aerocapture, and rapid airship inflation (during the descent). Many of these capabilities are
complementary to previously and currently considered Mars architectures, and their
development would be enabling to voyages to either planet. Ultimately, with its relatively
hospitable upper atmosphere, Venus can play a role in humanity’s future in space.
I. Introduction
UMANS will become a spacefaring civilization. Humans explore to satisfy their curiosity of their environment,
to acquire resources to support an expanding population, and to start a new life in a new location. Venus has the
potential to be a critical part to that spacefaring civilization as both a destination in itself and as a stepping stone to
sending humans to Mars [1].
Venus is the nearest planet to Earth, nearly half the distance between Earth and Mars at closest approach. This
corresponds to shorter communications time delays, thus mitigating the challenges of autonomy. Also, there is an
abundance of resources to be used by eventual human explorers. These include solar energy (Venus is 0.72 AU from
the Sun, and therefore has 2,643 W/m2 of solar energy flux in orbit), carbon, oxygen, and nitrogen (the atmosphere is
96% CO2 and 4% N2). Trace amounts of hydrogen are present in the form of sulfuric acid [2].
Venus exploration would also provide a useful stepping stone to Mars. The orbital mechanics are such that shorter
missions (approximately 14-15 months total duration) are possible with similar propulsive requirements. Also, unlike
Mars, an abort option exists after arrival at Venus. At Mars, once the crew arrives, they must remain there until the
planned return flight (approximately 500 days for a conjunction-class mission). The easier conditions at Venus allow
it to serve as an easier mission prior to attempting a crewed mission to Mars. Similar technologies and capabilities
would also be required for both missions (e.g. long-duration habitation, aerocapture/aeroentry, carbon dioxide
processing, and propulsion systems) [3].
Looking back on the history of Venus exploration, Venus was the second celestial body visited by robotic probes,
dating back to attempts in the early 1960s by both the United States and Soviet Union. Figure 1 presents an overview
of the successful missions to Venus. The Soviet Venera and Vega programs were extensive in their exploration of the
atmosphere and surface through the 1980s. The United States, through the Mariner, Pioneer, and Magellan programs,
explored the atmosphere both in-situ and from orbit over long durations. Much of our understanding of the Venusian
atmosphere derives from these missions. Since Magellan ended in 1994, exploration of Venus has come through
flybys by probes on their way to other planets and the Venus Express mission, which ended on December 16, 2014.
1
Aerospace Engineer, Space Mission Analysis Branch, MS 462, AIAA Member.
2
Aerospace Engineer, Space Mission Analysis Branch, MS 462, AIAA Member.
H

2
American Institute of Aeronautics and Astronautics
Figure 1: Overview of Successful Robotic Missions to Venus
In order to support human exploration of Venus, a resurgence in the robotic exploration of Venus will need to occur.
Understanding the environment that the humans will encounter will be critical to reducing mission risk. Compared to
Mars, where cumulative surface exploration time is over 31 years and cumulative orbital exploration time over 49
years, cumulative surface exploration time of Venus is approximately 9.7 hours, cumulative atmospheric exploration
time is just over 4.5 days, and cumulative orbital exploration time is over 28 years.
A. Mission Objectives
The mission objectives for HAVOC are divided into two groups: science objectives and human objectives. The
science objectives were derived from the Venus Exploration Analysis Group (VEXAG) [4]. The human objectives
were developed by the study team to define objectives that would reduce risks and advance technologies that would
be necessary for human exploration of the Solar System. The VEXAG science objectives are divided into three
groups, while the human objectives are divided into two additional groups:
 Understand atmospheric formation, evolution, and climate history on Venus.
 Understand the nature of interior-surface-atmosphere interactions over time, including whether liquid water
was ever present.
 Determine the evolution of the surface and interior of Venus.
 Demonstrate the ability for humans to survive and operate in deep space and around planetary bodies.
 Develop advanced technologies that will enable humans to visit planetary destinations.
To determine the platform that would effectively address these objectives, first a number of potential platforms
were identified. An orbital platform, similar to previous missions, can provide some measures of the upper
atmosphere, but lacks the in-situ measurement capability that can explore the deeper atmosphere and surface.
Persistent aerial platforms at high (>60 km), mid-level (45-60 km), low-level (15-45 kmg), and near-surface (<15 km)
altitudes can better explore these atmospheric regimes, but face environmental challenges that can impact the kinds of
instruments that can be used. Entry probes provide short term exploration of the entire atmosphere during their fall to
the surface. Surface systems, whether landers or mobile systems, can study the atmosphere and surface, but must
survive the harsh surface environment.
These platforms were rated on their ability to address the identified science objectives, based on analysis in the
Planetary Science Decadal Survey [5]. The results of this scoring is presented in Figure 2. The lander platforms

3
American Institute of Aeronautics and Astronautics
scored very highly due to their ability to analyze the atmosphere during descent and the surface. However, using near-
term technologies, sustaining humans on the Venusian surface would likely be infeasible, and even robotic systems
are limited to missions on the order of hours. The technology development efforts to support human life at or near the
Venusian surface lead to a low scoring of those concepts on the human objectives. Therefore, excluding these options,
the mid-level aerial platform scores the highest of the remaining options.
Figure 2: Comparison of Potential Platforms' Ability to Address Mission Objectives
The environment at the 45-60 km above Venus is one of the most Earth-like environments in the Solar System.
At 50 km, the baseline altitude for both the robotic and human concepts in this study, the conditions are relatively
benign and supportive of human exploration. Figure 3 presents a comparison of conditions at potential destinations
for human exploration on Venus and Mars with conditions at Earth. Temperature, pressure, and gravity are very
similar to Earth, while the atmospheric density and solar intensity is higher than at Earth. The increased density allows
gases such as breathable air to serve as a lifting gas in an airship/balloon and gases such as helium to function even
better as a lifting gas. The solar intensity increases the output of photovoltaic systems, providing sufficient power for
instrumentation and habitation systems.
Figure 3: Comparison of Potential Destinations for Human Exploration

4
American Institute of Aeronautics and Astronautics
B. Venus Evolutionary Exploration Program
Based on this desire to explore Venus at approximately 50 km above the surface, a five phase approach to an
evolutionary Venus exploration program was developed. This evolutionary approach, shown in Figure 4, grows from
the initial robotic missions that would characterize the environment and expand the knowledgebase about Venus
through the potential ability for permanent human presence in the Venusian atmosphere.
Initially, during Phase 1, robotic airships would explore the atmosphere at 50 km in order to characterize the
environment that eventual human explorers would experience and to learn about the workings of the Venusian
atmosphere as well as its surface. After the initial robotic missions, Phase 2 would deploy human-scale missions to
Venus. These missions could be large, human-scale airships deployed into the atmosphere without crew onboard or
could be short duration missions to Venus orbit to tele-operate assets in the atmosphere. Phase 3 would build upon the
missions in the previous phases to send the first human crew into the atmosphere of Venus for 30 days onboard an
airship. The mission length for this phase is constrained by the interplanetary orbital mechanics, where the opportunity
to return to Earth is open from the time the crew arrives and remains open for approximately 30 days. Phase 4 would
send a crew to the Venusian atmosphere for 1 year by returning to Earth on the next available return window. This
phase would advance new technologies and capabilities to eventually enable Phase 5, where there would be a
permanent human presence in the Venusian atmosphere.
Figure 4: Evolutionary Venus Exploration Strategy
II. Concept of Operations
The primary focus for the analysis was on the Phase 1 mission and the Phase 3 mission; this led to an understanding
of the challenges associated with the vehicle concept as it would appear in multiple phases, as well as the systems
required to support humans in the Venusian atmosphere. The Phase 1 (robotic) mission would be a one-way trip to
Venus, where the vehicle would aerocapture into Low Venus Orbit (LVO) before entering into the atmosphere,
deploying the airship, and operating. There would be no return segment. The Phase 3 (human) mission would be a
roundtrip mission with an outbound and return segment linked by a stay time at Venus.
A heliocentric view of a roundtrip mission is presented in Figure 5. This trajectory shows the worst case mission
(in terms of total ∆V) over a two decade period. The outbound trajectory would have a time of flight of approximately
110 days, require a ∆V from Low Earth Orbit (LEO) of approximately 3.9 km/s, and an entry speed at the Venusian
atmosphere of approximately 11.3 km/s. After a stay time of 30 days at Venus, the return flight would have a time of
flight of 300 days, which reduces the required return ∆V. The return ∆V is approximately 4.1 km/s and the Earth
entry velocity is approximately 14.2 km/s. This set of trajectories represents the Phase 2 or 3, 30-day human missions.

5
American Institute of Aeronautics and Astronautics
However, the Phase 4, 1-year human missions, have the opportunity for a fast return (on the order of 100 days) with
lower performance requirements (return ∆V is approximately 3.6 km/s) because the planetary alignment is more
favorable.
Figure 5: Roundtrip Mission to Venus
The trajectory for the Phase 1 mission would only include the outbound segment, as this concept does not include
a return element. The mission design for the robotic mission has many options, including low thrust spirals with
electric propulsion, using a chemical propulsion stage in Earth orbit to get the appropriate ∆V, or to have the launch
vehicle directly insert the payload into the interplanetary trajectory. This study does not include a thorough
comparison of these options, and used the high thrust options for the baseline analysis.
Figure 6 contains the concept of operations for the baseline robotic mission. The airship, packaged in its aeroshell,
are launched on a commercial launch vehicle, as it is small enough not to require a Space Launch System class launch.
To place this vehicle onto the appropriate interplanetary trajectory, a chemical propulsion stage could be used to
provide the appropriate ∆V in LEO or the launch vehicle can directly place the airship into the interplanetary trajectory.
At arrival, the airship would aerocapture into LVO before performing the Entry, Descent, and Inflation (EDI)
maneuver. Aerocapture is assumed in this baseline concept of operations because it provides an opportunity to
advance that capability in preparation for the latter human missions, which will require aerocapture. Once inflated in
the Venusian atmosphere, the robotic airship can begin the atmosphere operations.

6
American Institute of Aeronautics and Astronautics
Figure 6: Concept of Operations for Baseline Phase 1 Robotic Mission
Table 1 presents three preposition options for the Phase 3 concept of operations, where the human mission would
consist of the airship (along with its surface habitat and ascent vehicle) and the in-space vehicle (habitat and propulsive
stage). Qualitative comparisons on mass, operational complexity, and abort options are presented for each option.
The red text indicates the significant challenge(s) that a given option presents.
The Direct or Earth Orbit Rendezvous (EOR) option would aggregate all of the elements in LEO before departing
together. This departure stack would be very large and likely prohibitive to aerocapture at Venus using near-term
technologies. Operationally, this option would be similar to other large-scale assemblies around Earth and provide
fast abort options during the integration.
The Venus Orbit Rendezvous (VOR) option would preposition the airship in LVO and the crew would arrive in
the in-space vehicle afterward. The size of the two vehicles separately is within the feasible realm for aerocapture at
Venus. Time delay during rendezvous may present some issues, and if there were an issue before the crew transferred
to the airship, the option to abort to Earth (after the 300 day transit) is available.
Table 1: Phase 3 Preposition Options
Direct (or Earth Orbit
Rendezvous, EOR)
Venus Orbit Rendezvous
(VOR)
Venus Atmosphere
Rendezvous (VAR)
Mass
• Earth departure stack is
large
• Aerocapture stack is likely
prohibitive in near term
• Aerocapture stack is large
but feasible
• Could use ISRU for ascent
propellant (reduce
delivered mass)
Operational
Complexity
• Rendezvous in Earth orbit
similar to other in-space
assembly operations
• Rendezvous in Venus
orbit poses time delay
issues
• Atmospheric rendezvous
is challenging for early
missions
Abort Options
• Quicker abort during
rendezvous/ integration
operations
• Abort to Earth from
Venus (~300 days) during
rendezvous operations
• No abort options during
rendezvous (cannot
ascend to TEI stage)

7
American Institute of Aeronautics and Astronautics
Finally, the Venus Atmosphere Rendezvous (VAR) option could reduce the vehicle mass with In-Situ Resource
Utilization (ISRU) once in the atmosphere. ISRU could be used to produce ascent propellant, breathable air, and
lifting gas which would not need to be sent from Earth. However, the crew would need to rendezvous with the
deployed airship in the atmosphere, which would pose significant operational (time-critical events, high latency with
Earth, no abort options during rendezvous, etc.) and technical challenges for the early missions. For the eventual
missions in Phases 4 and 5, this atmospheric rendezvous would be needed, so this capability would need to be
developed to proceed into those phases.
The EOR option requires an aerocapture system beyond what will likely be available in the near-term; thus, the
VOR option is presented as the baseline concept of operations for the Phase 3 mission. Figure 7 presents the
preposition portion of the mission, while Figure 8 presents the operations and return portions of the mission.
Figure 7: Concept of Operations for Phase 3 Human Mission (Part 1)
The Phase 3 mission begins with aggregation of the airship, entry system, and Trans-Venus Injection (TVI) stages
in Earth orbit. From there, the TVI stages perform the burn(s) to escape the Earth sphere of influence on the outbound
interplanetary trajectory described in Figure 5. At Venus arrival, the vehicle aerocaptures into LVO, where it remains
until the crew arrives. Next, the transit habitat, Trans-Earth Injection (TEI) stage, aerocapture system, and TVI stages
aggregate in Earth orbit. Once those systems are integrated, the crew of two ascends on a separate crew launch and
rendezvouses with the transit habitat. The capsule that the crew used during ascent returns to Earth. Once aggregated
with the crew onboard, the TVI stages place this vehicle on the interplanetary trajectory. Depending on the launch
cadence, aggregation campaign, and risk posture, the crew flight could occur either in the same opportunity as the
airship or in the subsequent opportunity.
After performing aerocapture at Venus, the transit vehicle rendezvouses with the airship vehicle in LVO. The crew
transfers into the airship vehicle in LVO and begins the EDI maneuver. At the end of the EDI, the airship will be fully
inflated at the 50 km altitude and the crew will begin the planned atmosphere operations. The transit vehicle remains
in LVO during the atmospheric operations, where it waits for the crew return and functions as an orbital asset for
communications and other functions. At the end of atmosphere operations, the crew ascends from Venus using the
ascent vehicle. The airship remains in the atmosphere operating indefinitely, serving as a robotic observational
platform until the vehicle is no longer operational. After the crew transfers back into the transit habitat, the TEI stage
places the vehicle en route to Earth, where the vehicle aerocaptures into Earth orbit (the return velocity from Venus is
higher than typically seen in Mars missions, hence the lack of direct entry). A dedicated crew vehicle retrieves the
crew in Earth orbit and reenters in a dedicated crew capsule. The transit habitat can remain in Earth orbit for
subsequent missions or be discarded in lieu of a new transit vehicle for the next mission.

8
American Institute of Aeronautics and Astronautics
Figure 8: Concept of Operations for Phase 3 Human Mission (Part 2)
III. Vehicle Concept
A. Airship Vehicle Overview
The baseline vehicle concept (for Phase 3 mission) is shown in Figure 9. The envelope is a 3.8:1 fineness ratio
airship containing helium lifting gas. Due to the density of the Venusian atmosphere, helium provides more lift
capability than an equivalent volume on Earth. Air and hydrogen are also viable lifting gasses to be considered for
this concept, but the increased mass and volume of air required presents a significant burden for the transportation
system and hydrogen presents issues with transportation from Earth to Venus.
For propulsion and control, solar panels on top of the envelope collect power to drive electric propellers and fin
control surfaces. The power is also used by the payload attached to the bottom of the envelope in the gondola. For
the Phase 1 mission, the gondola could consist of scientific instruments, any deployable probes, or even surface landers
deployed from the airship during atmosphere operations. In the Phase 3 mission, the gondola would consist of the
atmospheric habitat where the crew of two live for the mission duration and the ascent vehicle which take the crew
back to orbit to rendezvous with the transit vehicle.
Figure 10 presents an overview of the Phase 1 robotic airship concept and key performance parameters; 1 kWe of
the power generated by the solar power system is available for payloads. Figure 11 presents an overview of the Phase
3 human airship concept and key performance parameters; the total payload mass (which includes atmospheric habitat,
ascent vehicle, ascent habitat, and science instruments) is approximately 70 t. Of the power generated by the Phase 3
airship, 20 kWe are used by the payload for habitat power and cryogenic management of the ascent vehicle. Figure
12 presents a comparison of the size of the baseline vehicle concepts with existing or proposed aerospace systems.
The robotic airship, when fully inflated at 31 m long, is approximately half the length of the Goodyear blimp. The
human airship at 129 m long is approximately double the length of the Goodyear blimp or half the length of the
Hindenburg. When packaged in the aeroshell, the human airship fits within a shroud diameter and length envelope of
10 m diameter by 30 m length.
B. Atmospheric Mission Overview
The super-rotation of the Venusian atmosphere leads to eastward winds of 85 – 110 m/s with respect to the surface
along the equator at 50 km altitude. This leads to an average apparent local solar time of approximately 110 hours.
Along with these longitudinal winds, poleward winds of approximately 5 m/s would push the vehicle toward the poles.

9
American Institute of Aeronautics and Astronautics
During the atmospheric operations at Venus, the airships “ride” the longitudinal winds while using electric propulsion
to counter the poleward drift.
Figure 9: Baseline Vehicle Concept Overview
Figure 10: Phase 1 Robotic Airship Concept and Performance Parameters

10
American Institute of Aeronautics and Astronautics
Figure 11: Phase 3 Human Airship Concept and Performance Parameters
The airship operates in one of two modes depending on the perceived day/night cycle. The nighttime operational
mode assumes the longest night (consistent with 85 m/s winds) of 66 hours. During this operational mode, the energy
storage system powers the payload and propulsion system capable of achieving 3 m/s. Because the poleward winds
are approximately 5 m/s, the airship will drift away from the equator during this time. The daytime operational mode
assumes the shortest day (consistent with 110 m/s winds) of 44 hours to collect power via the solar arrays. The arrays,
in addition to powering the payloads and charging the energy storage system, power the propulsion system, which is
designed for a velocity of up to 15 m/s. This higher velocity enables the airship to overcome the poleward drift and
reach the equator before the nighttime operations begin.
The payloads for the airship vary depending on the individual mission type (human or robotic). The robotic mission
would carry science instruments and secondary payloads to deploy sensors into different parts of the Venusian
atmosphere or even the surface. The human mission would have science instruments, but would also carry the
atmospheric habitat to support the crew for the duration of their stay in that atmosphere and the ascent vehicle with its
habitat to return the crew to orbit after their stay.
C. Instrumentation and Secondary Payloads
To address the science goals presented in VEXAG and understand the environment for future human missions,
onboard instrumentation and secondary payloads have notionally been included in the airship payload. The payload
allotment for the Phase 1 robotic mission is 750 kg while the allotment for the Phase 3 human mission is 100 kg.
A notional suite of instruments was taken from the Venus Flagship Mission Study report from JPL in 2009, which
contained an atmospheric balloon concept. This notional suite is included in Table 2, along with estimates of the mass
and power required for the instruments. The instrument package was selected to address atmospheric science
objectives similar to those presented in VEXAG. At 17 kg and 52 W for science instrumentation, the payload
allotment enables significant secondary payloads in both robotic and human configurations.

11
American Institute of Aeronautics and Astronautics
Figure 12: Size Comparison of Baseline Vehicle Concepts with Other Aerospace Vehicles
Table 3 presents options for the transfer vehicle, the airship itself, and secondary payloads that could be deployed
from an airship platform. The potential contribution to the VEXAG goals for each platform and secondary payload
option is also rated in the table. The airship platform provides a major contribution to understanding the first VEXAG
goal on understanding the atmospheric formation, evolution, and climate history. It is also naturally situated to study
the environment that the future human missions would experience. As an orbital platform, the transfer vehicle would
contribute to specific investigations on the energy balance and outgassing of Venus, but its likely impact would be as
a mission support asset (e.g. communications relay). Drop balloons and drop probes would be effective secondary
payloads due to their relatively small size and mobility. They would contribute knowledge about different parts of
the atmosphere that the airship platform is not exploring, and provide some understanding on the altitude variance of
the atmosphere, even near the surface where the surface-atmospheric interactions would occur. Finally, while a lander
would not provide significant contribution to understanding the atmosphere, it would provide the ideal platform to
explore the surface, interior, and surface-atmosphere interaction VEXAG goals. A notional mass range for each
secondary payload option is presented in the table, and a lander would require most if not all of the payload allotment.
Drop probes and balloons could be small enough to include multiple on a single robotic mission or 1-2 on a human
mission. While this report does not specify an individual manifest, there is enough flexibility in the airship platform
to fly multiple missions with different science instruments and secondary payloads on each to address different science
objectives.

12
American Institute of Aeronautics and Astronautics
Table 2: Notional Instrument Suite (Source: Venus Flagship Mission Study (2009))
Instrument
Mass (kg)
Power (W)
Source or Proxy
Gas Chromatograph Mass Spectrometer
11
40
Huygens, VCAM
Thermocouple, Anemometer,
Pressure Transducer, Accelerometer
2
3.2
MVACS, ATMIS
Net Flux Radiometer
2.3
4.6
Galileo Probe
Magnetometer
1
2
JPL internal studies
Nephelometer
0.5
1.2
Pioneer Venus
Lightning Detector
0.5
0.5
FAST
TOTAL
17.3
51.5
Table 3: Secondary Payload Options for Venus Atmospheric Exploration
D. In-Space and Atmospheric Habitats
During the Phase 3 human mission, the crew would occupy three habitats during various periods in the missions.
An overview of these three habitats is presented in Figure 13. The transit habitat contains the crew during the 110-
day outbound segment and the 300-day return segment. During the 30-day atmospheric mission, the transit habitat
loiters unoccupied in LVO. This habitat is similar in design to the Deep Space Habitat (DSH) concepts that have been
developed for human missions to Mars. The atmospheric habitat loiters with the airship in LVO until the crew arrives
to begin the atmospheric mission. The crew stays in this habitat during the 30-day atmospheric operations, between
descent and inflation and crew departure. This habitat is similar to that used in the Space Exploration Vehicle (SEV)
concepts for lunar, asteroid, and Mars missions. Finally, the ascent habitat is only occupied by the crew during Entry,
Descent, and Inflation (EDI) and ascent.
E. Venus Ascent Vehicle
The Venus Ascent Vehicle transfers the ascent habitat with the crew from the 50 km operating altitude to LVO
where the transit habitat is loitering. Figure 14 provides an overview of the two stage, LOX/RP-1 system that weighs
approximately 63 t excluding the ascent habitat. Due to the similar gravity and atmospheric density at 50 km, the
required ascent performance is comparable to Earth ascent. The concept is similar to the Pegasus rocket. To initiate
ascent, the ascent vehicle drops away from the airship. Then, the first of two stages ignites to start ascending to LVO.

13
American Institute of Aeronautics and Astronautics
Aerodynamic surfaces on the first stage provide lift and controllability during flight in the Venusian atmosphere. The
second stage provides the final burn to place the ascent habitat in LVO, where it rendezvouses with transit habitat.
F. Aerocapture, Entry, Descent, and Inflation
A key element of this study focused on the design, modeling, simulation, and analysis of two segments of the
HAVOC trajectories: aerocapture, and entry, descent, and inflation (EDI). During aerocapture the vehicle takes
advantage of the drag created by flying through the atmosphere to reduce the speed of the vehicle, without the use of
a large propulsive maneuver. Bank angle modulation is used to adjust the lift vector and guide the vehicle toward the
desired target apoapsis, using a small propulsive burn to clean up any targeting errors. Once the vehicle is in the
desired circular orbit, atmospheric entry may begin. During Entry, the entry vehicle (EV) is guided through the middle
and lower atmosphere and may be maneuvered to prevent excessive heating and aerodynamic loads. Once the EV has
slowed to supersonic velocities, the Descent phase begins when an aerodynamic decelerator, such as a parachute or
ballute, is deployed to further reduce descent rate. The Inflation phase begins when the EV velocity is sufficiently
reduced such that the airship may be exposed to the oncoming flow. The airship is pressurized during parachute
descent and reaches a nominal altitude of approximately 50 km. Figure 15 shows the HAVOC concept of operations
from aerocapture to inflation and operations. Though not shown in the figure, the human mission requires two vehicles
to undergo aerocapture: the crew and airship/ cargo are launched separately and arrive at Venus in separate vehicles,
then rendezvous in orbit before performing the EDI maneuver.
Figure 13: Habitats Used in Phase 3 Human Missions
Design and analyses of the entry and descent phases of the trajectories were conducted using the Program to
Optimize Simulated Trajectories II (POST2). Three-degree-of-freedom simulations run with POST2 used
aerodynamic and aeroheating databases generated with CBAERO for the scaled mid-L/D “ellipsled” from the NASA
Entry, Descent, and Landing Systems Analysis (EDLSA) study [6]. The nominal atmosphere was modeled using
VenusGRAM 2005 assuming no winds [7]. The Venus gravitational field was modeled up to the J4 zonal harmonic.
Entry interface was defined as a geodetic altitude of 200 km, and the entry point was defined as the intersection of the
equator and the Venus prime meridian (0° latitude, 0° longitude). The conditions at 50 km altitude are roughly
equivalent (though warmer) to atmospheric conditions at sea level on Earth; the nominal density, pressure, and
temperature are 1.5948 kg/m3, 106,679 Pa (1.0528 atm), and 350 K (170°F), respectively.

14
American Institute of Aeronautics and Astronautics
The sharp density gradient, coupled with the very high entry mass of the human mission, present significant
trajectory design challenges. Among these are:
1. Atmosphere skip-out. Skip-out results when the EV is traveling at too high a velocity (or has too much lift)
to be “captured” and exits the atmosphere at a reduced speed, and may or may not re-enter. When controlled,
this may be used as a technique for aerocapture and aerobraking.
2. Excessive lofting. Lofting is a repeated increase and decrease in altitude due to excessive lift that may result
in atmosphere skip-out, and is usually more pronounced in trajectories with shallow entry flight path angles.
3. Excessive aerodynamic forces (g-loads). Excessive g-loads may injure crewmembers and can be fatal at high
levels over extended periods. For this reason, NASA has established g-load limits for human crews that are
functions of length of exposure and orientation of the crew relative to the acceleration vector.
4. Excessive aerodynamic heating. Atmospheric entries are characterized by high heat rates and loads that
require the use of thermal protection systems (TPS) to dissipate heat and protect the EV. It is advantageous
to minimize heat rates and loads, thereby reducing structural mass.
Figure 14: Venus Ascent Vehicle Overview
To address these challenges, a variety of different trajectories were analyzed by establishing a parameter design
space and varying specific independent parameters relating to the trajectory. The result was thousands of simulated
trajectories from which parameters of interest were extracted and analyzed for trends. The independent parameters of
interest were entry flight path angle, total angle of attack, and bank angle. Dependent parameters of interest included
peak g-load, peak heat rate, and total heat load. A trajectory was considered “valid” if it did not skip out and if the
peak g-load did not exceed predetermined limits. Peak heat rates were monitored, though not used as a selection
criteria, since the thermal protection system is designed around these values.
Initial analyses were conducted with an unguided vehicle with a theoretical control system that maintained a
constant total angle of attack and bank angle through the trajectory. A target “landing” location was not considered
due to the highly mobile nature of the payload/airship, which does not require injection at a precise altitude or latitude.
Later analyses implemented a simple logic algorithm to modulate bank angle to mitigate aerodynamic loads.
The overall trajectory design was an iterative process. First, the initial trajectory design space was generated and
candidates were selected from the “pool” of initial conditions that satisfied the trajectory and mission requirements,
e.g., presented low heat rates and g-loads. These candidates were deemed representative of nominal trajectories. These
nominal trajectories, one for both the robotic and human cases, were then used to size the TPS as previously discussed.
The TPS sizing tool produces a new vehicle mass that is then used to re-run the trajectory design space, and the process

15
American Institute of Aeronautics and Astronautics
is repeated until the TPS mass converges. Although further iterations would be required to fully close the design, the
current results shown here represent HAVOC trajectories that deliver the desired payloads to the target altitudes.
Analysis of point cases drawn from the human mission design space showed that dynamic pressures at parachute
deploy exceed the valid environment due to the relatively low altitude at which the vehicle reaches Mach 2.1, and
resultant high density. The high dynamic pressure is beyond the survival capability of commonly used parachutes
(such as disc-gap-band) used in current planetary exploration. Thus, a different technology such a ribbon parachute or
ballute must be used to slow the vehicle enough to permit the airship to inflate while under a parachute. In the case of
the robotic lighter-than-air mission to Venus, it was determined that such a mission is feasible using current and near-
future technology, such as a PICA thermal protection system. Future work will include implementation of guidance
and control systems aboard the entry vehicle to aid in refining the reference trajectories. For the human mission this
may improve the dynamic pressure at the Mach 2.1 condition and enable the use of a conventional supersonic
parachute [8].
Figure 15: HAVOC aerocapture, entry, descent, and inflation concept of operations [8].
IV. Conclusion
The HAVOC study presents an evolutionary exploration plan for Venus that can meet scientific objectives for
planetary science while offering another destination for human exploration beyond Earth. Initial analysis has shown
that both robotic and human exploration of the Venusian atmosphere is feasible contingent on the development of key
capabilities: human-scale aeroentry vehicles, high dynamic pressure supersonic decelerators, long-duration cryogenic
storage, Venus and Earth aerocapture, and rapid airship inflation. Many of these capabilities are complementary to
previously and currently considered Mars architectures, and their development would be enabling to voyages to either
planet. Deeper dives into payload and vehicle sizing, trajectory analysis, and operations planning would refine
understanding of both the architecture and the multiple vehicles. Ultimately, the authors conclude that Venus, with its
relatively hospitable upper atmosphere, can play a role in humanity’s future in space.
Acknowledgments
The authors would like thank the Systems Analysis and Concepts Directorate at NASA Langley Research Center
for funding this study.

16
American Institute of Aeronautics and Astronautics
References
1. Landis, G. “Colonization of Venus.” Conference on Human Space Exploration, Space Technology & Applications
International Forum, Albuquerque NM, Feb. 2-6 2003.
2. de Bergh, C., Moroz, V., Taylor, F., Crisp, D., Bezard, B., Zasova, L. “The Composition of the Atmosphere of Venus
below 100 km Altitude: An Overview.”Planetary and Space Science Vol. 54, p.1389-1397, 2006.
3. Drake, B., “Human Exploration of Mars Design Reference Architecture 5.0,” Tech. Rept. NASA-SP-209566, NASA,
2009.
4. Herrick, R., Baines, K., Bullock, M., Chin, G., Grimm, B., Kiefer, W., Mackwell, S., McGouldrick, K., Sharpton, B.,
Smrekar, S., Tsang, C. “Goals, Objectives, and Investigations for Venus Exploration: 2014.” Venus Exploration
Analysis Group. 2014.
5. Committee on the Planetary Science Decadal Survey. Vision and Voyages for Planetary Science in the Decade 2013-
2022. National Academies Press, Washington DC. 2011.
6. Dwyer Cianciolo, A. et al. “Entry, Descent, and Landing Systems Analysis Study: Phase 1 Report,” NASA Technical
Memorandum, NASA/TM-2010-216720, July 2010.
7. Justh, H., Justus, C., Keller, V., “Global Reference Atmospheric Models, Including Thermospheres, for Mars, Venus,
and Earth,” AIAA/AAS Astrodynamics Specialist Conference and Exhibit, AIAA 2006-6394, August 2006.
8. Lugo, R., Ozoroski, T., Van Norman, J., Arney, D., Dec, J., Jones, C., Zumwalt, C. “High Altitude Venus Operations
Concept Trajectory Design, Modeling, and Simulation.” 25th AAS/AIAA Space Flight Mechanics Meeting. AAS-
15223. 2015.