Report on the Effects of Parachutes on Risk Mitigation 

to Third-party Property and Individuals 

Prepared by

Allen K. Mears

Futron Corporation

Prepared for

Office of Commercial Space Transportation

March 1993
Report on the Effects of Parachutes on Risk Mitigation 

to Third-party Property and Individuals 

This report is a response to an OCST request that a "quick-turnaround" study be conducted on

the subject topic. The research was conducted under the auspices of an existing contractor

instruction on Additional Data Sources and included a review of References 1-17.

Issue definition: A study on the public's exposure to risk caused by an object or payload

landing upon its arrival from an orbital or suborbital flight trajectory. An investigation was

conducted looking for research that may have been done or information that may have been

gathered that is able to qualitatively or quantitatively discuss the magnitude of risk, or

differential (delta) due to the presence of a parachute, to people or property on the ground in

comparison to the risks of an object descending in free fall; essentially ballistically. The basic

question the study seeks to answer is this. "Why is it safe to release an object on a parachute

while it is unsafe to release it without a parachute?" 

Discussion: The study did not reveal any existent research that included a side-by-side

comparison of objects; one with an aerodynamic decelerator (parachute for this study) and one

in ballistic free fall. However, the research conducted and the references and sources

contacted do allow for certain comparisons and observations to be made. 

Table 1 summarizes those comparisons and findings. The sections that follow Table 1 contain

a more detailed discussion of those comparisons and observations and contain the rationale for

the statements in the Table. The text also contains information on parachute reliability and

concludes with a bibliography of 

References 1-17.

1
Table 1. Comparison of various factors as they relate to the

presence or absence of an operating parachute

 Factor Without a parachute With a parachute

Landing dispersion More precise calculation of the

object's landing site and therefore

less uncertainty about where the

object will land.

Less precision while

determining landing area. The

landing area predicted 

therefore will be larger.

Potential injury or

casualty causing area

The object's casualty causing area is

roughly double the object's frontal

area in any calculation that uses

such information. 

The object's casualty causing

area is larger than the object's

frontal area but not to the point

of being doubled.

Impact forces Impact force or kinetic energy is a

function of velocity2

. With an

object of any size this can be a

considerable force.

Objects will have much lower

kinetic energy due to having

greatly reduced velocity.

Sheltering Only structures of a certain size and

strength can provide adequate

sheltering and protection from even

relatively small objects.

Depending on the object's

weight, most structures will

provide some or total protection

for its occupants.

Objects hitting the

ground

Trees, buildings, poles, etc. will

have slight effect on reducing an

object's ground impact force and

almost no effect on the likelihood of

the object actually reaching the

ground. 

Almost any object that extends

above ground level will have a

tendency to either snag or slow

the descent of an object.

Location Persons located outdoors will not

see or be aware of the object prior

to its impact. 

Persons outdoors are likely to

see the object during its

descent. 

Visual detection The object will not lend itself to

being visually detected because of

its relatively small size and high

velocity.

Detection is possible because of

the parachute's size and

coloration and the object's

much slower velocity.

Collision with aircraft Extremely unlikely but if it did

occur, the aircraft probably would

be destroyed.

Even more unlikely to occur

but if it did, the aircraft might

not be lost.

Weather Weather will have little effect on the

object's descent path.

Humidity, pressure,

precipitation and especially

wind will have an effect.

2
Discussion and Rationale

Landing Dispersion:

The absence of a parachute or other aerodynamic deceleration devices will result in a

more reliable prediction of an object's projected landing point. The projected impact

dispersion of a ballistically returning object can be mathematically calculated and

plotted. The landing dispersion of an object on a parachute is less straightforward to

calculate and when displayed, will encompass a larger area.

This is due to two factors that serve to shrink the dispersion pattern for a ballistic

object. The first factor is that the object will, by itself, be smaller and will be less

effected by atmospheric effects such as winds and air density. The second factor is

that the time length of exposure to any atmospheric phenomena will be considerably

less for an object that is not being mechanically decelerated. On the one hand is an

object coming down on a parachute at 20-25 feet per second and on the other hand is

one in free fall, attaining a terminal velocity probably on the order of 300 miles per

hour (440 feet per second). Similarly, between objects coming down on parachutes

but at different velocities, the system that takes longer to reach the ground will have

the larger landing dispersion area.

Potential Injury or Casualty area:

The absence of a parachute or other aerodynamic deceleration devices, from an object

that requires such a device in order to ensure its survivability, will have the effect of

destroying the object, in this case a payload of some sort, either by impact forces or

because of aerodynamic loads encountered. This has the effect of doubling the

payload's potential injury-causing area either because the in-flight break up of the

payload will create multiple reentering pieces or because a relatively intact payload

will ricochet or break apart on landing and serve to double the area. The exception to

this would be the payload that is partially or totally consumed by the reentry process.

An object on a parachute can also add to the potential injury-causing area by virtue of

the horizontal component of its descent. The convention is to add one foot to the

radius of the object when computing this potential injury-causing area. 

Impact forces:

The absence of a parachute, assuming an object's terminal velocity on the order of 440

feet per second (fps), (300 mph), compared to a descent on a parachute at 25 fps, (17

mph), will increase the impact by a factor of 300 times. For example, let us consider

an object weighing 1,000 pounds. At a descent rate of 25 fps its KE (KE=1⁄2 mv2

) will

be about 9,700 foot-pounds. In free fall at 440 fps, its KE will be 3,000,000 foot3
pounds. This increase in impact energy will have a substantial effect on the

survivability of a person hit or struck a glancing blow by the payload as well as

having a substantial reduction in the protection that may be offered by any structures. 

Thus an object on a parachute will present an impact of much less kinetic energy than

will the same object in free fall. 

Another factor that might reduce the impact force would be any feature, in addition to

the parachute, that would mitigate the kinetic energy forces. A system such as a

deployable airbag would accomplish this and logically would only be installed on an

object descending on a parachute or other decelerating device. An object expected to

come down in free fall would not be expected to have such a device.

 

Sheltering:

The literature is fairly consistent on the benefit of sheltering. Reference 1, as revised

by Reference 2, indicates that an object, if retarded by an operating parachute system,

may not pose a hazard to people sheltered by buildings of various sizes and

construction. The literature indicates that various type structures provide differing

levels of protection as shown below:

Three levels of protection

Type 1: Buildings with concrete or reinforced roofs or floors except for the top floor

in a multistory building.

Protection: Protects against KE from a minimum of 6,200 ft-lbs.

Type 2: Single-story buildings such as houses and trailers and the top floor of

multistory buildings.

Protection: Protects from a minimum of 100 ft-lbs to a maximum of 

3,200 ft-lbs.

Type 3: Unsheltered

Protection: None offered

Additionally, there are other factors that can contribute to how a structure enhances

survivability. For example, the structure may be more sensitive to force per unit area

(pounds per foot2

 or inch2) than to overall kinetic energy and the descending object

might be large to the point of exerting a relatively low force on impact. The structure

may also be designed in such a way as to result in glancing blows to more of its

exposed surfaces. When a glancing blow does occur, the normal force exerted on the

structure is reduced as a function of the slope or angle of the structure. When this

happens, the effective protection offered by the roof, or other part of the structure, is

increased.

4
Objects hitting the ground

There is a well-founded belief that risks to people and property will be mitigated by a

parachute's tendency to become entangled in trees, on buildings, telephone poles and

the like. This is especially true in areas of significant and large foliage and it should

be possible to estimate that probability of entanglement for any specific area once the

percentage of forestation is known. The analysis would need to bear in mind that trees

of differing sizes will have varying effects on the lethality or injury causing

consequences of a ground impact. However, impact with a tree or other vegetation of

almost any size will reduce the object's velocity and hence, its impact force on the

ground. Those same trees would have some effect on an object in free fall but not to

the same degree.

The same belief in mitigation of risk caused by entanglements holds for an urban

complex. Just as a payload and parachute system would probably become entangled in

a tree during a drifting (with the wind) descent, the same would happen in an urban

area. Buildings could be hit and snag the parachute, poles, towers and spires could

also arrest the parachute and the sheer mass of structures could likewise effect the

parachute's descent. Additionally, as discussed elsewhere, the buildings themselves

will have a significant effect on safety of the occupants.

Location:

Reference 1 (NASA) estimated the location of people in rural areas as shown below:

Day Night

Outdoors 50% 5%

Indoors 50% 95%

Reference 15 (Air Force) contains a detailed analysis on percentages of people who

would be under protective covering during working hours (Table B-2), only in much

greater detail than the NASA report. The average in Reference 15 was that about 88

percent of the people are indoors at any given time during the day. The difference

between the two references is caused by the inclusion of more typically urban

occupations in the Air Force study. OCST can feel safe using the Air Force value

when looking at urban or suburban communities. 

The location of people does not bear on the presence or absence of a parachute but it

is useful information on the percentage of the population exposed to risk in typical

demographic profiles. (For those people who are located outdoors during the daylight

hours, the belief is that most people will be able to see an object or objects on a

parachute. Most people would not see an object in free fall. See below.) 

5
Parachute visibility:

The probability of persons outdoors detecting an object descending on a parachute is

estimated at 90 percent, for both day and night. That percentage was based on the

presence of a large, brightly-colored chute and an operating strobe light. This comes

from References 1 and 17. The parachutes discussed in those references are colored

orange or with orange stripes. Reference 17 indicated that for other parachutes, the

likelihood of being seen will vary. 

There is also a belief that multiple parachutes or multiple objects, each on their own

chute or chutes, will be more visible than a single chute. At its simplest, this would

be because multiple chutes will scribe a larger visibility arc than would a single chute. 

There is a logical assumption that someone in a crowd of people would detect a

parachute and alert others around him. This type of generic alerting would probably

not occur with respect to an object in free fall unless the object were purposely being

looked for.

For night time, the presence of a parachute, especially one without a strobe light, will

have little effect on detection until the object is close enough to the ground, and any

lighting, to be sufficiently illuminated. 

Collision with aircraft:

The hazards to aircraft caused by a descending object were not quantified in the

references but were viewed in the literature as being "acceptable" 

(Reference 1). Whether coming down by parachute or ballistically in free fall, the

probability of any object (or any returning space debris for that matter) 

hitting an aircraft is extremely remote. An object on a parachute would be regarded as

simply another conflict to traffic for which a pilot routinely and consistently scans. 

Professional pilots consider their environment as one in which conflicting traffic, and

objects, are encountered as part of flying; which is why pilots are advocates of the

"see and avoid" approach to safety.

Weather effects on parachutes:

Reference 11 was developed to assess the optimum drop altitude that would minimize

damage to cargo but it did contain a brief section on weather effects. It stated that the

effects on the descent of parachutes are humidity, pressure, precipitation and wind,

with wind having the greatest effect. The Reference describes different wind states

that might be encountered and includes the observation that "A non-gliding parachute

will always horizontally decelerate to the horizontal velocity of such an air mass,

regardless of the velocity of the air mass with respect to earth." Simply stated, this

means the object will travel with the same velocity as the wind itself. An obvious

statement but one that needs to be said.

6
Parachute reliability:

In the NASA balloon program, the specific parachute failure rate for those parachutes

used on non-heavy payloads is about .4 percent (Reference 17). This is a reliability of

.996 (1.0-.004). As reported in Reference 16, the nominal failure rate for parachutes

used to drop supplies and equipment is .7 percent; 19 failures out of 2,694 drops in the

1st quarter of 1992. This is a reliability of .993 (1.0-.007). These are all excellent

reliability figures and indicate that if an object is supposed to come down by a

parachute, it will do so in nearly all cases. 

Summary:

The research indicates a large amount of qualitative information that allows one to conclude it

is safer for an object to come down on a parachute than for it to come down in free fall. This

applies not only to the object itself but to the people and property on the ground. There are

many factors relating to parachutes that serve to protect the public as well as protecting the

object descending on the parachute. If safety of the public is a concern, then operating

parachutes are a material benefit. 

It is also significant to note that there exists an extensive amount of research and

mathematical support dealing with ballistic objects, those in free fall, while a much smaller

amount exists for objects descending on a parachute. An inference might be drawn that the

risks attendant to an object on a parachute are not significant enough to warrant such detailed

mathematical analysis.

Researched and Referenced Documents:

1. Safety Analysis Report for Balloon Programs Conducted by the National Scientific Balloon

Facility, Palestine, Texas, Robert J. Beyma, NASA/GSFC, February 1985.

2. Safety Review for Balloon Programs Conducted by the National Scientific Balloon Facility,

Palentine, Texas, Robert J. Beyma, Ground and Flight Safety Section, September 1988

3. Atmospheric Factors Study Draft, OCST, SRS Technologies, August 1991. 

4. Parachute Systems Study Draft (September 21, 1991), OCST, SRS Technologies, August

1991.

5. A Key to a Successful Recovery, R.W. Roider, AIAA 5th Aerodynamic Deceleration

Systems Conference, November 17-19, 1975.

6. Space Shuttle Solid Rocket Booster Deceleration Subsystem Drop Test 3 -- Anatomy of a

Failure, R.E. Runkle, Marshall Space Flight Center, and W.R. Woodis, Martin Marietta

Corporation, 1979.

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7. Aeroballistic Characteristics of Sonobouy Parachute Decelerators Limited to Three Feet,

C.T. Calianno, AIAA 7th Aerodynamic Decelerator and Balloon Technology Conference,

October 21-23, 1981.

8. The Design and Flight Testing of a High-performance, Low-cost Parachute System for a

1,000 lb. Payload, Vance L. Behr, Sandia National Laboratories, 1991.

9. Design and Performance of a Parachute for the Recovery of a 760-Lb Payload, Donald E.

Waye, Sandia National Laboratories, 1991.

10. Recent Advances in Parachute Technology, D.R. Dennis, Royal Aircraft Establishment,

May 18, 1983.

11. Trajectory Analysis of the G-11 Family of Clustered Parachutes to Determine Minimum

Altitude, Steven Kunz, U.S. Army Natick Research, Development and Engineering Center,

November, 1991.

12. Risk Assessment and Its Application to Flight Safety Analysis, David I. Keese and William

R. Barton, Sandia National Laboratories, 1982.

13. Ground Impact Point Forecast for Stratospheric Balloon Floating Payload, O. Cosentino,

J.C. Varlet, ESA Symposium on European Rocket and Balloon Programmes and Related

Research, May 27-31, 1991.

14. Review of Orbital Reentry Risk Predictions, OCST, The Aerospace Corporation, 

July 15, 1992.

15. (U) Recovery Safety Criteria for Air Force Space Vehicles, Commander Space Systems

Division, Air Force Systems Command, The Aerospace Corporation, January 1965.

16. Airdrop Review and Malfunction/Safety Analysis, U.S. Army Quartermaster Center and

School, Volume I 1992.

17. Phonecon Robert J. Beyma, NASA Ground and Flight Safety Section and Allen K. Mears,

Futron Corporation, January 29, 1993.

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