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NASA PROGRAM APOLLO WORKING PAPER NO. 1203

HAZARDS ASSOCIATED WITH A LEM ABORT

NEAR THE LUNAR SURFACE

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N70 -35 73 1 ---_"(ACCESS,ON_L',V,_ER) (THOU)

0 _ (PAGE'S') r.) (CODE1 __ ,,. .

_¢ (NASA CRORTMX OR AD NUMBER) (CA1EGORY)

NATIONAL AERONAUTICS AND SPACE ADMINISTRATION

MANNED SPACECRAFT CENTERHOUSTON, TEXAS

"' June 24, 1966

XXX

NASA PROGRAM APOLLO WORKING PAPER NO. 1203

HAZA/SSS ASSOCIATED WITH A LEM ABORT

NEA/:THE LUNAP SUP2ACE

_e_aredb_# _ 7"_._"Charles Tei'xeirR"

AST, Mission Feasibility Branch

Authorized for Distribution:

_g b_axime A. Faget /Director for Engineering _dDevelopment

i: NATIONAL AERONAUTICS AND SPACE ADMINISTRATION

_. MANNEDSPACECRAFT CENTER

.;,. H_USTON, TEXAS

,'-.,_' June 24, 1966442'

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CONTENTS

_ Section Page

:t:_-_ SUMMARY ........................... i

INTRODUCTION ...................... 1

SIMBOLS ........................... 2

DISCUSSION .......................... 4

Impact Conditions ...................... 4

Propellant Hazards ..................... 5

Stagnation Pressures .................... 9

Fragmentation ....................... 7

CONCLUDING _ ...................... 8

REFERENCES ......................... i0

APPENDIX A .......................... A-I

APPENDIX B .......................... B-I

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T.a_LES

Table Page

I KINETIC ENERGY OF DESCENT STAGE AT IMPACT ........ ll

FIGURES

Figure Page

i Nominal Descent Trajectory - Phase III .......... 12

2 Descen_ V_locity - Phase IIl ............... 13

3 Horizontal Velocity L Phase III ............ 14

: 4 Altitude Time History 15

_ Pressure, Density, and Gas Velocity Pro_i!e ....... 16

6 Stagnation Pressure versus Distance .......... 17

7 Propellant Remaining versus Time ............. 18

..... 8 Ascent-Descent Stage Separation versus Time ....... 19

1 9 Stagnation Pressure Hazard Area versus Time ....... 20

/i i0 Probability of Being Hit by a Fragment versus Distance . . 21

ii Fragment Hazard Area versus Time ............. 22

B-I "Dead Man Curves" Based on Recontact Considerations . . . B-2

B-2 Ascent-Descent Stage Separation,

2.0 Second Staging Time ................ B-3

B-3 Ascent-Descent Stage Separation,4.0 Second Staging Time ................ B-4

B-4 "Dead Man Curves" Based on Pressure Considerations .... B-5

B-5 Probability of Beir_gHit by a Fragment .......... B-6

B-6 Nominal Trajectory versus "Dead Man Curves '' • • • • • • • B-7

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,_ HAZARDS ASSOCIATED WI_H A LEM ABORT

T

_j .,_J_R THE ID'NAR SURFACE

By Charles Teixeira

• A LEM abort near the lunar surface can be extremely hazardous during

, a portion of the descent due to the propellant remaining onboard the de-i_ scent stage at staging. Subsequent impact of the descent stage with the

lunar surface can rupture the propellant tanks prossibly resulting in

propellant mixing and an explosion, k_Isuing expansion of the products

" _': of detonation can result in high stagnation pressures and the ejection

i of a considerable numbe_: of fragments.• An indication of a malfunction requiring an i,_nediate abort between

53.7 and 20.0 seconds of thrusting time until touchdown (_ 265 - lO0 ft

-: above lunar surface) will result in the ascent stage being subjected to

pressures exceeding an assumed _ psi pressure limit. An assumed proba-bility of crew safety goal of .999 (entire flight) will be violated if

an abort is performed between 65.0 and 20.0 seconds of thrusting time

: #_ until touchdown (338-100 ft) due to the high probability of being hit

i ,_ by a fragment._ This exploratory study is intended to expose the possible hazards

associated with an abort near the lunar surface. The advantages of a

fuel dump in reducing or possibly eliminating the hazards are discussed.

INTRODUCTION

An abort during the final phase of the L_'s descent can involve

hazards caused by the propellant onboard the descent stage when the

stage impacts with the lunar surface. The hazards consist prin_rily ofstagnation pressures and fragments. The purpose of the study was todefine these hazards.

The procedure employed was to determine the ascent-descent stageseparation at the time of the latte:_'s impact with the lunar surfacefor various assumed malfunction times during a "nominal descent. '" _he

_m

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separations in conjunctionwith pressure-distance relationships enableddetermination of the pressure experienced by the ascent stage as afunction of the time of malfunction. An a_sumed pressure limit estab-lished the latest time a malfunction could be detected and not result

in the assumed pressure limit being exceeded.

_he probability of being hit by a fragment was determine_ on thebasis of the ascent-descent stage separation in conjunctionwith theratio of target to background area and the number of fragments a_tici-pated.

S_O_

a = acceleration, ft/sec2

c = constant

AEFF = effective area of target (ascent stage), ft2

= area of sphere of radius H (= 4_2), ft2

g = gravitational acceleration (moon) = _.14 ft/sec2!

H = ascent-descent stage separation at descent stageimpact, ft

ih = altitude above the lunar surface, ft

•_ H = vertical (descent)velocity, ft/sec

_' = vertical acceleration, ft/sec2

h° = altitude at time of malfunction, ft

H° = vertical (descent) velocity at time of malfunction, ft/sec

HI = descent velocity at time of ascent-descent stageseparation, ft/sec

i KI = cross sectional area of target, ft2_ K2 .. perimeter of target, ft,' _ = surface area of donar, ft 2

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m = 11_.SS j S 1U.gS

N = numb_,r of fragments

_ PCL = probability of crew" loss/

_ PCS = probability of crew safety (goal)

" PEX = probability of obtaining an explosion

P_AT = probability tha_ a hit by a fragment is fatal

PHXT = probability of being hit by a fragment

PS = stagnation pressure, psi

R = range, ft

R° = range to intended landing point at time of malfunction

T = thrust, Ibs

TGO = thrueting time till touchdown, seconds

t = time, seconds

tb = ascent stage engine burn time, seconds

td = sta_ing time, time to detect failure, initiate an abortand achieve ascent-descent stage separation, seconds

tH = time from abort initiation until descent stage impact,s_conds

vH = horizontal velocity, ft/sec

vV = vertical velocity, ft/sec

W = weight _ ibs

p = density, ibs/ft3

I

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DISCUSSION

A cor,-iderable quantity of hypergolic prc_ellants (50-50 mixture

of hydrazine and unsymmetrical dimethylh_Irszine_ nitrogen tetroxide

oxidizer) will be onboard the descent stage at impact. Whether or not

the propellant is released to the vafmum environment producing hazardousconditions depends on the severity cf the stage's impact with the lunarsurface.

Impac% Conditions

_he initiation of an abort will result in thrust termination of

the descent engine and subsequf.nt separation of the ascent and des-.ent

stages, be descent stage will follow a basically parabolic trajectory

until impact with the lunar surface, be impact energy is a functionof the i_Itial conditions at staging which were determined from the

"nominal" descent profile given in figures 1 through _ (ref. i) for the

final 60 seconds (_ 305 ft) of descent. _he impact energies are givenin table I for staging times of 2.0 and _.0 seconds.*

be high impact energies involved can be expected to result in

severe structural damage to the descent stage for malfunction times of20 seconds (until touchdown) or sooner. Even if the most liberal as-

sumption were made_ namely touchdown of the de_cent stage on its landingBar, the energies involved would _e considerably above the gear's energy

absorbing capability of approximately 20 000 ft-lbs/gear (design capa-bility). (It is highly improbable that the stage will impact on itslanding gear because of the angular ra_s which are likely to 'be. inducedat separation. ) Consequently_ for the purposes of this stm_y_ it wasassumed that a malfunction requiring an abort at or before 20 second_

before touchdown (_ i00 ft) would result in sufficient structural d_ma@e

to the descent ste_e to cause rupturiu_ of the propellant tank_.

__s the time of detecting a failure,

initiating an abort and achieving assent-descent sta_e separation.

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Propellant Hazards

be exposure of the hypergolic propellants to the vacuum environ-ment will result in one of two possible events. Rapid boiling, evapora-tive cooling and freezing can occur at such a high rate that an explosivemixture cannot occur, his is particularly true if the rr,_ella_t re-

_!_,. lease is complete_ unconfined. If the propellants are re_'easedinto

_ a somewhat confined region, the evaporative cooling can suppress any:_ in_ediate reaction and allow a detonable fuel-oxidizermixture to ac-_._..._-: cumulate, be latter situation can conceivably occur since the release_,' will be caused by impact with the lunar surface and the surface together

",._,,j,_. with the descent stage structure (debris) may provide the degree ofconfinement necessary to obtain an explosive mixtur_. It must be em-'_" phasized that a definite statement cannot be made concerning the possi-"_, bility of obtaining an explosion due to the lack of data proving or

disproving the possibility even under controlled cor_itions, In ad-_!! dition, there are _e unknowns concerning the degree of confinement_ of the released pr_pellants. For purposes of this study, it was assumed

•._:._ that an explosion would occur in order to consider what is presently

I believed to be a worst case.

_, Expansion of the products of detonation (or the hypergolie pro-

pellants themselves) to the vacuum enviro_nt can result in two basichazards:

i. A gas "cloud" of high velocity and stagnation pressure

i 2. Tank and structural fra_aents as a result of the above.

Stagnation Pressures

,_ E_sion of the products of detonation will be in the form of agas "cloud". Any _x_eacted propel_ant will participate in the expan-sion in the form of crystalline r_rticles, be influence of thesecrystalline particles on pressure, density, et cetera, are not knowndue to the lack of data and aual_tical techniques enabling its effectsto be determined.

be ex_iom of th_ gas "cloud" will occur at hi_ velocities(several thouss_ ft/sr_c) and will result in sta_tio_ pressures

(p i) i e impingement on a surface normal, ps approximated by _ p uponto the flow. A surface parallel to the flow will not ex_A,ience anypressure since the pressure arises solely from bringing the particlesto rest.

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2he pressure exerted on a surface as a function of time will varydue to the changlr_ cloud density. An_ical studies (refs. 2 and 3)have le_ to general density and pressure-tlme histories as shown infigure 5. _he relatively slow rate of pressure buildup will not resultin amplification factors co.non to the loading produced by a shock wavein the atmosphere. 2herefore, the peak pressure was treated as a staticpressure and it will be used as the loading criteria in this study.pressure limits, particularly in term of the pressure profiles in ques-tion, were not available at the time of the study. _e study was per-formed rJssumir_a limit of 5 psi.

In order to determine the stagnation pressure the ascent stagewould experience, the following had to be determined:

i. _Ihe stagnation pressure as a function of distance from thecenter of expansion for propellant quantities corresponding to the as-sumed malfunction times.

2. be separation between the ascent and the descent stage at thetime of the descent sta_ 's impact with the lunar surface.

Knowledge of the ascent-descent stage separation at the _ime of thedescent stage's impact (for various malfunction tin_s) enables deter-mination of the pressure experienced by the ascent stage from f_epressure-distancerelationships_ which were obtained by using the pro-cedure discussed in reference 3. _he resulting stagnatlon pressure-distance curves are given in figure 6 for various assumed malfunctiontimes. 2he quantity of propellant available at stagin6 was determinedfrom figure 7 for the descent profile of reference i. In calculatingthe pressure-distance relationships, the propellant quantity was doubledto account for a hemispherical expansiou rather than the spherical oneassumed in reference 3.

be separation (H) between the ascent and the descent stages atthe time of impact was determined for the initial conditions (altituAeand velocity) at the time of mmlfunction for the n_ninal descent profileusing the equations in Appendix A. _ lel_aration(_[)is plotted infigure 8 for the sta6ing times of 2.0 _u_ 4.O seconds. _he sel_Lration(H) consists almost entirely of altitude _ince both the ascent and thedescent sta_s have essentially the s_e h_risontaX velocity tmtili_mact. (_he abort trajectory is a vertical thrusting o_e durln_ thetime span under consideration.) FISttre9 combines the results of fig-ures 6 and 8 and gives the pressure the ascent stage would experience

as a function of re]function tlme. k malfunction at or after TOO

*' _ = 32.6 seconds (TOO - thrnstinl_ tie until touchd=rn) viii result in

, pressures on the ascent sta@e exceeding _ l_i for a staging time of

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2.0 seconds. A staging time of 4.0 seconds will result in excessi',e

pressures for TGO = 55.7 seconds or i%,_. In arriving at these results,

it was assumed that the gas cloud reached the ascent stage i:mtantane-

ously. _his is a reasonable assumption since the arrival times are

/ generally less than .Ol sec_nds due to the high cloud velocity and the

relatively slow rate of separation of the ascent stage.

_ _e pressures that the ascent stage would experience were alsodetermined parametrically for various combinations of initial altitude

and velocity at the time of malfunction. -_aiswill enable determination

of the pressures experienced by the ascent stage for descent trajectoriesdifferent from the one considered. The results are given in Appendix B.

Fragmentation

A proper analysis of the fragmentation hazardc would require know-

ledge of th spectrum of fragments as to size, weight, d spersal pattern,et cetera. Such information is extremely diff_ cult to obtain and was

not available at the time of the study. As a result, a simplified ap-

proach was taken that based the probability of being hit by a fragmenton an analysis using target area considerations.

_he probability of being hit by a fragment _PHIT)' assuming an

abort is required and the descent stage is destroyed releasing the pro-pellants, is given by:

AS

where

, N = number of fragments

_, _ = effective target area

AS = area of sphere of radious H centered at the centerof expansion

_is sSmplified approach assumes the fra_nts are uniformly dispersedand are of the same size. _e equation is defined in more detail in

Appendix A.

_e results of the above equation are plotted in figure i0 as afunction of separation for various numbers of fra_nents. _he number

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of fragments that can be produced is the big variable in the above

equation. Work done by Dr. D. C. Gerneth suggests that a conservative

(high) estimate of the number of fragments (N) is in the order of 500.Combining the results of figure iO (N = 500) and figure 8, the proba-bility of being hit was determined as a function of malfunction time

for staging times of 2.O and 4.0 seconds for the nominal descent tra-

Jectory of reference i. _e results are given in figure ii. In order

to use figure ii, an acceptable probability of being hit must be deter-mined. _his can be accomplished in the following manner:

PCL = i - PCS = PEX X PHI_ X PFAT

assume:

PCS = .9999 (portion of flight under consideration)

PEX = .001')" / "'_'i

' = .50! PFAT

: " PCL = .0001 = .0005 PHIT

.: _ PHITallowabl e-' = . 20

....( For a staging time of 2.0 seconds, an abort between TGO = 44.0 and

•_ . _ 20.0 seconds would result in a probability of being hit by a fra_nent

i! greater than .20 (20 percent). A _taging time of 4.0 seconds resultsin a probability of a hit greater than .20 between _GO = 65.0 and

20.0 seconds. (Below 20.0 seconds the impact energies were assumed to be

insufficient to cause the propellants to be released to the environment. )

_e probability of being hit by a fragment was also determined

parametrically for various combinations of altitude and velocity. _eresults are given dm A_endix B.

CONCLUDXNGi

l' ' J' l ' {' A L_ abort during .portions of the final phase of descent can be

,, ._,< extreme,ly hazardous if the descent stage propellants are allowed to

' _''l_ 4 _t' mix and explode. An abort between 5_. 7 and aO. 0 seconds (thruating.

• "i_.' ;_,'_

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_ time to go until touchdown) will result in stagnation pressures greaterthan the assumed limit of 5 psi (_.0 second staging time). An abortbetween 65.0 and 20.0 seconds will result in a probability of being hitby a fragment of 20 percent or greater (4.0 second staging time). _hesehazardous conditions are not peculiar to the descent trajectory con-sidered but would apply reasonably well to any practical trajectory in

• the altitude region considered.

_, _e critical time spans are rather short and may not Justify elabo-_ rate remedial action. However, the hazards do exist and must be either_•_ removed or accepted. A possible means of eliminating the hazards ap-

pears to be the elimination of the descent stage propellant prior to r_I_,#_its impact on the lunar surface. A fuel dump could be initiated at r_0

abort initiationwhich would dump overboard the propellant at as high _ l_"a rate as safely possible. _he fuel dump would decrease and possibly (5_ rc_

eliminate the propellant available at impact. _he hazards caused by ,_i__i the descent stage propellants would consequently be lessened and pos-

sibly eliminated. _'_

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REFERENCES

1. Cheatham, D. C. ; a_ Price, T. G. : A Proposed L_ Descent Tr_eetoryand Fuel Budget. MBC Internal Note No. _-EG-29, Dee. 11_ l_.

2. C_emical Explosions in Space. Houston Research Institute, Feb. i, 965.

3. Hypergolic Explosion and Blast in a Vacuum_ Atlantic ResearchProposal, 196_.

4. L_ FamiliarizationManual, _ 790-1, Grunman Aircraft EmgineeringCorp., Tuly 15, 1964.

5. Lutzky, M.: Explosions in Vacuum. NOLTR 62-19_ Nov. 1962.

6. Payne, J. D.: Trajectory Profile for the LD_ Powered Descent.MSO Memo, Apr. 30, 1965.

7. Personal Co,mmnication with Dr. D. C. Gerneth.

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.1

XXX-023

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XXX-024

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XXX-025

22

"_ A-I

APP_.nVDIXA

. ASCENT - DESCENT STAGE SEPARATIONS%

,_: _he basic equations of motion for constant acceleration were used_. to derive an expression for the ascent.<lescent stage separation (H) at_' the instant of the descent stage's impact with the lunar surface.

A-2

Ascent

T = 3900 ibs

wm = g W _ i0 200 ibm (earth)

1 640 ibs (lunar)

_T= 2.13(lunar)T-W=m W'* L

T a- - 1 =- m = assumed constant

, W g (propellant experuliture

a=_=g -

, , tB = burn time (ascent stage): tB tB

f f• "" ..-." t = o, _ = _i = c,.., ._ _ = _'dt= g - dt o

i o o

.... .. _ =gT . 1 +c

,-- _ = g T I tB+ _i

• : , H = 9.14 (2.13 - i) tB + BI = 9.81 tB+ n I (AI)

,t'" ?(,.h = Jo _dt = 81 t + _ dt

h = 2.90 tB2 + _i tB + c

-.' h --2.9ot_+_l tB+h t.o,h - h1. c (A2)

..:.... ,, - . ,%_-R 1_. °

×××-028

j-

To find H:

I gtH2 (descent stageh = 0 = 5o_ " _ + h° free fall)

_o2+ 2 s °_ "_o_. tH = -- -g _ = time from abort

till impact

_, _ = tD + tB tD = staging time

tDfrom (A2)

h = 2.90 tB2 + _itB + hI

where

conditions at ignitionof ascent stage after

i 2 free fall for tD secondshI = _otD - _ gtD + ho

_ 'h =2.90t + _o _otD'_ +ho

substituting tB = _ - tD

_he initial velocity induced on the descent stageby the ascent engineat staging is not considered in the above equation.

m

XXX-029

.F '

A-4

PROBABILITY OF BEING HIT BY A FRAGMENT

_he probability of being hit by a fragment is given by.

PHIT°@S

; where

_: N = number of fragments

, AEFF = eff_.ctivetarget area

A = area of sphere of radius H, = 4_s

_e effective target area (AEFF) is determined by the target. geometry.

. .. AEFF =KI + 2K

where

•..• KI = cross sectional area of target, 144 ft2

K2 = perimeter of target, 48 ft

= surface area of the descent stage, plus t_okage _ bu!l_eadarea, _ 810 ft2

• (

: "'. = _ 11.48 +. _,'_I

.%_..::

: "2 ='_'i4'md '_ l

×××-030

J

9

4

B-I

APPENDIX B

The pressure and fragmentation hazards were determined for various

combinations of initial altitudeCho_ and vertical velocity_ ,_3 in

order to evaluate the hazards for\-/de"scenttrajectories differn_e from

:. the one given in reference i.

_; The procedure was to determine the ascent-descent stage separation

(H) at the time of the latter's impact for combinations of h and_ o o

which did not violate the so=called "dead man's" curves. _hese curves_._. specify the abort conditions which will result in recontact of the ascent

stage with either the lunar surface or the descent stage (or its debris)' when the latter impacts on the lunar surface. This area has been investi-

gated previously (e.g., ref. 4). For reference purposes, a set of the. so-called "dead man's" curves is given in figure B-I. These particular

._ curves are based on the criteria of the ascent stage being able to arrestits vertical velocity by an altitude of at least 25 feet to avoid re-

contact with the descent stage.

i The separation H for acceptable combinations of h and _ areo o

i given in figures B-2 and B-3 for staging times of 2.0 and 4.0 seconds,

• respectively. Combining these results with those of figure 6 deter-

mined the stagnation pressures experienced by the ascent stage as a

._' function of various combinations of h° and lio. The results are given

I_ in figure B-h and can be interpreted as "dead man's" curves. (In using

',_'-_ figure 6, the thrusting time to go (TGo _ was approximated by using the

initial altitude Cho) versus TC_ of the "nominal" descent trajectory_ of reference i. )•y.

The fragmentation hazards were also determined parametrically asfunctions of various combinations of h and _ by combining the re-

o o

sults of figures B-2, B-3 and figure lO. _he results are given in

figure B-_ which gives the probability of being hit by a fragment _PHIT)]

_ as a function of the initial conditions at the time of malfunction.

These results can also be interpreted as "dead man's" curves by estab-

lishing a maximum allowable PHIT"

Figure B-6 s_mnarizes the pressure and fragmentation hazards to-

gether with the nominal descent trajectory of reference i_

XXX-031

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×××-032

XXX-033

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