Document
NASA TN D-45S I
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TECHNICAL
NOTE
D-453
LANDING ENERGY DISSIPATION FOR MANNED REENTRY VEHICLES By Lloyd J. Fisher, Jr.
Lau_ey Research Center Lah_ley Field, Va.
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION
WASHINGTON September 1960
/ / _4 IJ NATIONAL AERONAUTICS AND SPACE ADMINISTRATION TECHNICAL NOTE D-453 LANDING ENERGY DISSIPATION FOR MANNED REENTRY VEHICLES By Lloyd J. Fisher, Jr.
SUMMARY L I Analytical and experimental investigations have been made to deter- mine the landing-energy-dissipation characteristics for several types of landing gear for manned reentry vehicles. The landing vehicles are considered in two categories: those having essentially vertical-descent paths, the parachute-supported vehicles, and those having essentially horizontal paths, the lifting vehicles. The energy-dissipation devices discussed are crushable materials such as foamed plastics and honeycomb for internal application in couch-support systems, yielding metal ele- ments as part of the structure of capsules or as alternates for oleos in landing-gear struts, inflatable bags, braking rockets, and shaped surfaces for water impact.
It appears feasible to readily evaluate landing-gear systems for internal or external application in hard-surface or water landings by using computational procedures and free-body landing techniques with dynamic models. The systems investigated have shown very interesting energy-dissipation characteristics over a considerable range of landing parameters. Acceptable gear can be developed along lines similar to those presented if stroke requirements and human-tolerance limits are considered.
INTRODUCTION The landing vehicles for manned reentry are considered in two categories: those having essentially vertical-descent paths, the parachute-supported vehicles, and those having essentially horizontal paths, the lifting vehicles. Because of the nature of the operation, numerous maintenance free landings are not required; consequently, one-shot landing gears having replaceable elements are particularly interesting. This paper presents some brief results from analytical and experimental investigations of energy dissipation with such landing gear in order to give a general idea of feasibility.
STATEMENT OFPROBLEM
The major variables of landing energy dissipation are velocity and
stopping distance and the quantities to be determined as far as man is
concerned are maximum acceleration, duration_ and onset rate of accelera-
tion. (See ref. i.) Possible acceleration time histories for reentry
landings are shown in figure i. For orientation purposes typical accel-
erations are shownby the broken lines. Maximum acceleration and dura-
tion are apparent on a time history but onset rate is not so obvious.
For purposes of this paper onset rate is considered as the ratio of
L
plateau acceleration to time for reaching plateau. The plateau value
i
is obtained by approximating the more complicated time histories with a
simple trapezoid as shownby the solid line.
Acceleration and onset rate determine man's tolerance to impact
and the physical relationship of these parameters showswhat compromises
can be madebetween the two for the stopping distances available. These
relationships are shown in figure 2 where maximum acceleration in g units
is plotted against stopping distance in inches. (See ref. 2.) The data
shown are at an impact velocity of 30 ft/sec, a value familiar for
parachute-supported vehicles. The lower curve is for an infinite onset
rate and the curves for onset rates of 9,000, 1,500, and 400 g/see are
from trapezoidal acceleration time histories. The dashed curve repre-
sents a linear buildup to maximum acceleration at maximum time and with
the curve for infinite onset rate forms a limit for the given conditions.
A frequently quoted tolerance for manwith load applied sternumward is
shown by the point at 40g_ I_500 g/sec_ and sbout _ inches stopping
distance. It should be realized that the stopping distances shown in
this figure are the absolute minimumfor the _iven conditions.
DISCUSSION A short motion-picture film supplement illustrating the effects discussed in this paper ha_ been prepared and is available on loan. A request card form and a description of the fi[m will be found at the end of this paper; on the page immediately preceding the abstract and index _}g_gcs.
Various energy-dissipation devices are b_ing considered for manned reentry vehic _ '_ _eceiving most attention presently are yielding metal elements _ _,_ _I' the structure of ca0sules or as alternates for oleos in landing _ _ar _truts, inflatable bags, crushable materials such as foamed plast_c_1 anJ h,_neyccmbs_ braking rockets, and shaped surfaces for water impact.
Couch Support The crushable materials are receiving most attention for internal application in couch-support systems. It is difficult to scale such materials; therefore, full-scale testing appears best. Results from drop tests using a combination of semirigid plastic and aluminum honey- comb are given in figure 3. (See ref. 2.) The drop-test model con- sisted of 4 inches of each material with a metal plate separating the two. The static loading for the test weight was I psi. Aluminum honey- comb is an efficient material for impact load alleviation since up to 80 percent of its depth is usable and there is little rebound. However, the stiffness of the material results in high onset rates of accelera- o,.I cO tion. These may be controlled by reducing the initial area of contact, o _-I by precrushing, or by combining with foamed plastic as shown here.
I Plateau acceleration was about 30g and the onset rate was about 2,500 g/sec for this combination. The initial shape of the acceleration followed the simple one-degree-of-freedom spring constant for springs in series. One of the problems inherent in work of this nature is shown by the sharp peaks in the record indicating that the test weight "virtually" bottomed before the impact velocity had been completely dissipated.
Vertical-Landing Vehicles It usually is feasible to absorb only part of the landing energy with coach or seat supports; therefore, some external absorption must be provided. The inflatable bag lends itself very well to energy absorption for the vertical-landing vehicle. Included in this category are the emergency escape pods and the reentry ballistic capsules. A number of bag shapes such as a cylinder, sphere, or torus might be used depending on design requirements. (See ref. 3.)
Torus-shaped bag.- A drawing of a model of a torus-shaped landing bag is shown in figure 4. This bag is divided into eight compartments, the partitions of which are shown by dashed lines in the plan view of the torus. The compartments are needed in cocked landings to permit pressure buildup under that part of the capsule impacti_ first. Each compartment has a blowout patch so that air can escape from the bag to regulate acceleration and prevent rebound. The patches are designed to blow out at scale pressure.
Sequence photographs of landings of the torus-bag model a_e give_ in figure 5- Figure 5(a) shows the model in a vertScal flight path.
The blowout patches (little white disks) can De sec_ Just al_er blowout in the fourth picture of the sequence. Figure 5(b) shows the model in a 63 ° flight path. An additional air bag has been used in this condition to ease turnover impact. Turnover results if horizontal velocity is too great but turnover is a secondary problem which can be solved by the same technique used for the main air bag.
Figure 6 gives full-scale accelerations for torus bag landings at several attitudes in a flight path that would result from a horizontal wind velocity of about 9 knots. The positive and negative attitudes and axes of the capsule are illustrated by the sketches with direction of flight path shown by the arrows. These data are for a vertical impact velocity of 30 ft/sec, a flight-path angle of 60 °, a vehicle weight of 1,200 pounds, and a 3.5-foot torus section diameter. The acceleration along the X-axis shows a maximum of about 30g (full scale) at a 0 ° landing with a decrease in acceleration as attitude is changed.
The acceleration along the Z-axis is zero at a 0 ° landing and increases !
in magnitude as attitude is changed. Maximum onset rate for this air O bag was about 600 g/sec.
Co rO Vertical-cylinder ba6.- A drawing of a vertlcal-cylinder landing- bag model is shown in figure 7. The air chamber upper body of the model is used to improve scaling accuracy and is not a part of an actual vehi- cle. The air bag is installed between the air chamber and a heat shield and is opened from a collapsed position by the weight of the heat shield.
There is essentially unrestricted flow between the air bag and air chamber. Orifices which are always open are located around the upper part of the bag. The bag is dimensionally relresentative of a proto- type 6 feet in diameter, 4 feet long, used wilh a 2,000-pound capsule.
An acceleration time history for this configuration in a landing on concrete at a flight-path angle of 90 ° (vertical) and a 0 ° contact attitude is given in figure 8. Computed and experimental results are in good agreement at model scale and scale up to full size as shown here. Comparisons with large-scale model tests also indicate agreement.
This bag was designed for low accelerations and results in a maximum value of about 10g. Onset rate was about 200 g/sec. Landings at other fllght-path angles and attitudes have shown similarly acceptable acceleration.
Computations of acceleration for the various systems discussed herein have been made at the 0 ° attitude, vertical flight-path condi- tion only.
Compliable metal less.- A drawing of a mo_el used to investigate energy dissipation and scaling methods for compliable-metal-leg shock absorbers is shown in figure 9. The model consisted of a steel weight attached to a wooden base by 3003-H14 aluminum-alloy legs and weighed about 30 pounds. The legs were made of rectangular strips bent as shown. This initial bend was used in an attem0t to reduce onset rate and to control the location of bending during impact. A large-scale model similar to this one and weighing about 200 pounds was also tested.
Figure lO gives an acceleration time history for the compliable- metal-leg models in landings on concrete. The small-scale-model data are shown by the long-dash--short-dash llne and the large-scale-model data by the dashed line. The agreement is very good. The computed acceleration time history shown by the solid line uses a value of yield stress about one-fourth greater than the handbook value because of work hardening during fabrication, high strain rates during impact, and plastic flow.
oJ The onset rate of acceleration for these configurations is high, about 10,000 g/sec. Other investigations using tapered legs have given !
onset rates of about 2,000 g/sec for the same design maximum accelera- tion. A photograph showing the tapered compliable metal legs installed between a capsule and heat shield is shown as figure ll. The tapered legs are prebent into a modified "S" curve. Typical sequence photo- graphs of a landing on concrete are given in figure 12.
Water impact.- Another method for landing a space capsule is the water landing. (The configurations discussed previously have lower accel- erations in water landings than in hard-surface landings.) The water landing provides a way of obtaining long stroke without onboard devices, that is, by shaping the vehicle for water penetration. It is also of interest where accuracy for hitting a landing area is a question. Peak impact accelerations for two capsules landing on water are given in figure 13. (See refs. 4 and 5.) One capsule impacts on a 126-inch spherical radius heat shield and the other on a 53 ° conical heat shield.
Both model and full-scale experimental investigations were made for the 126-inch spherical radius model. The model results are shown by the solid line and full-scale results by the dashed line. Experimental model results for the 53 ° conical model are shown by the lower solid line.
Computed results for both configurations, by using well established procedures, are shown by the circles. Excellent agreement is indicated between model, full scale, and computation. The curves sho_ that con- figuration shape has a significant influence on landing acceleration.
At 0 ° contact attitude the 126-inch spherical radius model has a blunt shape, water penetration is small, and the peak acceleration is high.
Onset rate is about 20,000 g/sec. As contact attitude changes from 0 °, the accelerations decrease rapidly and onset rate also decreases to about 800 g/sec at a 30 ° contact attitude. The difference is due to the more pointed shape of the impacting surface as attitude increases.
The 53 ° conical model has fairly low accelerations, about 10g, and shows little change with change in attitude. Onset rate is about 400 g/sec. The accelerations in the 10g region for both models result from fairly deep water penetration or in other words relatively long strokes.
p anding is the braking rocket. Fnis system employs rocket thru-s9%L.:_._._._._ .__ _ f_._i_9.i_O_:)_]h_K_s _ gg_D_eC_n_;i_ man enviir__ngt_<_,._e:,,_i _i_ a@,9_e_o_.j_@._.__ ._$ _ _Le _akiag:.._,x s rocke%_ _%]_]je ,___r_.._k_ rgt_,&._o_uc_ssi_ £ _iq_lr<.,,_b acceleration oz- aoddt 3g occurred in these te:_ts. A change inc,l%n41ng,_Lq condition such as an increase in speed would result in higher accelera- L tlons _i___,_w_l_m__ _m_,_ _}_st_ _ _._9_i_ ._n _a_oi_;iThe (1 test'.,_ _ __ _D_ _d o _ _ .... ui_ - _ _.._, ' e £,. ' e 0 , _" o,-2 i .
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Each of the s rstems discussed h_s special advantages_ as an example the cgmpliab$_._t_l, e n_ • -b . o La. 4 ,ble-_o s _en_" • men_s _nan certain _ner t s el 1 _ sn°c_-_Rsorp,@r S_ml}_, ._,_ aBPliqat_U q_ .c- e.' . - rine': . - ayn zc mo.qe_.o .an i t_ -. a r n _ i "' .... " ,, metal s_r_s,,._r_ *i_c_'_ate_ In_-t_ gear/._ t_,a .. e • fo i ("- _',,,:<f_[_',,,',:."_, tr::s _:3.l-._:_,u"r__EoL,:<! e,,.d!' .[gf.<m a;)ik, n"{ l>:5.-":c,;i,i:: S:_n,_-<_ :i z{rst, t_- e _o _ tr S d0,w_, e, _ i,: t - , , -flj, O J_.,_,J "',,'O_{_ _<,vqlJ'_ O/fl _ ._OiJi3_uq_o9 _)ll_ tgiz_OS f.-[_J']. ,it;_J:H] ;Ig_3_;,tiWt ccezeyaz Qns 6 e - _ n _. 1_ t ' runway -an zn9 O _ _ i * - ......... r._ Z ose-_es_ sno ;_ 8 ru__ _ % " t, • c_ am R u e a r eac r akcce ra r _ _ ....
ceier zzons, z t ,_ ' o-• , m, y e±a s_ress oz_ )'one ,_rN._ ma . - , , o..... , r o r/na_ , s_r e _ f ts , • n _ne s c . e a _m _v u Rf , , - z t iun h_ds" up _g_e ,ecause_ gear oo,,omed. _°imum landi_c_9_e_9_ ' tlon always occurred at nose-gear impact and was about 5g- Moving the nose wheel aft or replacing the wheel with the nose skid had little effect on acceleration or stability.
CONCLUDING REMARKS It appears feasible to readily evaluate landing-gear systems for internal or external application in hard-surface or water landings by computation methods and free-body landing techniques with dynamic models.
L The systems investigated have shown very interesting landing-energy- i dissipation characteristics over a considerable range of landing param- eters. Acceptable gear could be developed along lines similar to those presented if stroke requirements and human-tolerance limits are considered.
Langley Research Center, National Aeronautics and Space Administration, Langley Field, Va., April 12, 1960.
REFERENCES 1. Eiband_ A. Martin: Human Tolerance to Rapidly Applied Accelerations: A Summary of the Literature. NASA MEMO 5-19-59E, 1959.
2. O'Bryan_ Thomas C._ and Hatch_ Howard G., Jr.: Limited Investiga- tion of Crushable Structures for Acceleration Protection of Occu- pants of Vehicles at Low Impact Speeds. NASA TN D-158, 1959.
3. Esgar3 Jack B. 3 and Morgan_ William C.: Analytical Study of Soft Landings on Gas-Filled Bags. NASA TR R-75, 1960.
4. Vaughan 3 Victor L., Jr.: Water-Landing Impact Accelerations for Three Models of Reentry Capsules. NASA TN D-145, 1959.
9. McGehee_ John R._ Hathawayj Melvin E. 3 and Vaughanj Victor L., Jr.: Water-Landing Characteristics of a Reentry Capsule. NASA MEMO 5-23-59L, 1959.
TYPICAL ACCELERATION TIME HISTORIES ....... TYPICAL ACCELERATIONS / \ __ PLATEAU • ACCELERATION_ ACCELERATION, t_ I g UNITS Po /// ONSET RATE= QPLATEAU \ to tl TIME, SEC Figure i VARIATION OF MAXIMUM ACCELERATION WCH STOPPING DISTANCE IMPACT VELOCITY =30 FT/.CEC 120 ONSET RATE, g/SFC '°°F '_-_9,ooo ACCELERATION, MAXIMUM 801 II ",_ \ g UNITS
/ \\ "-,.;,,soo
/ \\ _Fh..J- FREQUENTLY QUOTED 4°r kk T ''" HUMAN TOLERANCE I "" - 400 J i J I J -j 0 4 8 12 16 20 24 STOPPING DISTANCE, IN.
Figure 2 23" C_ ACCELERATION TIME HISTORY FOR COMBINATION OF ALUMINUM HONEYCOMB AND FOAMED PLASTIC VERTICAL IMPACT VELOCITY, 30 FT/SEC ; STATIC LOADING, I PSI LEAD WEIGHT -f-- m_rl=B_6_ ____- SEMIRIGID PLASTIC 60- 4 'N.__F_ *,'_ METAL PLATE "_ I[_ ALUMINUM HONEYCOMB ACCELERATION, oJ CJUNITS 20 ,-I !
-20 I I I I 1 L 0 02 D4 .06 TIME, SEC Figure 3 TORUS-SHAPED LANDING-BAG MODEL PLAN VIEW OF COMPARTMENTED TORUS BLOWOUT PATCHES --_ -_....__ TORUS BAG Figure 4 LO SEQUENCE OF TORUS BAG LANDINGS ON CONCRETE FLIGHT-PATH ANGLE, 90 ° ; CONTACT ATTITUDE, 0 ° 2 3 4 [X c I r- co po 5 6 7 L-59-1501 Figure 5(a) SEQUENCE OF TORUS BAG LANDINGS ON CONCRETE FLIGHT-PATH ANGLE, 63 ° ; CONTACT ATTITUDE, -26 ° t 2 3 L-59-6105 Figure 5 (b) ACCELERATIONS FOR TORUS AIR BAG LANDINGS ON CONCRETE VERTICAL IMPACT VELOCITY, 30 FT/SEC; FLIGHT-PATH ANGLE, 60°; WEIGHT, 1,2OO LB; TORUS SECTION DIAMETER, 3.5 FT NEGATIVE ATTITUDE \ --/-_,X ATTITUDE POSITIVE
,o 7-
II Ill,Ill Ill/liar _'__ACC ELERATION ok1 PEAK cO ALONG X-AXIS O ACCELERATION, ,-4 I g UNITS ACCELERATION ALONG Z-AXIS I I I -20 -40 -20 0 20 CONTACT ATTITUDE, DEG Figure 6 VERTICAL-CYLINDER LANDING-BAG MODEL AIR CHAMBER 3ES AIR HEAT SHIELD Figure 7 b ACCELERATION TIME HISTORY FOR VERTICAL-CYLINDER BAG LANDING ON CONCRETE IMPACT VELOCITY, 30 FT/SEC; FLIGHT-PATH ANGLE, 90 ° ; CONTACT ATTITUDE, O; WEIGHT, 2,000 LB 12- ACCELERATION ALONG X-AXIS, - _COMPUTED 9 UNITS !
(FROM MODEL) I I I I 0 .04 .08 .12 .16 TIME, SEC Figure 8 SMALL-SCALE COMPLIABLE-METAL-LEG MODEL ACCELEROMETER -_ _" SIEEL PLATE f I:_"--. _<'/:_!_ I.I IN.-_ _- / _----ALUMINUM LEG / '_- WOOD BASE Figure 9 ACCELERATION TIME HISTORY FOR COMPLIABLE-METAL-LEG MODEL LANDINGS ON CONCRETE IMPACT VELOCITY (FULL SCALE), 3(3 FT/SEC FLIGHT-PATH ANGLE, 90 ° :, CONTACT ATTITUDE, 0 ACCELERATION _T.._._.._. PUTED 2O ALONG X-AXIS, g UNITS - - ..,,- LARGE- SCALE OJ -- _ MODEL r--I I0 I \_ SMALL-SCALE _ MODEL I I I 0 .02 .04 .06 .08 TIME (FULL SCALE), SEC Figure i0 DYNAMIC MODEL WITH COMPLIABLE METAL LEGS BETWEEN CAPSULE AND HEAT SHIELD L-60-2464 Figure ii t4 SEQUENCE OF COMPLIABLE-METAL-LEG MODEL LANDING ON CONCRETE FLIGHT-PATH ANGLE, 90°; CONTACT ATTITUDE, 30 ° 2 3 I rO 5 6 L-59-6203 Figure 12 ACCELERATIONS FOR CAPSULES LANDING ON WATER IMPACT VELOCITY (FULL SCALE), 30 FT/SEC; FLiGHT-PATH ANGLE, 90% WEIGHT (FULL SCALE), 2,000 LB 50- 0 COMPUTED ..... FULL SCALE MODEL 4O IN R _"'_ PEAK ALONG X-AXIS, g UNITS I0 I I J L | I I I 4O -40 -30 -20 -I0 0 I0 20 30 CONTACT ATTITUDE, DEG Figure 13 :]-5 SEQUENCE OF BRAKING-ROCKET MODEL LANDING ON CONCRETE FLIGHT-PATH ANGLE,60 ° 1 2 L-60-1899 Figure 14 DYNAMIC MODEL OF HORIZONTAL-LANDING REENTRY VEHICLE L-59-6533 Figure 15 SEQUENGE OF MODEL LANDING WITH MAIN-SKID NOSE-WHEEL LANDING GEAR I 2 L-60-1720 Figure 16 LANDING TIME HISTORY TUDE
2°y
ATTITUDE, ^ /-/ATTI I I IO r I _,_-.m I /-- ACCELERATtONS I/i,._,,_ AT NOSE GEAR
o[ ....I .... • --__-_
I t ACCELERATION, I I g UNITS I I "* "T ,,"ACCELERATIONS 5r- I '_L'='-'a;r _/AT MAIN GEAR 0 I ......
.I 0 .I .2 .3, .4 .5 .6 TIME, SEC Figure 17 NASA- Langley Field, Va. L-I082