Ablation sensor for measuring char layer recession rate using electric wire

Le Bel, P. J.

English

NASA Technical Reports Server

REPLY TO 
ATTN OF: GP 
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION 
WASHINGTON, D.C. 20546 
March 2 7 ,  1971 
TO : USI/Scientific & Technical Information Division 
Attention: Miss Winnie M. Morgan 
FROM : GP/Office of Assistant General 
Counsel for Patent Matters 
SUBJECT: Announcement of NASA-Owned 
U,S. Patents in STAR 
In accordance with the procedures contained in the Code GP 
to Code USI memorandum on this subject, dated June 8 ,  1970, 
the attached NASA-owned U . S .  patent is being forwarded for 
abstracting and announcement in NASA STAR. 
The following information is provided: 
U . S ,  Patent No. : 3,357,237 
Corporate Source z Langley Research Center 
Supplementary 
Corporate Source 
NASA Patent Case No.: XLA-0174 
Gayle Parker a 
NASA-HQ 
https://ntrs.nasa.gov/search.jsp?R=19710012111 2020-03-17T02:49:19+00:00Z
Dec. 12, 1967 3,352, 
\ 
k ; 
BLATION SENSOR 
Filed J u i ~  + 
I 
I- -
INVENTOR 
FIG. 2 PETER J. LEBEL 
BY 
3,357,237 United States Patent Office patented 12, 1g67 
1 2 
The ablation sensor that constitutes the present inven- 
tion consists essentially of two wires embedded in the 
ablation material. One end of each of these wires is em- 
bedded in the material at the same predetermined level. 
The other ends of these two wires protrude out of the 
ablation material. When the char layer interface recedes to 
Filed June 17,1965, $ere No. 464,880 the said predetermined level, the char layer electrically 
PO Claims. (Cl. 73-86) connects the two embedded ends of the wires together. 
The invention described herein may be manufactured An electrical circuit means is connected to the other ends 
and used by or for the Government of the United States of the two Wires to sense when the embedded ends are 
of America for governmental purposes without the pay- lo connected together. A series of these sensors stepped in 
ment of any royalties thereon or therefor. depth in the ablation material provides means for meas- 
This invention relates generally to an ablation sensor uring the rate of recession of the char layer interface. 
and more specifically concerns an ablation sensor for Other objects and advantages of the invention will fur- 
measuring the char layer interface recession rate of a ther become apparent hereinafter and in the drawings, 
charring ablator. in which: 
In developing ablation heat shield materials for space FIG. 1 is a schematic drawing of the invention; and 
flights, it is necessary to make certain measurements of FIG. 2 is a schematic diagram of the electrical circuit 
the thermal characteristics of the ablation materials. means 18 shown in FIG. 1. 
Some of these measurements include rate of recession of In describing the preferred embodiment of the inven- 
the char layer interface and rate of recession of the ablat- 2o tion selected for illustration in the drawings, the number 
ing surfaces of the ablation materials. The char layer be resorted to for the sake of clarity. However, it is not 
interface is the intersurface of a char layer that its imme- intended to be limited to the specific terms so selected and 
diately below the ablating surface of a charring ablator. it is to be understood that each supecific term includes 
In the past, there have been no devices for measuring the 25 technical equivalents which operate in a similar manner to 
rate of recession of the char layer interface. However, accomplish a similar purpose. 
there are several previous ablation sensors that are used Turning now to the specific embodiment of the inven- 
for measuring the rate of recession of the ablating surface. tion selected for illustration in the drawings, the number 
Two of these previous sensors are the breakwire sensor 11 designates generally a piece of charring ablation ma- 
and the variable capacitance sensor. The breakwire abla- 3o terial whose char layer interface recession rate is to be 
tion sensor utilizes a series of fine wires stepped in depth measured. A surface 12 is the ablating surface of the 
in the ablation material. When the material surface re- ablation material. Since ablation material 11 is a charring 
cedes to the level of the first wire, the wire burns thereby ablation material, a char layer interface will always 
interrupting its continuity. As the surface ablates to the precede the ablating surface 12. The rate of recession of 
levels of the successive wires, similar occurrences result 35 the char layer interface is what is to be measured by this 
whereby means are provided for making ablation rate invention. Opposite surface 12 there are several holes 
measurements. The disadvantages of the breakwire abla- 139 13b, 13c, and 13d drilled into the ablation material 
tion sensor are twofold: the sensor output is affected by 11. A sensor plug 14 is inserted into each of these holes 
the layer of plasma formed around a spacecraft during 13: sensor plugs 14a, 14b, 14c, and 14d are inserted into 
its reentry into the earth’s atmosphere which shorts the 40 holes 13a, 13b, 13c, ;tnd 13d, respectively. A sensor plug 
wires with a continually varying resistance, and the sensor 14 consists of a plug ‘of ablation material into which a 
is not usable in charring ablators because of the shorten- wire 15 and a wire 16 are embedded. One end of wire 
ing effect of the electrically conductive char layer. 15 and one end of wire 16 are embedded in the ablation 
The variable capzcitance ablation sensor is a concen- material at the same predetermined level. The other ends 
tric cylinder capacitor oriented so that the axis of the 45 of wires 15 and 16 protrude out of sensor plug 14. Wires 
cylinder is perpendicular to the ablation surface. As the 15 and 16 are ten mil platinum wires. The embedded ends 
material surface recedes, the height of the concentric cyl- of wires 15 and 14 have thirty mil peens 17 to provide 
inder decreases at the same rate thereby changing the larger surface area for good contact with the char layer 
area of the capacitor plate and thus varying the capaci- interface. Platinum wire was used because it does not form 
tance. This ablation sensor also has two distinct disad- 50 an oxide which will insulate the wires from the char as 
vantages: it sometimes produces erroneous readings be- most metals do when heated. Each pair of wires 15 and 
cause its capacitor plates become shorted by the layer 16 is connected electrically to a corresponding electrical 
of plasmas formed around a spacecraft during reentry into circuit means 18 that produces a voltage output when 
the earth’s atmosphere, and it cannot be used in a char- the embedded ends of wires 15 and 16 are connected to- 
ring ablator because of the shorting effect of the char 55 gether by the char layer. This voltage produced by electri- 
layer. cal circuit means 18 is applied to a telemeter 19 where 
It is therefore an abject of this invention to provide it is transmitted back to earth. 
an ablation sensor that can be used to accurately and It should be noted that each sensor plug 14 has only 
reliably measure the rate of recession of the char layer one pair of wires 15 and 16 embedded in it. However, in 
interface of a charring ablation material. 6o actual practice, there are many such pairs of wires 15 
A further object of this invention is to  provide an abla- and 16 embedded in each sensor plug 14. Each of these 
tion sensor that will sense when the char layer interface pairs of wires is embedded at a different predetermined 
of a charring ablation material has recessed to a predeter- level and each pair of wires is connected to a different 
mined layer. electrical circuit means 18. There are several ways in 
Another object of this invention is to provide an abla- 65 which the wires 15 and 16 can be embedded in the ablation 
tion sensor for measuring the rate of recession of the material of each sensor plug 14. One way is to drill pairs 
char layer interface of a charring ablation material during of holes in the plug to different predetermined levels, in- 
reentry into the earth’s atmosphere that is not affected by sert the wires and peens into the holes, and then fill up 
the layer of plasma surrounding the reentry body. any voids in the holes with an epoxy resin. Another way 
Still another object of this invention is to provide an is to use a soft ablation material for the sensor plug 14, 
ablation sensor that utilizes the char layer of an ablating and slice the plug into two or more sections, place the 
charring surface. wires between the different sections, and then compress 
3,357,237 
ABLATION SENSOR 
peter J. &e Bel, Newport News, vas, assignor to the 
United States of America as represented by the Admin. 
istratsr of the National Aeronautics and Space Ad- 
ministration 
3,357,237 
3 
the sections together to  embed the wires in the sensor 
Plug. 
Referring now to FIG. 2, one of the electrical circuit 
means 18 in FIG. 1 will be described in detail. A sensor 
plug 14 is Connected in series with a resistor 21 and a 
resistor 22. This series circuit is connected in parallel 
with a voltage source 25 having its negative terminal 
grounded. Another series circuit consisting of a switch 
24, a resistor 25 and a resistor 26 is also connected in 
parallel with voltage source 23. Switch 24 is a three-tcr- 
minal PNPN semiconductor. Its construction is similar to 
a PNPN diode except an ohmic connection is made to its 
isolated P region. The insertion of a pulse of current into 
this region causes the device to switch from its high 
impedance “OF’ state to a very low impedance “on” state, 
thus allowing control of current, much as in a thyratron, 
The connection 27 to switch 24 is the connection made 
to its isolated P region. Once the switch 24 is actuated to 
its “on” state by a pulse applied to connection 27, the 
switch will not return to its “off” state. A PNP transistor 
28 is connected in series with a resistor 29 across voltage 
source 23. The base of transistor 28 is connected to the 
junction of resistors 21 and 22. A capacitor 30 is con- 
nected in parallel with resistor 22. The collector of tran- 
sistor 28 is connected through a diode 31 to the connec- 
tion 27 of switch 24 which is connected to ground through 
a resistor 32. The output of the circuit in FIG. 2 is at a 
terminal 33 across resistor 26. Resistors 21 and 22 form 
a voltage divider that applies voltage to  the base of 
transistor 28 when the two wires in the sensor plug 14 
are connected together by the char layer interface. Re- 
sistor 21 is not absolutely necessary, but it is desirable 
since it limits the current in the circuit. The resistor 32 is 
a bias resistor for switch 24 and the resistor 25 is a re- 
sistor to drop the voltage output at terminal 33 to a de- 
sired level. As has been mentioned previously, there will 
be several pairs of wires in each sensor plug 14. Conse- 
quently, there is a large amount of capacitance linking 
these different circuits together. Because of this capaci- 
tance when one of the circuits 18 is conducting there is 
a great likelihood that it might cause another circuit 18 
to conduct prematurely. This is the purpose of capacitor 
30. It is to counteract the capacitance between the pairs 
of wires in the sensor plug 14. With capacitor 30 the 
different electrical circuit means 18 are less likely to be 
actuated prematurely. 
In operation, before the char layer interface reaches 
the embedded ends of the two wires of a sensor the 
switch 24 will not be conducting and the potential at ter- 
minal 33 will be at ground. As soon as the char layer 
interface reaches the two ends of the two embedded wires 
of the sensor the char layer will electrically connect the 
two ends and connect resistors 21 and 22 across voltage 
source 23. The resulting voltage drop across resistor 22 
causes the base of transistor 28 to become negative with 
respect to the emitter of the transistor thereby causing 
it to conduct. This conduction causes a positive potential 
to be produced across resistor 29 which is applied through 
diode 31 to connection 27 of switch 24 causing it to be 
actuated to its “on” state. This causes current to flow 
through resistors 25 and 26 which produces a positive 
potential at terminal 33. The potential at terminal 33 is 
applied to the telemeter 19. Inasmuch as there are many 
of the sensors 14 at different predetermined levels in 
the material 11, the rate of recession of the char layer 
interface can be determined. Further changes in sensor 
resistance due to the plasma will not affect the output at 
terminal 33 since switch 24 remains in its “on” state. 
The advantages of this invention are that it provides 
a means for measuring the rate of recession of the char 
layer interface of a charring ablation material; it is not 
#affected by the plasma formed around a reentry space- 
craft; it makes use of the electrical shorting effect of the 
char which impairs the operation of previous sensors; it 
has a small thermal mass which has no effect on the 
4 
physical properties of the ablation material; and it has 
no moving parts, is extremely simple and highly reliable 
and will withstand the severe environment associated 
with spacecraft. 
It is to be understood that the form of the invention 
herewith shown and described is to be taken as a pre- 
ferred embodiment. Various changes may be made in 
the shape, size and arrangement of parts. For example, 
equivalent elements may be substituted for those illus- 
trated and described herein, parts may be reversed, and 
certain features of the invention rmay be utilized inde- 
pendently of the use of other fe’atures, all without de- 
parting from the spirit or scope of the invention as de- 
fined in the following claims. 
5 
15 What is claimed is: 
1. An ablation sensor for sensing when the char layer 
interface of a charring ablation material has recessed to 
a predetermined level csmprising: a first wire embedded 
in said ablation material such that its first end is at said 
20 predetermined level and such that its second end is pro- 
truding out of said ablation material; a second wire em- 
bedded in said ablation material such that its first end is 
at said predetermined level close to the said first end of 
said first wire and such that its second end is protruding 
25 out of said ablation material; and electrical circuit means 
connected to the said second ends of said first and second 
wires for detecting when the char layer interface electri- 
cally connects together the said first ends of said first and 
second wires thereby indicating that the char layer inter- 
30 face has recessed to said predetermined level. 
2. An ablation sensor in accordance with claim 1 
wherein the first ends of said first and second wires have 
peens to provide laTger surface area for better contact 
with the char layer. 
3. A ablation sensor for sensing when the char layer 
interface of a charring ablation material has recessed to  a 
predetermined level comprising: a first wire embedded in 
said ablation material such that its first end is at said 
predetermined level and such that its second end is pro- 
40 truding out of said ablation material; a second wire em- 
bedded in said ablation material such that its first end is 
at said predetermined level close to the first end of said 
first wire and such that its second end is protruding out 
of said ablation material; a voltage source, a first resistor 
45 and a second resistor connected in series across the sec- 
ond ends of said first and second wires; a switch and a 
third resistor connected in series across said voltage 
source; and electrical circuit means coMected between 
the junction of said first and second resistors and said 
50 switch to actuate said switch when said char layer inter- 
face electrically connects the first ends of said first and 
second wires together whereby a voltage is produced 
across said third resistor thereby indicating that said char 
layer interface has receded to said predetermined level. 
4. An ablation sensor in accordance with claim 3 
wherein said switch remains in its actuated state once it 
has been actuated thereby maintaining a voltage across 
said third resistor. 
5. An ablation sensor in accordance with claim 3 
6. An ablation sensor in accordance with claim 3 
wherein said electrical circuit means includes a PNP tran- 
sistor and a resistor connected in series across said volt- 
age source. 
7. A plurality of ablation sensors for determining the 
rate of recession of the char layer interface of a charring 
ablation material comprising: a plurality of pairs of wires 
embedded in said ablation material such that the first 
ends of each pair of wires is located at a dfierent pre- 
determined level and such that the second ends of said 
pairs of wires protrude out of said ablation material; and 
electrical circuit means for each of said pairs of wires 
connected across the second end of each of said pairs of 
75 wires to detect when the char layer interface electrically 
35 
55 
60 wherein said switch is a PNPN switch. 
65 
3,357,237 
5 6 
connects the first ends of said pairs of wires whereby the receded to the predetermined level of the first ends of 
rate of recession of the char layer interface can be de- the pair of wires. 
termined. 10. A plurality of ablation sensors in accordance with 
8. A plurality of ablation sensors in accordance with claim 9 wherein said switch is a switch that remains in 
claim 7 wherein the first ends of said pairs of wires have 6 its actuated state once it has been actuated thereby main- 
peens to provide larger surface area for better contact taining a voltage across said third resistor. 
with the char layer. 
9. A plurality of ablation sensors in accordance with References Cited 
claim 7 wherein said electrical circuit means comprises a UNITED STATES PATENTS 
211963 Weaver _ _ _ _ _ _ _ _ _ _ _ _ _ _  73-86 first resistor, a second resistor and the second ends of a 10 3,078,707 pair of wires connected in series across a voltage source, 3,201,973 8/1965 Fitzgera,d et al. _ _ _ _ _ _ _  73-35 a switch and a third resistor connected in series across 3,267,721 8/1966 Jacobs et al. _ _ _ _ _ _ _ _ _ _  73-35 said voltage source. and means for actuating said switch 
when a voltage drop occurs at the junction-of said first 
and second resistors whereby a voltage occurs across said 15 
third resistor indicating that the char layer interface has 
JAMES J. GILL, prima,.,, ~ ~ ~ ~ i , , ~ ~ .  
SALZMAN, Assistant Examiaer. 


Ablation sensor Patent

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