Spacecraft Heat Shield Testing Method

Abstract

A process for measuring the service life of heat shields on re-entry spacecraft after each flight is disclosed. A low electron-energy level, beta emitting, isotope, having a reasonably long half-life, preferably promethium-147, is uniformly dispersed throughout a refractory heat shield coating. The beta particle emission level at the coating surface is measured to provide a base reading. After each space flight, the beta emission level at the coating surface is again measured, such as by autoradiography or with a Geiger-Muller counter. Erosion, abrasive wear, spalling or other damage is detected and measured to determine whether additional flights can be made without re-coating.

Description

BACKGROUND OF THE INVENTION

Re-entry spacecraft generally are provided with a heat shield to protect against thermal and abrasive effects of re-entering the earth's atmosphere. Typical heat shields employ relatively thick organic ablative materials or thin radiative materials which may employ coatings.

Where a spacecraft employing a radiative thermal protection system is designed for a single trip, it is merely necessary that the coating have sufficient thickness and integrity to survive one re-entry. However, where it is intended that the spacecraft be used for multiple trips into space, as with the "Space Shuttle," it is necessary that either the heat shield be refurbished after every trip or that it have sufficient service life (e.g. thickness, strength and structural integrity) to survive the intended number of re-entry flights.

Re-coating the heat shield is difficult and expensive. Removal of remnants of the previous coating is difficult, since the coating adheres tenaciously and is generally highly resistant to chemical or abrasive removal techniques. Merely applying a new full coating over the remnants of the prior coating is unsatisfactory because of the adhesion problems, increase in weight in some areas after a number of flights, and the possibility of hiding structural defects in underlying coating layers.

Thus, it is preferred that the initial coating have sufficient service life for a number of flights. However, it is essential that the coating be examined and measured after every flight to assure that no defects have developed and that sufficient thickness remains in all areas.

Attempts have been made to monitor recession of relatively thick ablative heat shields or missile nose cones by inserting radioactive plugs into holes in the shield, then measuring the decrease in radioactive emission due to shield losses. A typical system of this sort is described in U.S. Pat. No. 3,461,289. However, these systems merely measure losses at one location and are incapable of detecting varying wear patterns across a large heat shield surface. Also, these systems are incapable of detecting dangerous surface defects, such as cracks, which may develop over a series of space flights. While these systems provide useful experimental information, they do not provide the required measurement of the entire heat shield surface which is necessary if the heat shield is to be used for a series of space flights. Furthermore, these systems generally use isotopes which emit gamma radiation. Gamma radiation is difficult to shield and is dangerous to ground personnel working on or around the spacecraft between flights.

Heretofore, non-destructive techniques for measuring the thickness and quality of a thin coating have not been sufficiently accurate or rapid to meet the requirements of the reusable heat shield. It is essential that all defects, such as cracks and spalling be detected and that the thickness of the entire coating area be rapidly and accurately measured.

Thus, there is a continuing need for improved methods of non-destructive testing which will provide rapid determination of heat shield reusability.

SUMMARY OF THE INVENTION

It is, therefore, an object of this invention to provide a heat shield measuring method overcoming the above noted problems.

Another object of this invention is to provide a method of rapidly and accurately measuring heat shield coating thickness and detecting coating defects.

Still another object of this invention is to provide a method of measuring heat shield characteristics which may be quickly performed over a large heat shield area without hazard to personnel in the area.

The above objects, and others, are accomplished in accordance with this invention by incorporating into the heat shield coating a low-electron energy level, beta radiation emitting radioisotope, having a half-life of at least one year. After the heat shield coating is applied, measurements of beta radiation emitted by the shield surface are taken by any suitable technique, such as autoradiography or measurement with a Geiger-Muller counter. After each space flight, the radiation level at the heat shield surface is again measured. After allowing for natural decay of the radioisotope over the time period since the previous measurement, any decrease in emission levels is indicative of decreased shield thickness. In addition, autoradiographic examination will reveal coating irregularities, and defects such as surface cracks.

BRIEF DESCRIPTION OF THE DRAWING

The basic requirements of the method of this invention will be further understood upon reference to the drawing, in which:

FIG. 1 is a flow chart detailing the steps performed in the novel method; and

FIG. 2 is an illustration of the loss of radio activity with decreasing coating thickness.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to FIG. 1, there is seen a flow chart illustrating the basic steps in the method of this invention. As shown, initially a ceramic coating slurry is prepared as shown in box 11 and mixed (box 12) with a dispersion (box 10) of the selected radioisotope in an appropriate carrier. It is highly desirable that this mixing produce a very uniform dispersion of the isotope throughout the coating slurry. Any suitable mixing technique may be used.

The heat shield substrate is then uniformly coated with the slurry as indicated by box 13. While any suitable coating technique may be used, spraying or dipping is typical depending on shield geometry. After drying, the coating is preferably fired at about 1,500.degree.C. Next, a base reading of the beta emission levels over the heat shield surface is taken as indicated in box 14. This may include a scan of the entire surface with a Geiger-Muller counter and/or autoradiographic examination of the surface. In addition to establishing base emission levels, these measurements will provide process controls in that they will show original coating thickness, uniformity, freedom from surface defects, etc.

After exposure to the service environment (box 15), which in the case of a spacecraft heat shield would be reentry into the atmosphere, the measurements of beta emission at the shield surface are repeated, as indicated in box 16. The re-measurement will show any loss in coating thickness and will disclose any defects, such as spalling or surface cracking, in the coating. Then, the steps of boxes 15 and 16 may be repeated a number of times, until the service life of the coating has been completed.

FIG. 2 shows a typical curve of loss of radioactivity plotted against loss of coating thickness. As can be seen, in general the decrease in emission counted is approximately logarithmic with respect to decrease in coating thickness. Desirably, tests (of the sort described in Example I) are made of a particular combination of isotope, carrier, coating and substrate with mechanical abrasion of the surface. Once the curve is prepared from the tests, remaining heat shield thickness after exposure to the service environment (e.g. heat shield atmosphere re-entry) may be determined from measured emission levels. Loss of radioactivity is not directly proportional to loss in thickness because of such effects as diffusion of the isotope into the substrate. The curve shown in FIG. 2 is corrected for these variables.

The heat shield coating may comprise any suitable material formed on any suitable substrate. Typical heat shields comprise a thin inorganic refractory ceramic coating over a suitable substrate. Best results have been obtained with layers of disilicides and subsilicides. These are primarily MSi.sub.2 and M.sub.5 Si.sub.3, where M is the combination of the refractory base alloy and various additions to the silicon. During high temperature service as a heat shield during reentry, disilicides are slowly converted to lower order subsilicides as the refractory alloy substrate diffuses outward, while stable silicon dioxide is formed in the coating through the infusion of oxygen. If desired, modifier elements may be included in the coating to form other stable oxides and improve oxidation properties.

While theoretically the life of the coating depends on the rate at which stable SiO.sub.2 is formed, the coating does not resist oxidation because of thermo-chemical preference for a reduced state. For satisfactory service, the most significant characteristic is the formation of an adherent oxide film on the surface of the coating which either prevents or reduces the infusion of oxygen. Coating life depends on a number of conditions and variables, among which are coating thicknesses and composition, oxidation temperature, and pressure.

The heat shield coating may have any suitable thickness. In general, thicknesses of up to about 250 microns are useful. Thicknesses in the range of 75 to 100 micrometers are preferred. The energy of the beta emitter is selected to provide a range beyond the nominal coating thickness. Accordingly, the beta particle count at the surface of the coating is proportional to the thickness of the coating. The scattering of the beta particles in the coating provides a technique for obscuring subsurface defects while providing a very strong signal from critical defects in the surface of the coating.

While these coating materials may be coated onto any suitable heat shield substrate, high temperature resistant materials including carbon composites and refractory alloys of columbium and tantalum are preferred.

Preferably, the radioisotope dispersed in the coating material is a radioisotope which emits beta radiation at low electron-energy levels, and has a half-life of at least one year. Ideally, the isotope should not emit gamma radiation because of the danger to personnel maintaining the shielded spacecraft, difficulties in providing proper shielding and difficulties in separating signals from opposite sides of the shield. Beta radiation at low energy levels is generally harmless to personnel working in the area of the shielding, and can easily be shielded with a strippable plastic film coating, is necessary. The half-life of the isotope is preferably greater than two years, so that decay will not seriously deplete the quantity present over the useful life of the spacecraft. Typical radioisotopes which emit beta radiation include promethium-147, thulium-171, thallium-204, and europium-152. Of these, promethium-147 is preferred since it emits beta particles having a maximum energy level of 0.223 Mev, emits no gamma radiation, has a half-life of 2.62 years and has a high boiling point (about 4,000.degree.C for Pm.sub.2 O.sub.3).

The average energy of beta particles emitted by promethium-147 is about 70KeV per disintegration so that 1 millicurie of the isotope (3.7 .times. 10.sup.7 disintegrations per second) emits about 4.15 ergs/sec. Since the maximum range of these beta particles is about 55 mg/cm.sup.2, the actual range in air (at a density of about 1.29 mg/cc.) is less than 43 cm and in skin is such that all of the radiation dose is delivered to the skin. For illustration, if a hand with an area of 250 cm.sup.2 is placed in direct contact with a surface loaded with 1 microcurie of 147 Pm per cm.sup.2, the dose rate to the skin of the hand will be about 1 erg/sec-25gm, or about 2.5 rems/hour. Since the allowed dose to the hands per calendar quarter is 18.75 rems, a direct contact exposure of 7.5 hours could be tolerated. Thus, probably no radiation shielding of the heat shield surfaces would be necessary during maintenance of the spacecraft. However, if desired a strippable plastic film could be sprayed over the heat shield during spacecraft maintenance. A coating of most plastics having a thickness of a few thousandths of an inch would absorb all of the beta radiation emitted at the heat shield surface. Alternatively, gloves having an area density of at least about 55 mg/cm.sup.2 could be worn to eliminate the radiation hazard.

While the isotope may be used in any suitable form, the oxide is preferred since it is stable, simply prepared and is compatible with the coating materials. Generally, it is preferred that the isotope be dispersed in a carrier material which is in turn dispersed in the coating slurry. Use of the carrier improves the uniformity of the final dispersion, since so little isotope is used. While any suitable carrier material may be used, lanthanum oxide is preferred for use with promethium oxide because of its similar orbital structure and excellent dispersion characteristics.

Although any suitable concentration of isotope in the coating material may be used, from about 1 to about 2 micro-curies per cm.sup.2 is preferred in the case of promethium-147. While increased amounts of the isotope will speed measurement techniques such as autoradiography, they will also increase the radiation hazard. A small fraction (about 0.1 percent) of the beta particle energy is converted to bremsstrahlung (energetic photons) having energies up to 0.223 MeV. An increase in radioactive loading may impose limitations on the time unprotected workers could remain in the vicinity of the coating.

The isotope-carrier mixture may be dispersed in the coating material, and the substrates may be coated by any suitable method. After careful mixing, the slurry is typically applied onto the substrate, then heated in a vacuum to form the silicide coating.

DESCRIPTION OF PREFERRED EMBODIMENTS

The following examples define preferred embodiments of the method of this invention. Parts and percentages are by weight, unless otherwise indicated.

EXAMPLE I

To an acetone slurry containing about 5 grams solid, finely divided coating agents comprising about 20 percent hafnium, 10 percent chromium, 5 percent iron and 65 percent silicon is added about 9.8 milligrams of lanthanum oxide having dispersed therein about 10 microcuries of promethium-147. The mixture is extensively shaken and mechanically mixed, then sprayed onto a 5 cm. square columbium coupon. After coating, the coupon is allowed to dry, then is fired at about 1,500.degree.C in a vacuum to form a radioactively tagged silicide coating on both sides of the coupon. Since about 20 percent of the slurry remains on the coupons, each side of the coupon contains about 0.5 microcuries (1.1 .times. 10.sup.6 disintegrations per minute) of radioactivity. A comparison of the weight of the coated coupon to its weight before coating shows an increase of about 0.451 grams. The radioactive loading is about 0.02 microcuries/cm.sup.2. The radioactivity at the surface of the coating, counted with a Geiger-Muller counter having a 3.2 cm. window, is 4051 counts/min (front) and 3110 counts/min (back). This indicates that the front coating weighs about 0.254 grams and the back coating about 0.197 grams. Each surface is than autoradiographed by contact with Eastman Kodak Type T film for 110 hours. The resulting pictures show slight segregation and unevenness in the coating. The coated surfaces are then abraded with carborundum paper in eight steps. After each abrasion step, the radioactivity of the surface is measured. Results show that the reduction of beta radiation detected at the surface is proportional to the decrease in coating thickness. It is found that coating thickness changes can be measured using a single 3.2 cm. diameter Geiger-Muller detector to a precision of better than 1 percent with a 10 minute counting period. The autoradiographs are found to be capable of resolving details at least as small as 0.005 cm. in the coating.

EXAMPLE II

To an acetone slurry containing about 20 percent chromium, 10 percent titanium and 70 percent silicon is added about 2microcuries of thallium-204 in a nitride carrier. Following mechanical stirring to break up material aggregates, the mixture is extensively shaken. The mixture is applied to a carbon composite substrate, outgassed for about 1 hour, then fired at about 1,400.degree.C in vacuum to form a radioactively-tagged silicide coating on both sides of the specimen. The spray utilization during coating application is gauged to be about 20 percent and the amount of isotope contained by the coating is estimated to be about 0.02 microcuries/cm.sup.2. Using a 1 cm.sup.2 end window Geiger-Muller counter, the count rate is found to be about 300 counts/second at the surface of the coating. The radioactive loading at the surface of the coating is measured following abrasion in equal steps of about 12 micrometers. The results show that the decrease in counting rate is approximately logarithmic. It is determined that the range of the beta radiation is sufficient to provide meaningful measurement of coating thickness up to about 200 mg/cm.sup.2 or actual coating thickness of about 0.05 cm.

EXAMPLE III

About 300 square feet of columbium heat shield area of a re-entry spacecraft is coated with a slurry of radioactively tagged silicide heat shielding material to a dry thickness of about 0.008 cm. The surface is fired at about 1,400.degree.C in a vacuum to form the silicide coating. The slurry contains about 20 percent chromium, 20 percent iron; 60 percent silicon to which is added about 10 percent lanthanum oxide carrier containing about 500 .times. 10.sup.12 atoms/cm.sup.2 promethium-147. The radioactive loading level is estimated to be about 2 microcuries/cm.sup.2. The heat shield surface is completely autoradiographed, using Eastman Kodak Type AA film, and the entire surface is scanned with a Geiger-Muller counter having 3.2 cm. diameter aperture. Scanning with a single counter and a 0.2 minute count of each incremental area requires about 20 hours. The autoradiography and scanning are repeated after each of several trips into space during which the heat shield is subjected to the stresses of re-entry. The decrease in coating thickness indicated by a decrease in beta radiation detected at the heat shield surface is found to closely correlate to actual destructive thickness measurement tests. The autoradiographs are found to clearly reveal defects developing in the coating surface and damage sustained from impact with flying objects.

Although specific ingredients, components and proportions have been described in the above description of preferred embodiments, other suitable materials and conditions may be used, where suitable, with similar results, as indicated above. In addition, other materials may be included in the heat shield coating to enhance or otherwise modify its properties.

Other modifications and ramifications of the present invention will occur to those skilled in the art upon reading the present disclosure. These are intended to be included within the scope of this invention, as defined in the appended claims.

Claims

I claim:

1. A method of testing protective coatings on re-entry spacecraft which comprises:

a. forming a coating on a surface to be protected, said coating comprising a composition selected from the group consisting of disilicides, subsilicides and mixtures thereof having dispersed therethrough a composition comprising an oxide of promethium-147 and a lanthanum oxide carrier, said coating having a thickness from about 75 to 100 micrometers and containing from about 1 to about 2 microcuries of promethium-147 per square centimeter of coating surface;

b. measuring the beta radiation at the surface of said coating;

c. exposing said surface to a high temperature atmospheric re-entry environment likely to cause loss of coating protection;

d. again measuring the beta radiation at said surface, to detect losses of coating protection.

2. The method according to claim 1 wherein said beta radiation is measured by autoradiography whereby defects in said coating are revealed.

3. The method according to claim 1 wherein said beta radiation is measured by scanning said surface with a Geiger-Muller counter.