The topic here is how experimental experiences, with ramjet
ablative combustor liners, relate to the
unexpected and unpredicted entry heat shield problems seen on the Artemis-1 test
flight. There actually is some
correspondence, the overlap suggesting a
“fix”. What might actually be done to
correct the heat shield problem is presented here.
The author is qualified to speak on this topic, having spent 20 years in aerospace defense
new product development work, most of it
in solid rockets and ramjet propulsion.
He is no narrow specialist, as
the next 20 years in teaching (at all levels),
in civil engineering, and in
aviation work demonstrates. He is now
long-retired, but occasionally
consults.
The Problem
The Artemis-1 heat shield partially failed by unexpected and
unpredicted cratering, but did not
have a burn-through. It was
constructed of Avcoat tiles bonded to the capsule structure. Chunks of the charring heat shield spalled
out, creating alarmingly-deep craters in
it. The same Avcoat material was used as
the Orion EFT-1 flight test heat shield,
and on the Apollo capsule years ago.
The difference with Artemis-1 was that the Avcoat of those
earlier heat shields was hand-gunned into the numerous cells of a fiberglass-type
hex attached directly to the capsule structure.
The change to bonded tiles was intended to greatly reduce the
labor cost of the hand-gunning, and also
reduce its inherent variability.
The bonding of the tiles to the Artemis-1
capsule, and the gaps between them, are not in question here. Those performed quite well. Nor is the cost reduction in
question, it is quite real, obtained by casting blocks of Avcoat that
were final-machined to tile shapes.
The
only real difference is the question explored here: those blocks of Avcoat had no reinforcing hex
in them!
A Place to Look is Outside the Organization!
One needs a “fresh pair (or pairs) of eyes” to see past the thinking
“ruts”! The problem with that is that
one may not really know the people who are from outside. Therefore,
one tends to trust such outsiders less!
Another aspect of that is the “not invented here” prejudice! One must strongly resist that!
Ramjet Ablatives Is a Related Experience
Unlike solid rockets,
ramjets have very long burn times.
And unlike rocket motor free volumes out near their case diameters, ramjets have high fluid shear forces all
along their combustor walls! The ramjet
has long proven to have the more challenging environment for ablative
protection. The reinforced-rubber
insulations used in rocket cases are quite inadequate for ramjet combustors!
The reentry heat shield sees hotter local effective
temperatures, experiences lower surface pressures
(because the atmosphere is so thin at entry peak heating altitudes), and quite likely sees fluid shear forces
along the heat shield surface that are on the same order as those of the ramjet
combustor. That last seems likely
because both are subsonic (behind the bow shock for the capsule, with higher sound speeds but lower densities).
This author ran some experimental ablative candidates
through short-burn, full-scale tests in
a combustor of the size for a ramjet replacement of the AIM-120 AMRAAM
motor. In the process of understanding
the results, he identified some 5 key
issues to consider, as are indicated
by notes in red in Figure 1 below.
(All figures are at the end of this article.) These issues were porosity for gas release, the polymer pyrolysis temperature, the “glue” effect of viscous melt
materials, the “reinforcing aggregate”
properties of particulate solids, and
the reinforcing effects of fibers reaching into (or within) the char layer.
The baseline ramjet insulator, Dow Corning’s DC 93-104, did the best,
as expected. The shortfalls of
the experimental alternatives were understood,
once those issues were all identified.
These issues were all addressed in the formulation of the DC 93-104 (as
indicated by the blue notes), but were
not all addressed in the other materials.
Those included a Japanese “equivalent” to DC 93-104, and two pre-preg cloth layups.
These were short-burn tests,
all under 1 minute long. It was
already known that many ramjet applications needed longer burn times than
that, but that thicker layers of the
ablative were simply not a design option,
because of the enclosed volume reductions. The solution to the burn time problem came
from “in-house”, not Dow Corning, and is illustrated in Figure 2. As used in the 20-inch diameter ASALM-PTV
combustor at the same liner thickness,
the retention ribbons held the char in place as an insulator, for up to 15 minute burns! While not as “good” an insulator as the
virgin material, the retained char was
“good enough” to essentially achieve acceptable steady-state results in ground tests
and in flight tests!
That kinked-ribbon retention feature was not included
in any of the experimental insulation tests discussed here, primarily because all the burn times were
under 1 minute! Not being present
in these tests, it cannot obscure or
impact the fundamental ablation phenomena behind the results obtained! Furthermore,
the two pre-preg cloth alternatives had to be wrapped onto an inflatable-bladder
mandrel for installation, more like
rocket motor insulations. They could not
use the kinked-ribbon retention feature anyway!
The hope was that the cloth fabric would provide enough retention. But it did not.
Applying This Knowledge to Ramjet Combustors
The designer must deal with all 5 of the issues in the
ramjet environment, but only 1 of the 5
issues actually dominated the picture,
as indicated by the gold stars in Figure 3. This dominant issue proved to be randomly-oriented
high-temperature-capable fibers tying the char to the virgin beneath. The need for enough porosity to release the
pyrolysis gases through the char was not much of an issue in the thin layer
designs we were using. In heat
shields that are thicker, it
might be more important! There is
some impact of the presence (or not) of sticky,
viscous melt on the surface, to
prevent the more rapid erosion of small particles by the fluid shear.
If ramjet combustor design was all that we were
considering, then the loss of some char
chunks as seen with the Japanese material,
might be countered either by the long-burn kinked-ribbon retention
feature, or by adjusting the formulation
to include more carbon fiber, or both. Even so,
it provided useful 1 minute burn time,
even with some chunk loss.
Applying This Knowledge to Heat Shields
There is definitely overlap of the physics identified
between the ramjet combustor and the heat shield applications! While
heat shields do not deal with all the same issues, the porosity and char loss do indeed overlap
a little. But the need for radial fiber reinforcement would
seem to dominate both applications,
preventing as it does the loss of chunks of char! This is illustrated for ramjets in Figure 4.
Quite apparently the real mistake made with the Artemis-2
bonded-tile heat shield design, was
deleting the reinforcing hex from those bonded tiles! And that says the most important thing NASA could do is to look for
ways and means to put that reinforcing hex back into those very bonded tiles! Preferably doing it without doing the
labor-intensive and expensive hand-gunning of the Avcoat into each individual
hex cell!
So, the main question
here is: can that really be done?
There Really Is a Practical Fix
The answer is “yes”!
But to even consider it,
one must avoid the trap of “either/or” thinking! That trap considers only tiles without hex
versus doing it hand-gunned completely like Apollo and Orion EFT-1. That thinking trap is illustrated in Figure
5, along with this author’s way out
of that trap: use an extrusion
press to load all the cells in the hex with Avcoat, all at once!
That
way, one still uses bonded tiles, and avoids all the hand-gunning labor! And the tiles with hex in them should
cease shedding chunks of char, no matter
how the entry is flown, skip or not! Plus,
the high micro-balloon content that leads to low densities, but also to high mix viscosity difficulties, is something the extrusion press can
handle. All that is needed is the right
tile mold tooling to hold the hex in place on the press. That is just tooling design. Plus,
the inherent human labor variability is eliminated.
Such a solution is unavailable to people hampered by the bad
habit of “either/or” thinking. The
author gave this solution to NASA more than a year ago! But they so very clearly did not use it, or even contact him about it!
Final Comments
These ramjet results and their application to heat shields
are as much “engineering art” as they are anything! One will not find this in published
reports or academic texts. A lot of
this is qualitative, not quantitative, and inherently so! The difference between “engineering art” and
“engineering science” is explained in Figure 6. Note the large fraction of the necessary
knowledge that is “art”, especially in
development work!
Note also that the engineering art is passed-on, one-on-one, on-the-job, from the old hands to the “newbies”. That is,
it is passed-on if, and only
if, there are any old hands on the
staff to do that teaching job! Too many
organizations prefer to hire only “newbies” that they can under-pay, instead of retaining the “old hands”, excusing this with “because they are too
expensive”. But it loses you half (or
more) of your essential knowledge!
The current status is this:
It is too late to fix the
Artemis-2 heat shield, that rocket is on
the pad to launch!
Artemis-2
is forced to risk worse or deeper cratering with a crew
Probability
of a fatal burn-through is not zero !
Had
almost 2 years, so why not? Schedule and money, same as Challenger!
Must fix the heat shields of
Artemis-3-on, to avoid a fatal
failure, sooner or later!
Must
not repeat Challenger mistake: must value
lives above schedule & money
Trust
results (even qualitative), not the same
sources that failed to predict cratering
What one has to learn and remember is this very harsh
lesson: There is nothing as expensive as
a dead crew! Especially one dead from a
bad management decision!
In different ways, this bad decision thing is really is the root cause of both the Challenger and the Columbia disasters! “Avoid that mistake like plague” is the best advice this author can give you, even if it means listening to outsiders, or overcoming “not invented here” prejudice! Please feel free contact him for more details. He has test data and lots of photos from those old ramjet tests.
Figure 1 – Related Experience with Ramjet Ablatives
Figure 3 – Only One of the Issues Dominated the Short-Burn
Ramjet Tests
Figure 4 – There is Physics Overlap from Ramjet Liners to
Heat Shields
Figure 5 – There Really Is a Practical “Fix”
Figure 6 – “Rocket Science” Really Is Not Just Science
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Search keywords aerothermo,
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