n the Spring of 1995 I envisioned a series of tests to evaluate the safety of radio transmissions in the
vicinity of rocket igniters and prepped rockets. My motivation for testing came from my dual
status as a rocketry hobbyist and amateur radio operator. Several people in the AERO-PAC rocketry group
(http://www.aeropac.org/)
to which I belong wondered whether RF (radio frequency) energy from nearby mobile or hand-held radios might
ever transfer enough radio energy to heat electrical igniters to their ignition point.
In May 1995 I developed an experimental protocol I hoped would shed some light on this question,
and I presented it to the AERO-PAC Board of Directors. They
indicated great support for the effort and provided samples of rocket igniters for me to test
with.
The protocol I envisioned was to attach rocket igniters directly to tuned, receiving dipoles and try to
ignite them at various distances with mobile and hand-held radios. My presumption was that a
receiving dipole would provide a degree of RF coupling to the igniter that was much greater
than the greatest degree of coupling one would reasonably expect in an actual launch
configuration. I reasoned, therefore, that any distance that proved safe using receiving
dipoles would certainly also be safe under real launch conditions where the RF coupling would
be expected to be much less.
CRITICISMS OF MY PROTOCOL
I posted a summary of my planned experiment to the Internet news group rec.radio.amateur.misc
to solicit feedback on my protocol from the amateur radio community. I received several email
replies, all of which were supportive and helpful, but most of which pointed out serious flaws
in my experimental protocol. Here is a paraphrase of the criticisms offered.
1. Individual rocket igniters have great statistical variation in current required for
ignition, even within the same type and brand. Any proper test would have to evaluate many
samples of each brand to produce a statistical curve of ignition properties.
2. Dipoles attached to igniters would not provide a realistic model of actual launch
conditions. I knew this, but many people felt it made the experiment not useful. In actual
conditions igniters are connected to the ends of very long cables which may act as long-wire
antennas. Such antennas may exhibit large variations in signal received depending on cable
length, routing, radio frequency, and other factors. An appropriate test would have to include
exhaustive testing at numerous launch sites with different configurations of all of the above.
3. Testing with dipoles and different powers provides no data that could not be calculated
using RF formulas. The real variable is the ignition current characteristics of the igniters. Tests
should be done by feeding electrical current directly to batches of igniters and making
precise measurements of the electrical current required for ignition. RF safety standards
could be extrapolated from that.
NOW WHAT WAS I TO DO?
Alas, all these criticisms were valid. It became clear to me that the tests I envisioned
would not amount to a proper, quantitative evaluation of the safety of rocket igniters in the
presence of RF. For a long time I pondered these issues knowing that I did not possess the
expertise in statistics or RF propagation theory to carry out the quantitative safety testing
that should really be done. I knew I was capable of performing the testing I had originally
proposed, but was now acutely aware that these tests would not produce the results for
decision making I had originally anticipated.
In the end, it was either a matter of doing the tests I knew I could perform, or not doing any
tests at all. I decided to go ahead with the testing as I originally planned but with a
reduced set of expectations for the usefulness of the results.
I therefore stipulate that the results of the tests I have performed do not provide a
quantitative basis for establishing RF safety guidelines at launch sites. The tests do,
however, begin to probe the issues, and I think they provide some useful information on which
to begin formulating tentative precautions until more quantitative information becomes
available.
MY TESTING AND RESULTS
I performed two types of tests. The first was the dipole testing I had originally proposed,
using igniters attached directly to tuned receiving dipoles with minimal-length leads. As I have discussed, this
provides a degree of RF coupling much greater than the greatest RF coupling one would expect in
an actual launch situation. I performed the test using Daves brand electrical igniters because they
are popular in high power rocketry. However, I have no basis for presuming this brand of igniter to be
better or worse in terms of RF hazard than igniters of other brands. During earlier
(non-documented) testing with Estes brand igniters, I obtained results similar to the ones documented
in this experiment.
With my RF transmissions in the amateur two-meter band at 147 MHz. I was able to ignite Daves
igniters with 50 watts of power from a distance of one foot. I was not able to cause ignition
with this power at distances greater than one foot. I was also able to ignite Daves igniters with 5
Watts of power from a distance of 6 inches. I was not able to cause ignition with 5 watts at
distances greater than six inches.
The second type of test was an all-out effort to ignite Daves igniters while they were attached
to their regular wire leads like they would be when inserted into a rocket motor. Out of the box,
Daves igniters have zip-cord (parallel wire) leads about six feet long. I tested Daves igniters with their
full-length leads, and also with the leads cut to 38 inches and to 19 inches. (1/2 and 1/4
wave lengths respectively at the frequency I was using) For each of these length
configurations, I tested with the lead ends shorted together (as they come packaged) and also
with the ends separated.
For each of the six combinations of lead length and short/open configuration, I wanded a 1/2
wave antenna emitting continuous 50 watts all around the igniter and leads. I tried different orientations
and different degrees of overlap from far away up to about 1 inch separation. I was not able
to ignite a Daves igniter at any distance as long as the igniter was in its normal zip-cord
configuration.
CONCLUSIONS?
As I have disclaimed already, the tests I performed do not provide a quantitative basis for
establishing a "safe" RF distance for two-way radios. My gut feeling, however, is that in lieu of further testing it is
probably reasonable for rocketry organizations to adopt tentative safety measures that would
keep prepped rockets motors (motors with igniters inserted) at some distance (30 or 40 feet
perhaps?) from parking areas or from other potential sources of high-power radio transmissions. A
launch-site rule that keeps the prep area on the launch side of the flag line away from the parking areas would seem to me to
accomplish this at most launch site configurations. Keep in mind that RF signal falls off
exponentially with distance so that increasing distance by 10 times reduces received power by
100.
Note that I do not suppose it to be desirable (or even possible) to prohibit mobile radio
transmissions in the parking areas or access roads around launch sites. Government vehicles,
for example, may drive up at any time and transmit at 120 watts or more. My gut feeling is
that any such transmissions 30 or 40 feet removed are not likely to be a serious safety issue.
I hasten to state again, however, that I do not feel my testing provides quantitative proof
for this.
My gut feeling is that hand-held radios are likely not a serious safety issue at launch sites in terms
of their RF heating potential..
Perhaps it wound be prudent for rocketry organizations to adopt tentative safety measures that
would prohibit transmitting with hand-held radios and cellular phones along the rocket line,
cable runs, and inside the prep areas. I would suppose, however, that more restrictive
rules are likely unwarranted. Once again, however, I do not feel my testing provides
quantitative proof for this.
Please note that my experiment addresses RF only in terms of its direct heating potential.
I have not considered the question of radio interference to electronic control devices. My
feeling is that any control device that is not known to be immune to RF interference should
simply not be used for a function in which safety is a concern.
While launch-site rules may be able to prevent the high levels of RF required to burn an igniter,
there is no way possible in the modern world to eliminate the low
and medium levels of RF energy present all around us that might interfere with electronic devices.
Control systems need to be inherently safe in this regard if their failure would pose a danger.
FURTHER STUDY
My testing barely scratches the surface of this issue. Professional, quantitative analysis of
the effects of RF emissions on rocket igniters would be highly desirable. I feel that the
manufacturers of igniters have some responsibility in this regard since they are the ones
developing the ignitor products. The amateur radio community should also be able to play a
role in this because of the pool of expertise there. Keep in mind, however, that potential
sources of RF at rocketry sites go way beyond amateur radio. RF sources include government and
commercial vehicles, personal services like CB and GMRS, cellular telephones, and commercial broadcast.
The highest RF coupling levels might be expected to come from government vehicles like police cars
that drive up to a launch site and transmit at high powers.
As I have stated, I do not posses the expertise or equipment to conduct this sort of professional
testing. I would be happy to continue the effort with letters and coordination to get some
serious testing started in this area. If anyone has ideas on who might be interested in
tackling this, I would be happy to do some footwork. I welcome any comments or further
criticisms anyone might have of my experiment or statements.
Copyright © 1996 Donald J. Irving
Document URL: http://www.irving.org/rocketry/igniters/summary.html
Permission to reproduce and distribute this document is granted providing the entire document is reproduced, including copyright, without change.
