I started working with N2O/HTPB hybrids about 15 years ago working with another
person. Built 2, 4, and 8" hybrids.Then a few years ago I was tasked with
building a LOX hybrid. I knew nothing about LOX hybrids and simply applied some
knowledge from the nitrous hybrids and what I knew about LOX from teaching
chemistry. I had just 4 months to do the building of 3, 4, and 6-inch motors
for testing with seven 4" and six 6" rockets ready to fly for the project. It
was very stressful as I had little time for testing and supplier issues. The
types of fuel were dictated by the customer.
The design was simple to keep costs down, I would have preferred using helium
to pressurize the LOX but we went with LOX self-pressurizing. Aluminum tanks
were made, simple stainless steel ball valves from McMaster Carr were used, and
the nozzles were an assortment of phenolic/graphite that I made or molded
commercial nozzles from RCS/Aerotech.
The tank had four openings, 3 on top and one on the bottom going to the motor
injector. The top three were 1) fill valve, 2) vent valve, and 3) safety
pressure release. The bottom valve was a larger stainless ball valve. All were
'non'cryogenic' rated to -40 if I recall. The tank vent had a dip tube that
went into the motor about 25% of the length of the tank.
The procedure was simple. Open the vent valve, connect the LOX Dewar to the
fill valve. Begin filling. Vapor came out of the vent valve until the LOX level
reached the dip tube and started squirting out LOX. The Dewar was turned off.
The hose was disconnected from the fill valve and allowed to drain out. The
fill valve was closed. When everything was ready to go the vent valve was
closed to start pressurization as we vacated the pad. The bottom valve to the
motor was remotely opened after we ignited one of my LollyPops...a Kabob skewer
dipped in sugar propellant that was stuck up the nozzle. The sugar burned for
approximately 10 seconds. We could easily see it burning as the smoke poured
out the nozzle. Never once failed to ignite about 20 LOX hybrids (3, 4, and 6").
The biggest problem we had was determining 'when to open' the motor valve as we
did not monitor tank pressure. It was guesswork and the ambient temperatures
from 75 to over 100F didn't help much. Measuring tank pressure would have been
a plus as we sometimes opening up too early with minimal tank pressure built
up. We once thought about allowing it to build up pressure to see if the safety
valves worked and what it would be like. Took about 20 minutes when it vented.
That gave us some reassurance.
When the right time presents itself, I will again return to LOX
hybrids...simple and inexpensive but as with any rocket propellant, you need to
exercise the caution necessary with the type of propellant you are working with.
This is a video of one of the tests we made. Remember, this was done on the
cheap. Instead of phenolic liners, we used Home Depot ABS sewer pipe and did
not prepare the inside surface for paraffin binding. The 'fuel grain' was
paraffin that could have used additional opacifying to limit radiant heating.
And the day was hot, well over 90 F and the grain probably was starting to
soften up. Shortly after motor start-up, the paraffin fuel grain completely
detached from the ABS liner and was extruded out the nozzle, flying off like
some angry snake awaken from his slumber. Then the LOX continued it combustion
of what was left, the ABS liner that actually burned quite nicely, After the
liner burned away, the still flowing LOX melted through the aluminum case
showering hot liquid aluminum droplets over the test stand concrete. When the
hole was sufficiently enlarged, the top couple of inches of ABS liner rolled
out of the motor through the hole.
The downside of LOX is finding a place that will sell small quantities. I can
get a 160-liter dewar from AirGas for around $200 and can fill my 20 pound N2O
tank for about $120 at a speed shop (my local one closed down) so I don't know
where or cost.
We did later fly one of these LOX hybrids two months later.
100mm (4") LOX / WAX hybrid rocket motor test 06-24-2015
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100mm (4") LOX / WAX hybrid rocket motor test 06-24-2015
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Rick
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From: roxanna Mason <rocketmaster.ken@xxxxxxxxx>
Date: Sun, 14 Mar 2021 15:46:37 -0700
Subject: [AR] Re: Hybrid High Powered Rocket
Troy, have you designed built and hot tested LOx systems yourself?
Just curious because all the minor negatives are easily dealt with. BTW,
LOx can be self pressurizing,
The first liquid rocket flight by Goddard in 1926 was done this way, he
used it to pressurize the fuel tank as well.
I wouldn't recommend it but it worked.
Being an engineer I don't believe in fixing something that works and if
someone is already using Nitrous safely then great. But if a newcomer to
liquid rockets asks what oxidizer to try out, I'd recommend LOx. It will
help him/her in their future career in aerospace more than nitrous
experience too.
Ken
On Sun, Mar 14, 2021 at 3:12 PM Troy Prideaux <troy@xxxxxxxxxxxxxxxxxxxxx>
wrote:
How to make hard-line clerics look like consummate pragmatists…
Now there’s a Best Seller just waiting for pen to paper…
Ken,
There’s a reason (no, multiple reasons) why N2O has been the only liquid
oxidizer approved for use with Tripoli and that’s not through the
hand-waving-chest-beating-table-thumping preaching of biased dogma. It’s
by pure **practicalities** and even the extra fuss needed for N2O proved
over time to rendered them unpopular against the vastly more practical
solids.
HPR hybrid flyers could go out and fly their birds without the need for
a dewar or the need to have it filled just prior to the launch or to deal
with cryo sealing or potential embrittlement of a neighbouring parts or
needing a separate pressurant and accompanying plumbing with that or the
requirement for more specialised injectors. The hardware was cheap and
simple. The assembly could be fairly simple.
Again, it’s horses for courses. Yes, beyond a certain size system, LOX
becomes more practical, but the intended audience was quite clearly spelled
out as HPR.
Troy
*From:* arocket-bounce@xxxxxxxxxxxxx [mailto:arocket-bounce@xxxxxxxxxxxxx]
*On Behalf Of *roxanna Mason
*Sent:* Monday, 15 March 2021 8:06 AM
*To:* arocket@xxxxxxxxxxxxx
*Subject:* [AR] Re: Hybrid High Powered Rocket
I wish certain folks would lose their cryo phobia, LOx is such a
superior oxidizer and the self pressurizing 'advantage' of nitrous is all
but lost with the very few cons.
If you have a misfire in a LOx system just let it evaporate away. Of
course all pressurized systems will have relief valves or burst discs or
both, which is my favorite passive doubly redundant design philosophy. A
40-50 gallon dewar of LOx can run anywhere between $75 and $300
depending on many factors.
A well maintained dewar will have only 1-2% loss per day so if you have a
week or two delay no problem.
My mom used to take me down to the welding supply store when I was in my
single digits age and get a liter of LOx or LN2 to 'play' with, price was
always $2. Get used to its properties and work your way up to closed
systems. When I was about 15 I asked an Aerojet engineer that was visiting
our group, "what is LOx like?" He said to me: "It's like water" He meant a
Newtonian fluid, the cold is largely inconsequential once understood. Some
seals/o-rings need changing and all non ferrous metals are com[patabler
with LOx. Insulation is usually non necessary with test stand components
only extended storage is insulation necessary nother like LH2 which
requires insulation under all circumstances.
My friend Glenn May died in a nitrous explosion along with two of his
co-workers. I guess you could argue I have nitrous phobia, maybe so but
it's based on fact not a misperceived fear of an industrial gas that is
produced, transported.and used by the thousands of tons per day world wide
w/o mishap.
Read and learn as much as it takes to understand the nature of any beast.
LOx Rocks,
Ken