Thanks for sharing, Rick. The afterburning on the deflector during the wax
phase shows that it was running very rich then. It seemed happier on the
plastic.
Another challenge with self pressurization is that as tanks get larger they
stratify more easily, and during depletion the pressure will drop off
faster than it would for an ideal propellant of consistent temperature.
It's probably possible to separate the pressurant role from the propellant
role with some clever tank design; have a section of warm propellant at the
top and cold at the bottom, helped along with insulation on the bottom and
maybe a heater on the top.
I'm not particularly a fan of nitrous or hybrids, but the availability of
small quantities (and long term storage for intermittent rocket weekends)
is a real advantage to nitrous. Making a really small LOX rocket is a fun
stunt but it's easier to do it with nitrous, as demonstrated by the
commercial hybrids and tribrids.
On Mon, Mar 15, 2021 at 7:22 PM Rick Maschek <dmarc-noreply@xxxxxxxxxxxxx>
wrote:
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
<https://youtu.be/nfftc3yNfbI>
100mm (4") LOX / WAX hybrid rocket motor test 06-24-2015
<https://youtu.be/nfftc3yNfbI>
Rick
------------------------------
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…liquid
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
oxidizer approved for use with Tripoli and that’s not through theIt’s
hand-waving-chest-beating-table-thumping preaching of biased dogma.
by pure **practicalities** and even the extra fuss needed for N2O provedspelled
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
out as HPR.arocket-bounce@xxxxxxxxxxxxx]
Troy
*From:* arocket-bounce@xxxxxxxxxxxxx [mailto:
*On Behalf Of *roxanna Masonvisiting
*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
our group, "what is LOx like?" He said to me: "It's like water" Hemeant a
Newtonian fluid, the cold is largely inconsequential once understood.Some
seals/o-rings need changing and all non ferrous metals are com[patablerwide
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
w/o mishap.
Read and learn as much as it takes to understand the nature of any beast.
LOx Rocks,
Ken