[SS-Propellant] Launch of the Propellant Development phase of SS2S

[Rich Nakka]

Welcome to the SS2S Propellant Development Team (PDT) e-mail discussion 
group...!  <>

Team members:
<>* Richard Nakka
* Randy Dormans
* Matthew McFarland
* Chris Richards
* José Miraglia (please confirm whether or not  you received the earlier 
test message)
* Vlad Mateica
* Antoon Vyverman (Antoon, have you signed up to the list? I am cc'ing 
you for now)  <>
---
What is on our agenda?  <>

First, the big news is that test firing of the ¼ scale motor  (BEM) will 
be taking place soon, tentatively at the beginning of December. With 
sorbitol based propellant (KNSB) being used, it can be expected that the 
motor thrust curve will experience the odd behaviour that has been seen 
in the past with KNSB motor. That is, the "triangular" thrust curve, 
rather than the predicted "flat" curve that BATES should provide.  <>

For those of you not familiar with this phenomenon (which I'll refer to 
as "flushing" coined by,  C.Knight ), I recommend reading the test 
report of the Kappa-SB motor:
http://members.aol.com/kappadx/ksb002.html#Analysis

Note that the use of primer coating and  pyrogen ignition does not 
eliminate this behaviour.

<>Also check out C.Knight's hypothesized explanation for this phenom:
http://www.nakka-rocketry.net/knsbchar.html  <>

Tony Vyverman has written an excellent article that also hyhpothesises 
certain causes such as partially-delayed igntion:
http://sugarshot.org/downloads/evaluation_of_kno3.pdf <>It is important 
that we avoid this type of behaviour for the SS2S motor, for two 
important reasons:

1) We need full thrust off the pad to get the rocket stable quickly to 
avoid any deviation from the desired flight path.

2)  The low average chamber pressure resulting from a "triangular" 
thrust significantly cuts into the delivered specific impulse, which we 
cannot afford. The sims for the flight are based on an average 130 sec. Isp.

 <>Suggestions are needed on how we can modify the KNSB propellant, or 
the motor design to achieve this goal.  <>

Miraglia has suggested the use of a sorbitol/sucrose (SB/SU) mixture. 
This is an interesting option. Although this may not eliminate the 
flushing problem (see Tony's report) it may nevertheless help, or such a 
formulation could provide other advantages. For example, sucrose has a 
greater density, which means more propellant mass for a given motor  
volume.  A 1 or 2 % performance gain might be realized, based on my 
calcs.  <>

My own feeling of late is that flushing might be a result of the grain 
segments being too close together, causing a sort of self-extinquishing. 
Perhaps we can do  static testing to see if greater spacing helps. Also 
consider instead of flat faces on segments, have end surfaces sloped 
(shallow cone). A really interesting suggestion put forth by Tony is the 
use of KNSU on segment ends. This would also boost thrust off the pad, 
due to KNSU's greater burn rate. Instead of KNSU, perhaps KNSB doped 
with  Iron oxide ? Thoughts?  <>

Additives such as carbon or iron oxide to act as opacifier  can also be 
considered/tested.  <>

Erosive burning behaviour of  KNSB should be investigated. Perhaps 
someone can come up with a test proposal.

Consider small modifications to the formulation to improve Isp (e.g. 
66/34 instead of 65/35)?  GUIPEP runs can be done to get an idea if this 
is worthwhile. Runs can also be done on KN/SB/SU formulations.

<>If the propellant formulation is changed in any way, testing will need 
to be performed to characterize burnrate & c-star, as well as other 
qualities such as hygroscopicity and safety with regard to overheating 
during casting.  <>

Much of the testing can be done using small, scaled down motors.  Vlad 
has volunteered to do some static testing. Randy, could you do some 
testing as well? I can do propellant characterization tests, such as 
strand burner. I may also be able to do limited static testing, say, 
with the A-100M motor.

<>The next major SS2S motor test will be the "short-stack" motor. Before 
this, we need to come up with a means of casting large propellant 
grains. The short-stack is slated to have 2 segments, each with a mass 
of about 34 kg (75 lbs), which is identical to the full-size motor. 
Also, curing the grains under pressure to ensure positive bonding to the 
casting tube will need to be investigated and testing done. <>So there 
is no shortage of work for the PDT, and no shortage of ideas that need 
to be put forth.

And  please respond to the ss-propellant list, not to me personally, to 
keep this a group project.

Richard Nakka

 


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