Fuel tests--


illustration 2 - main gig

These are my first experiments with Nitrous Oxide so I set out to do some basic tests to get a first hand feel for the oxidizer and some hybrid rocket principles. Thrust was not intended here.
illustration 3 - Setup for the tests.

The two fuel formulations tested were:

TEST 1 Epoxy 90%
  Zn 10%
   
TEST 2 Epoxy 20%
  Sorbitol 80%
Table 4

 

TEST 1

illustration 4
 Had problems casting the epoxy and Zinc mixture because of the compound being to fluid. Ignition was fast, exhaust dense and black
View video of test
 
TEST 2


illustration 5

 Easy to cast because epoxy sorbitol mixture was pasty. Nice clean burn with virtually no visible smoke.
View video of test
sorbitol_epoxy_small.mov (905 KB) sorbitol_epoxy_small.mpg (245 KB)

 

AFTER IMAGES

illustration 6
 Test 1 chamber exit area after 35 sec. burn. The grain was cast without insulation. Notice how the casing started to deform and burn from the heat transfer.

illustration 7
Test 2 chamber exit area after 60 sec. burn. On this test EPDM insulation was used between the casing and the grain. Notice there is virtually no deformation or burning of casing.

illustration 8

Test 1

Very noticeable smoke mark left on brick after burn, which suggests that combustion is incomplete and that the exhaust contains various long chained molecules.


illustration 9

Test 2

Very faint smoke mark left on brick after burn.

 

These two tests gave me a good idea of how to fuel and ignite a hybrid, but did not tell me very much about the performance of the fuels, except maybe that sugar burns cleaner than plastic (epoxy).

The next step, run simulations of different fuels on propep using two constants: NOX as oxidant and O/F ratio of 7/1.

PROPEP is a program for those interested in determining the characteristics of different propellant formulations. A copy of it can be obtained here: http://sunsite.unc.edu/pub/archives/rec.models.rockets/PROGRAMS.

Propep results
Fuel Isp
Sucrose, Epoxy. Carbon Black 200.8
Sucrose 194.7
Carbon Black, Epoxy 207
Epoxy, Zinc 207.9
Epoxy 215
Table 5

These results do not show much difference in performance between one fuel and the other, specially for the purpose of the PVC Hybrid prototype for this project. For such a reason, a sucrose (table sugar), epoxy, carbon black formulation was decided on, based primarily on the fact that it seams to be a cleaner and friendlier fuel.

On the same note, Hans Olaf Toft says:

"I have always thought that some kind of sugar filled polymer would be a good candidate for a hybrid propellant for the following reasons:

  1. high energy content
  2. The fact that KN/sugar propellants burn so readily while KN/HTPB or epoxy or whatever polymer binder needs a fair amount of help to burn suggests that the obtainable regression rate for sugar hybrids should be fairly good. I also think that carbon black or another opacifier would further increase regression rate.
  3. Sugar is inexpensive and non toxic.

The amount of smoke generated by Your epoxy+zinck experiment suggests that combustion is incomplete and that the exhaust contains various long chained molecules, while the clean sugar exhaust consists of smaller molecules."

Thus the final formulation for the fuel is:

Table Sugar 75%
Carbon Black 5%
Epoxy 20%
Table 6

 

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