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Hypergolic Ignition of Oxidizers and Fuels by Fuel Gelation and

Suspension of Reactive or Catalyst Particles ∗∗∗∗

Benveniste Natan†, Valeriano Perteghella

‡ and Yair Solomon

#

Faculty of Aerospace Engineering, Technion – Israel Institute of Technology, Haifa, Israel

The need for a storable, non-toxic and high energy hypergolic propellant points towards

the combination of kerosene with hydrogen peroxide. Hypergolic ignition of almost any fuel-

oxidizer combination can be obtained by gelling one of the liquids and adding the proper

material. The rheological characteristics of gels enable the suspension of reactive or catalyst

particles, uniformly distributed in the fuel, without compromising the energetic performance

of the system. In the present research, hypergolic ignition was achieved for a 92%

concentration hydrogen peroxide and a kerosene gel containing sodium borohydride

particles. Ignition delay times of less than 10 µs were observed.

I. Introduction

In general, for the selection of rocket propellant, its energetic performance is the most important factor.

However, modern propulsion systems require the development of propellants that may satisfy the increasingly

stringent requirements of safety, security and environment friendly fuels. Other issues, such as propellant toxicity,

storability and density, should be taken into consideration in the evaluation of the overall performance of a space

system and the possibility of success in its mission.

In the past, the solution seemed to be the utilization of hypergolic propellants, i.e., propellants that ignite upon

contact. Traditional solutions requiring storable hypergolic bipropellants have used a hydrazine based fuel, such as

monomethyl hydrazine (MMH), combined with nitrogen tetroxide (NTO) or inhibited red fuming nitric acid

(IRFNA).1 However, both NTO and the hydrazine-based fuels pose significant health hazards. NTO has a

permissible exposure limit of 5 ppm but a very high vapor pressure of 720 mm Hg whereas MMH, a carcinogenic

liquid, has a permissible exposure limit of 0.2 ppm and a relatively low vapor pressure of 36 mm Hg.2

One of the ways to handle the health hazards is to gel the liquids so they are less dangerous to handle. Gels are

liquids whose rheological properties have been altered by the addition of certain gelling agents (gellants) and as a

result, their behavior resembles that of solids. The gel surface hardens in contact with a gaseous environment. Hence,

in cases of failure in the feeding system or during storage, the leakage rate is reduced in comparison to liquids. In

cases of accidental spillage due to damage in the fuel and oxidizer tanks, burning will occur only at the fuel-oxidizer

interface, if they are hypergolic. When contact ceases, the rheological nature of the gels will prevent further flow

and chemical reaction. The volatility of gels is significantly lower than the volatility of liquids and in case of leak or

spill, much less vapors will be released, thus reducing toxicity hazards.

Several studies were conducted on hypergolic gel propellants.3-5

Rahimi et al3 examined the rheological

characteristics of gelled IRFNA and MMH and conducted firing tests. They achieved c* efficiencies of 90% and

significant improvements in storage and handling.

Other hypergolic bipropellants were developed, based on hydrogen peroxide (HP),4,5

a high density liquid

oxidizer that decomposes exothermically into steam and oxygen, as well as being environmentally safe and

relatively easy to handle. High concentration peroxide solutions have been suggested as oxidizers in combination

with different fuels in which suitable agents were dissolved and their ignition properties were studied in order to

attain the desired hypergolicity level.

Most ideas regarding hydrogen peroxide for various applications are based on its catalytic decomposition using

catalyst beds.6-10

Catalyst beds produce high temperature decomposed HP that can burn with a hydrocarbon fuel,

however, the system complexity and weight are both increased.

∗ International Patent Application PCT/IL2010/000527.

† Associate Professor, Head, Missile Propulsion Laboratory, AIAA Associate Fellow.

‡ Visiting Scientist.

# Research Scientist, presently graduate student at Purdue University.

46th AIAA/ASME/SAE/ASEE Joint Propulsion Conference & Exhibit25 - 28 July 2010, Nashville, TN

AIAA 2010-7144

Copyright © 2010 by B. Natan, V. Perteghella, Y. Solomon. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission.

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Another method is based on the idea of using catalytic or reactive material that is dissolved in a liquid fuel. The

reactive material, decomposes the hydrogen peroxide and ignites the fuel, so hypergolic ignition is achieved without

the use of a catalyst bed. However, this method requires fuels such as ethanol or methanol that serve as solvents for

the reactive material. All these solvents either used alone or with kerosene based fuels, have relatively low heat of

combustion; therefore, the energetic performance of the system is low.

Figure 1 shows the performance of several hypergolic bipropellants in standard conditions. The highest

performance achieved for IRFNA and N2H4 with an Isp of 279 s.

Figure 1. The energetic performance of selected hypergolic bipropellants.

A. Non-Newtonian, power-law fluids

Contrary to Newtonian fluids, in a non-Newtonian fluid, the viscosity, η, depends on the applied shear rate γɺ .

For the characterization of Newtonian and non-Newtonian fluids without a yield point, the power-law model

(Ostwald-de Waele model) is the most common correlation11

: n 1

Kη γ−

= ɺ (1)

where K is the consistency and n is the rate indexes respectively. For n=1, Eq. (1) describes a Newtonian fluid.

Figure 2 depicts the behavior of shear stress and viscosity as a function of shear rate for different Newtonian and

non-Newtonian fluids.12

Various fluid types are presented, such as a shear-thinning fluid for a power-law exponent

(rate index) 0<n<1 or shear-thickening (dilatant behavior) for n>1.

Gels envisaged for the application in rocket and air breathing propulsion systems should have a yield point in

order to enable the suspension of particles. In addition they should have a shear-thinning behavior to provide a

reasonable pressure drop when pumped from the storage tank to the chamber.

Figure 2. Shear stress and viscosity vs. shear rate for Newtonian and non-Newtonian fluids.

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B. Combustion of Gel Fuels

The phenomena involved in the combustion of gelled fuels were investigated for both organic and inorganic

gellants. Nachmoni and Natan,13

and Arnold and Anderson14

conducted an experimental study on the ignition and

combustion characteristics of inorganic gellant, non-metallized, kerosene-based, gel fuels. In addition, calorimetric

tests were conducted to evaluate the heat of vaporization of gels. The authors indicated that in general, gels obey the

d2-law of diffusion-controlled combustion and that the effect of the gelling agent on the burning time can be taken

into account through the heat of vaporization. The heat of vaporization of gels was found to increase with

increasing gellant content in the liquid fuel and gels burned at lower burning rates than the pure liquid. In gels, as in

liquids, burning rate was found to increase with increasing oxygen mass fraction. As regards ignition, increasing the

gellant content resulted in an increase in the ignition delay time and the higher heat input was required for ignition.

Solomon, Natan and Cohen15

studied the combustion of organic-gellant-based gel fuel droplets. The research

revealed that during the combustion, after the vaporization of part of the fuel, an elastic layer of gellant is formed

around the droplet that prevents further fuel vaporization from the outer surface of the droplet. This causes the fuel

to evaporate below the droplet surface producing bubbles, which results in droplet swelling, fuel jetting and finally

collapse of the remaining droplet. The process repeats itself until complete consumption of the fuel and gellant.

This is actually a periodic combustion phenomenon.

C. Scope of the present study

The scope of the present research was to suggest a “green” propellant, based on a gel kerosene-based fuel in

which catalyst or reactive particles are suspended that can ignite hypergolically with an oxidizer. In the present

study the hypergolic ignition of gelled kerosene with hydrogen peroxide was investigated.

II. Gelled Fuel with Suspended Reactive or Catalyst Particles

The present solution based on the rheological characteristics of gels. The existence of yield stress assures that

particles can be added without the affect of sedimentation. Gels enable the suspension of reactive or catalyst

particles, uniformly distributed in the fuel, without compromising the energetic performance of the system. The use

of suspended particles enables every combination of fuel and oxidizer as a hypergolic bipropellant by gelling one of

the liquids and adding the proper material.

In addition the suspended particles can be energetic materials as aluminum or boron particles that can increase

the energy of the combustion. Figure 3 shows the performance of several fuels with HP in standard conditions. The

highest performance achieved for RP-1/Al with Isp of 281 s.

Figure 3. Energetic performance of several fuels with hydrogen peroxide.

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(a) (b)

Figure 4. Energetic performance of several propellants; Isp (a) and ρρρρ · Isp (b).

The combination of nitric acid with hydrazine (IRFNA-N2H4) represents a standard hypergolic bipropellant,

which is use in several systems. It is the hypergolic propellant with the highest performance available but it poses

significant health hazards. RP-1 and O2 is one of the most common propellants for launching vehicles, it has very

high performance but the use of LOX involves complicated cryogenic systems. The RP-1/H2O2 combination is a

storable, non-toxic and high energy pyrophoric propellant. Introduction of Al into the RP-1/H2O2 combination is

feasible by gelation of RP-1 that enables the suspension of Al particles and catalyst particles that can make it

pyrophoric.

Production of hypergolic gel based on kerosene

A hypergolic bipropellant based on kerosene may be combined with most oxidizers. For the use of HP as an

oxidizer a wide range of ignition materials are suitable. A wide range of metal oxides can serve as catalysts that

decompose HP. Also, metal hydrides like sodium borohydride (NaBH4) or lithium aluminum hydride can serve as

reactive materials

For the use of a –NO2 based oxidizer with kerosene, a wide range of solid products of hydrazines with aldehydes

and ketones, namely, the hydrazones can serve as reactive materials. Jain15,16

examined the hypergolicity

characteristics of various hydrazones with –NO2 oxidizers as HNO3, WFNA and RFNA and got ignition delay times

of 9 µs for some of the hydrazones powders. The solid hydrazone particles can be suspended in the gel and serve as

reactive materials.

For the use of LOX, presumably alkalic metals and pyrophoric materials can serve as reactive materials.

III. Experimental System

The kerosene/HP combination is non-pyrophoric by its nature and there is no catalytic material that can dissolve

in kerosene. The use of silica gelant in kerosene enables the mixing of sodium borohydride particles in the kerosene.

The silica based gels are non-Newtonian with a long-term storage capability and silica is compatible with the

sodium borohydride; therefore the gel is storable.

A. Chemicals

• Kerosene based fuel (JP-8, JP-5 etc): Kerosene is a liquid mixture of carbon chains that typically contain

between 6 and 16 carbon atoms per molecule and approximate molecular formula C11H21 (Jet-A). The

density of kerosene is 810 kg/m3. The flash point is 52

°C. The boiling range is 165-265

°C and its auto-

ignition temperature is 220 °C. The heat of combustion of kerosene is 43.1 MJ/kg.

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• Nano-Silica fumed powder, 0.014 µm (SIGMA): Nano-silica has hydrophilic characteristics and is capable

of building up hydrogen bonds to create a gel when mixed with the pure liquid do-to his large surfers area.

• Sodium Borohydride (NaBH4) p.a., ≥96 % (SIGMA): Sodium borohydride is a metal hydride that reacts

spontaneously with Hydrogen Peroxide. The density of NaBH4 is 1074 kg/m3. The flash point is 70

°C. The

boiling point is 500 °C and its auto-ignition temperature is 220

°C.

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• Hydrogen Peroxide (O2H2) p.a., 92 %: Hydrogen peroxide is a liquid with a melting point temperature of -

43 °C, boiling point temperature of 150

°C and density of 1460 kg/m

3. Hydrogen peroxide decomposes

exothermically into water and oxygen. It has a heat of formation of −98.2 MJ/kmol.

B. Combustion system

In order to test the reaction characteristics of hypergolic substances, a suitable test apparatus was designed to

enable reliable measurements on a drop-like quantity of fuel.

Chemical reaction rates depend on temperature and pressure; therefore these parameters may affect the ignition

delay time. Hence a hermetically sealed cell, in which temperature and pressure were controlled, was used to

conduct the ignition tests. The test cell can be pressurized up to 20 atm using nitrogen as an inert ambient gas.

Ambient temperature was controlled by an electrical resistance inside the cell and it was measured using a

thermocouple located at the upper part of the cell. The gel droplet was placed on a glass plate and the hydrogen

peroxide droplet is dropped from a syringe on the gel fuel droplet. Two thick circular optical glass windows

have been fitted into two holes in the cell walls along the same axis in order to enable view through the cell and

focus the high speed camera on the fuel and oxidizer droplet impingement point.

Figure 4. The experimental system.

Figure 5. Test cell view.

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Figure 6. Test cell interior.

1 2

3 4

5 6

Figure 7. A sequence of high-speed photographs demonstrating hypergolic ignition of hydrogen peroxide with

kerosene. The time interval between sequent pictures is 2 µs.

plate holder

glass plate

heating

resistance

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The most important parameter to be evaluated is the ignition delay, defined as the time interval between

the moment the oxidizer drop and the fuel drop come into contact and the moment the fuel ignites. The experimental

system is presented in Figs. 4, 5 and 6. The ignition delay measured by a high-speed camera. The camera was set to

a frame rate of 500 fps, i.e., the time interval between sequent pictures was 2µs. It is possible to identify the frame

recording the moment the two drops come into contact and the frame recording the moment the ignition occurs. The

time lapse between these frames can be accounted as the ignition delay time.

IV. Experimental Results

Figure 7 depicts high-speed photographs (taken at 500 fps) of a hypergolicity test of 92% HP with a kerosene

gel containing NaBH4 suspended particles. The ignition delay is less than 8µs.

The experimental observations indicate that the idea of gelling kerosene and adding reactive particles that can

promote hypergolic ignition with oxidizers is feasible and can be used in “green” rocket systems.

V. Conclusion

Gelation of liquid fuel and suspension of reactive or catalyst particles enables hypergolic ignition of almost any

fuel-oxidizer combination. It allows using more energetic and environmentally safe hypergolic bipropellants as

hydrogen peroxide with kerosene. It eliminates the need for complicated and heavy ignition systems and the danger

of working with carcinogenic and toxic materials.

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