OHKLA Design/Propellant choice

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This document is a leaf node in the Design Task Tree for the OHKLA project.


[edit] Summary

The purpose of this design task is to select the propellants (fuel and oxidiser) for the OHKLA hybrid rocket engine. This involves selecting a solid fuel and a liquid or gaseous oxidiser.

[edit] Background

Bla, bla, bla.

[edit] Features/Considerations

[edit] Safety

The section of the CSTART Social Contract entitled "Dedication to Safe and Responsible Operation " states that "All rockets and spacecraft whose construction or launch are organized or financed by CSTART shall contain materials which are flammable, explosive, poisonous, toxic, corrosive, radioactive, carcinogenic or infectious only when their presence can be justified by legitimate engineering or scientific concerns, and then only when alternatives without these properties are for some reason unsuitable. Where used, these materials shall be present in the minimum quantities practical and their transport, storage and use shall be in accordance with industry standard safety procedures". These are important stipulations to bear in mind when selecting propellants for OHKLA. Propellants with the listed properties above not only make the project more dangerous (possibly causing problems with getting the required licenses/permits), they also make the project more expensive (since greater care and training is required to safely store and transport materials) and potentially less efficient (valves which are rated for cryogenic liquids are heavier than those which are not, for instance).

[edit] Simplicity

Simplicity is the essence of the CSTART Design Philosophy. There are several ways in which this will affect OHKLA propellant selection:

[edit] Fuel grain convenience

Ideally, the choice of fuel should be such that CSTART (and anybody else!) can readily cast its own fuel grains without excessive difficulty, cost, or special equipment. This means the availability and cost of fuel materials should be considered, as should various physical properties, for example:

  • Does the material expand or contract significantly when solidifying?
  • Does the material emit hazardous fumes when liquid?
  • Can combustion ports be drilled in the material without the grain crumbling?
  • Is the material able to withstand high acceleration?

Even fuels for which the answers to the above questions are the "wrong" answers can be made to work, but each "wrong" answer adds an additional layer of necessary complexity for things to work.

[edit] Oxidiser pressurisation

Most oxidisers must be forced out of their tank and into the combustion chamber by means of a pressurant gas, which is non-reactive and is stored at very high pressure in a separate tank (helium is a common choice of pressurant). However, some oxidisers are self pressurising, removing the need for a separate pressurant tank and valve and resulting in a simpler (and lighter) overall design.

[edit] Oxidiser vaproisation

If an oxidiser is stored in its tank in a liquid state, it is important that there be a sufficiently large and/or well-heated precombustion chamber in which it can vaporise before entering the combustion port(s) and taking part in combustion. If these conditions are not met, "blobs" of liquid oxidiser can enter the combustion port(s), causing combustion instability (sudden strong changes in combustion chamber pressure). If an oxidiser can be stored in a gaseous state, however, this risk does not exist, removing the need for a precombustion chamber (or at least allowing a smaller chamber) and the need for precombustion chamber heating.

[edit] Performance

The most obvious feature to take into account when selecting a propellant is performance, which in this case is best measured by specific impulse. High specific impulse is important and attractive, however it is not the end-all-be-all property to consider. The CSTART Design Philosophy explicitly states that "Simplicity is more important than performance". Density of propellants will also play an important factor in performance. High density solid fuels will result in the smallest possible fuel grains and hence the smallest possible combustion chambers. Since the combustion chamber must be able to withstand very high temperatures and pressures, it will be one of the heaviest parts of the rocket. Keeping the fuel grain size as small as possible will hence help to keep overall rocket mass low.

The table below provides the sea-level specific impulse for a number of common hybrid rocket propellant choices:

Combustion chamber pressure 500 psia, external pressure 14.7 psia

Fuel Oxidiser Optimum O:F Specific impulse (sea level)
Carbon LOX 1.9 249
Carbon N2O 6.3 236
HTPB LOX 1.9 280
HTPB N2O 7.1 247
HTPB N2O4 3.5 258
HTPB RFNA 4.3 247
HTPB FLOX 3.3 314
Paraffin LOX 2.5 281
Paraffin N2O 8.0 248
Paraffin N2O4 4.0 259
Polyethylene LOX 2.5 279
Polyethylene N2O 8.0 247
PMM LOX 1.5 259

[edit] Requirements

Formalisation of the above into hard numbers: maximum and minimum melting, boiling and flash points for all materials, minimum and maximum densities, tensile and shear strength measures, etc, etc.

[edit] Available Solutions

[edit] Fuels

[edit] HTBP

Pros: Cons:

[edit] Paraffin


  • Extremely easy and safe to acquire and work with
  • Has high regression rate due to liquid boundary layer effects


  • Expands during solidification process, complicating fuel grain casting
  • Is structurally weak at high temperatures, possibly causing grain to "slump" during high acceleration

[edit] Polyethylene

Pros: Cons:

[edit] Oxidisers

[edit] Dinitrogen tetroxide (N2O4)


  • Offers moderate specific impulse when burned with most fuels (relative to other oxidisers)
  • Storable


  • Not very safe: EU Dangerous Substances Directive ratings "Corrosive" and "Very toxic".
  • Requires use of pressurant gas (Helium etc.)

[edit] Liquid oxygen (LOX)


  • Offers high specific impulse when burned with most fuels (relative to other oxidisers)


  • Is cryogenic (raises safety and simplicity issues)
  • Not storable (due to boil-off)
  • Requires use of pressurant gas (Helium etc.)

[edit] Nitrous oxide (N2O)


  • Storable
  • Is quite safe: non-flammable, non-toxic, non-cryogenic, etc.
  • Is self-pressurising, so does not require pressurant gas (so less tanks, valves, etc.)


  • Offers low specific impulse when burned with most fuels (relative to other oxidisers)

[edit] Red fuming nitric acid (RFNA)


  • Storable


  • Offers low specific impulse when burned with most fuels (relative to other oxidisers)
  • Not very safe: US National Fire Protection Association health code 4 (Very short exposure could cause death or major residual injury).
  • Requires use of pressurant gas (Helium etc.)

[edit] Supporting material/calculations

[edit] Volume requirement calculations

Once we have specific impulse, optimal fuel to oxidiser ratios and fuel and oxidiser density figures for all propellant choices, it will be possible to find the required volume of fuel grain and oxidiser tank for each choice, which will be an important factor in making our decision.

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