Preliminary OHKLA design | Rocket body discussion | Forum

Post edited 1:24 am – February 11, 2010 by Luke Maurits

It's good to see this issue starting to receive some consideration.  My only thoughts are:

1. Is 2MPa inside the combustion chamber enough?  I thought we found out that 3-4 was a more likely value?
2. I think the oxidiser tank will be able to be thinner than the combustion chamber, due to lower pressure.  Here's a rough guidline, from here:
   

   At room temperature, the pressure of the Nitrous Oxide in a pressurised oxidiser tank is around 750 psi or 54 bar. The pressure in the tank will increase in warmer temperatures, and the tank pressure will decrease in colder temperatures. The result of this, is that in warmer temperatures, a hybrid motor will burn for a shorter amount of time at a higher level of thrust, and in colder temperatures, a hybrid rocket motor will burn for a longer amount of time at a lower amount of thrust. The following table provides an approximate guideline:

   Table 2: Nitrous Oxide Vapour Pressure / Temperature

   Vapour Pressure (psi)	Temperature   (° C)
   460	0
   520	5
   600	10
   680	15
   760	21
   860	27

   Maybe you can use these figures to get a more suitable oxidiser tank wall width?

3. I'm not sure what you had in mind when you spoke fo finding the volumes to be used for the oxidiser and fuel grain.  It is impossible to do this before we decide on propellants, because propellant-specific figures like specific impulse and density will be what determines the volumes.

Good work!

rpulkrabek said:

1. My mistake, this was done with 3MPa. I should start proofreading my posts! (I'll edit the original to say 3MPa)

2. If I use the value for vapor pressure at 27 deg C, which could be appropriate temperature If we try to launch it from a location as close to the equator as we can get, the the pressure is nearly 6MPa. This would mean an even thicker wall thickness if we were to use an aluminum alloy.

3. I meant that we can't determine actual dimensions until the decisions such as what propellants are used. I am just trying to avoid the misconception that we are making decisions already on the geometry and not documenting them correctly.

1. Good to know. :)
2. Oh wow.  I hadn't realised the N2O pressure was that high.  Maybe we should try to find figures for LOX oxidiser with helium or nitrogen pressurant.  If the LOX pressure is quite low then maybe the oxidiser tank could be less massive?  Another thing to bear in mind: with N2O we could use a composite material tank (SpaceShipOne did this) and save a lot of mass.  This is not an option with LOX because composite tanks for cryogenic storage are very experimental/immature at the moment: probably quite expensive and not as reliable.
3. Ah, understood, no problems.

Post edited 6:42 am – February 19, 2010 by Luke Maurits

rpulkrabek said:

 The materials were assigned only for the purpose of determining mass.

Does ProE have material values for cobalt by any chance?  This would be a more likely choice of nozzle material than aluminium, I think (it has a significantly higher melting point) and is considerably more dense.  The mass of the nozzle is probably a small contribution to the overall rocket mass, but we shoudl still try to get things approximately correct if we can.

EDIT: Graphite would be another good choice as well.

Another thought: all the current OHKLA images showing the interior structure of the rocket leave no room for at all for parachute stowage.  We will in fact need room for two parachutes, one for the nosecone with the avionics and one for the booster body (even if we don't intend to reuse any of the booster body, I don't think we'll be able to get away with just letting it free fall.  On a suborbital reentry there may not be enough heat to fully break it up, and the entire thing hitting the ground at terminal velocity would be extremely dangerous).  I have no intution at all for how large our chutes will be, but a few quick calculations of mine suggest that the fuel grain and oxidiser tank alone won't require 6m of height at all, so the parachute space may in fact be quite large.

The obvious arrangement seems to be [NOSECONE][NOSECONE PARACHUTE][BOOSTER PARACHUTE][BOOSTER BODY].  Maybe we should look into getting an estimate of chute space requirements so that the next generation of renders can include this.