Rocket Motor Throttling

[gtnick]

I went to astronautix and found the time to burn for a certain rocket motor, but I want to be able to throttle the engine through the use of a valve to the fuel or oxidizer, anyone know exactly how this will affect the time to burn? It should increase time to burn because you are throttling down the amount of fuel you are using but you are also burning at less than optimal efficiency so there will be losses to the length of time to burn as well. Any thoughts or references?

 

[gtnick]

Right, but how would this affect your overall time to burn?

 

[henryhallam]

If you know how the Isp varies at reduced thrust (due to the inefficiency you mentioned) you can work out the rate of propellant consumption and therefore the time to burn.<br /><br />It can be shown that mass flow rate = thrust /(g*Isp)<br /><br />If you are throttling the rocket to a constant reduced thrust for the entire launch, time to burn = (wet mass - dry mass) / mass flow rate.<br />If the rocket follows a variable thrust profile you will have to integrate over time using Isp as a function of thrust. This can be done numerically using Matlab (or excel, in a pinch). All this is valid for a single stage, of course. If it is a multi-stage rocket you have to do this for each stage separately.<br />

 

[barrykirk]

Use a smaller engine that is able to stay at optimal efficiency because it is at design thrust?<br /><br />Or did you want a variable amount of thrust during the burn?

 

[gtnick]

Yes, I need a variable amount of thrust.

 

[mlorrey]

Cluster many smaller rockets and use zero throttling. Just turn engines off as you need to throttle down (drop them off when you shut them down if you want to significantly improve performance).

 

[propforce]

<font color="yellow">...I want to be able to throttle the engine through the use of a valve to the fuel or oxidizer, anyone know exactly how this will affect the time to burn? </font><br /><br />There are solid rocket motors (SRM) available using hot gas valve to throttle, but they tend to very fairly short duration (less than 1 minute) type of application. Also the thrust are not very high with applications mainly for attitude control. <br /><br />Another method is if you know the throttling is a very repeatable, such as the Shuttle SRB for example, you can work with the solid propellant houses to shape the grain sizes, MR, and burn rate to have pre-determined throttling profile. <div class="Discussion_UserSignature"> </div>