RSE37 rse37

CONSTANT VERSUS DECAYING THRUST RSE # 37, MCD a continuation of #35

A simple vapor pressure propellant feed system with decaying tank pressure has a lot of cost and operational advantages, but it does reduce the end of burn thrust and reduces the end of burn ISP. The question as always is how much, and are the cost savings greater than the loss of performance. Will the added weight and expense of high pressure gas tanks and an expensive high flow regulator system added to the rocket to maintain propellant feed pressure overcome the added weight and cost?

In comparing two rockets, one using a gas pressurization system, and the other using propellant vapor pressure, the down sides are; a 52.2% reduction in end of burn thrust due to reduced flow (Torricilli's equation, v=sqr 2Gh) into the combustion chamber (39.2%) and reduced ISP (13%). However the actual total loss of overall impulse is 6.5% one half of the 13% ISP end of burn loss. Burn duration is extended 29% due to the reduced flow. This extended burn duration causes some increase in the gravity loss, but this is more than offset by the reduction in weight from the removal of the pressure tanks and regulator, giving an actual gain in performance. Just to be equal in performance the gas pressurization system must add less than 15% to the weight of the rocket, and that does not include the cost, which can add 25% to the price of the rocket. The upsides, an actual gain in performance, lower costs, simpler operation. This is called a winner.