Thrust and Weight

The hot gases that exit from the rocket motor provide thrust. Rockets are (usually) launched vertically. The thrust must be bigger than the weight for the vehicle to start moving upwards. Usually, the ratio of thrust to weight is about 1.3, which means the initial acceleration is 0.3 times that due to gravity (the force driving the rocket upward is thrust minus weight). As fuel is burned off, the weight (and mass) decreases and the acceleration increases. The final velocity is governed by two factors: the speed of the exhaust gases (another way of expressing S. I.) and the ratio of the total weight of the rocket to the final, empty weight, as in the equation cited above. This latter figure becomes very important for satellite launchers, where the payload may be around only 1% of the initial weight. A small increase in weight can have a considerable impact on payload.

Thus if the Blue Streak motors could provide a total thrust of 300,000 lb, its maximum lift-off weight would be around 230,000 lb. If Blue Streak itself weighs 183,000 lb, then 47,000 lb is left for the upper stages and payload, only around 25%. A satellite launcher usually consists of three stages, and as each stage burns out, it is discarded. To make this staging as efficient as possible, the first stage should be around a half or two thirds of the total. Blue Streak was designed as a ballistic missile, and so was not as efficient a satellite launcher as it might have been.

The same problem applied to the early American launchers, but one way of overcoming the problem is to add strap-on solid fuel boosters to augment the thrust for the first part of the flight. These boosters might then provide extra thrust for the first 30 seconds or so of flight, whilst fuel from the main stage is being burned off and so reducing its weight.