a continuing series for the gravitationally impaired.
Big space rockets taking off with lots of engines blasting away is a common sight to most space enthusiasts. Von Braun's early designs used large numbers of small engines, up to 51 for the first stage rocket. The Russian R7 has held the title for 'most engines' for 47 years, lifting off with 32 chambers firing. The Saturn I had eight engines which is about the most any US rocket has ever had. There are two reasons for multiple engines. One, big engines are hard to develop with turbopump and combustion instability problems. If you cannot build a big engine, use lots of small engines to get the job done, that's the philosophy behind the R7 design. The other idea is that with multiple engines, you can complete the launch even if one or two engines fail, the rest of the engines will continue to fire. In the early years of aviation a lot of planes landed with one or two engines out, the Lockheed Constellation had the reputation as the best three engine airplane ever built. Unfortunately, the engine out idea has only worked twice, once on a Saturn I booster and once on a Saturn V second stage, both in test launches. Usually when a rocket engine fails it does so catastrophically, taking out other parts of the launch vehicle and destroying the stage. An abort system works much better than an engine out system, and several Russian cosmonauts will agree.
There is also a small seldom mentioned problem with multiple rocket engine installations, the thrust of all engines must pass through the Center of Gravity of the vehicle which oddly enough is at the center of gravity of the vehicle. The Saturn I's center engines canted at 3 degrees, the outer engines at 6 degrees, the Space Shuttle's main engines cant at a quite obvious angle at liftoff. The reason, any thrust that does not pass through the CG will turn the rocket. Getting exactly balanced thrust from two rocket engines on opposite sides of the vehicle is very difficult, but with the thrust of all engines passing through the CG any imbalance will only cause a lateral movement of the rocket not a turning moment. Canting an engine slightly reduces the forward thrust (1-cosine). At 6 degrees the loss is 0.6%, at 15 degrees it's 4%, not a big deal. It has been pointed out that the structural problems of canted engines are more significant (sine of angle). At 6 degrees the lateral force is 10%, at 15 degrees it's 26%. If you keep your engines close to the vehicle structure this won't be too much of a problem. The Space Shuttle however has it's engines at the ends of long lever arms. The lateral thrust of the solid boosters and the main engines all pass up a long distance and then through the bottom of the external tank causing real flexure and structural design problems. Not good engineering.
With good design you can build large low pressure big dumb single chamber rocket engines which have all of their thrust in one line right up the center of the rocket, so there is an advantage to building single engine boosters and stacking stages with single engines.