Keeping Up Rhythmical Momentum
One thing many learning composers/orchestrators struggle with in orchestral music is to keep up a rhythmical momentum without relying on a drum kit or trailer music like ostinatos or trailer music like use of the percussion section (as a surrogate drum kit), which in all cases carry the rhythmical momentum on their own.
It is important to keep an eye on the overall movement of the music and make sure that it doesn’t stall on one or several spots. For instance if your main melody mainly moves in 8th notes but at the end of its melodic phrase holds a whole note, you will lose most of the rhythmical momentum that has been established beforehand in this one bar if you don’t counteract with another voice/instrument that takes over the 8th note movement in a side line etc.
In such cases, it helps to reduce all the voices down to their rhythmical activity and check whether the 8th note movement is kept alive by any voice at practically any moment you want the rhythmical momentum to push forward.
But also be warned to give back and forth the rhythmical activity in small chunks between several voices and instruments in a too active way as this will create a very nervous effect and should only be used in special cases. And also as usual, trust your ear. Sometimes it works perfectly fine to give the music a rhythmical rest and pick that up again after that break instead of keeping the rhythmical movement rolling.