  It is shown that multi M5-brane inflation in heterotic M-theory gives rise to
a detectable gravitational wave power spectrum with tensor fraction $r$
typically larger than the projected experimental sensitivity, $r_{exp} = 0.01$.
A measurable gravitational wave power spectrum entails a large inflationary
energy scale and a super-Planckian inflaton variation. They present serious
problems for particle theory model building resp. a reliable effective field
theory description. These problems are eased or even absent in multi-brane
inflation models and multi M5-brane inflation, in particular.
