Evening HP,
A clue to sweeten the pill:
What goes up must always come down (and then up, and then down, and then up again...
Could it be the DFS-332 research glider/rocket
I've only found a thumbnail and a drawing and neither give me much hope, but the description of its planned use kinda fits the clue
After being towed to altitude, the crew would make a series of dives at different speeds. However, to be able to perform tests at higher speeds, the aircraft was also fitted with two Walter HWK-R II/203 rocket engines, delivering 750 kg of thrust each. They were installed in the fuselages with rocket nozzles under the lower fuselage, aligned 10 degrees down.
Abend Reg,
Sorry about the photo mate
; but it was the only one I could find that looked like your challenge, and of course, I was never going to find the original 'coz it wasn't on the web!!
But it is now 
, Good challenge!!