 After the HP-18, Schreder tried his hand at manufacturing carbon-fiber composite wing spars, and abandoned it as too troublesome after making the one set, which was used by Henry Preiss to complete the one-off HP-19.Schreder's HP-20 combined the HP-19 fuselage with an innovative new wing spar consisting of leaves of aluminum laminated and bonded together under pressure.The HP-21 was intended to be a variable-geometry self-launching sailplane based on the RS-15 fuselage and featuring retractable sailcloth wing extensions along the inboard trailing edges of its constant-chord wing, but was abandoned before completion.