== New equation == 

 In the new section Formal expression the formula 

 : 

 is quoted from the paper General Theory of Optimal Trajectory for Rocket Flight in a Resisting Medium, with the explanation: 

 :"The first term on the right hand side is the part of the force which may be called "gravity drag"." 

 I am struggling to see the relevance of this equation to "gravity drag" because: 
 #The paper makes no mention of the terms "gravity drag" or "gravity losses" in relation to this equation (or anywhere else that I can see). 
 #In the equation quoted, the vector A is identified as "aerodynamic force" i.e. lift (see Figure 1 in the paper). As there is no lift in space, this term will be 0 a spacecraft in orbit. 
 #The vectors pr and pv appear from context to be momentum vectors, but their connectrion to physical characteristics of the spacecraft is not made clear. 
 Can anyone provide a clearer explanation of the relevance of this equation ?