I have explored the Approval workflow in depth since I had to write a detailed guide. You are aware that the workflow has a setting (that you can activate via sharepoint designer) that lets you 'start the workflow automatically' and/or "start the workflow manually" (the 2 options are not in opposition, as a matter of fact you can leave them both checked), however I have noticed that if you check only the 'start the workflow automatically, it will not show you the dialogue box with all the details (you will be shown that dialogue box only the first time, when you activate it), so the user is not able to change the approvers or the days required to approve etc.
If you use the 'manually start the workflow' option, the pop up window will show up every time.
The other option is to re-create the workflow entirely and leaving the approvers in the details of the workflow as fixed approvers.
However, by experience please consider the fact that one of the approvers might be on holiday, unavailable or sick. Your file will never be approved, as there is no way to force the approval of an absentee.
I do not have access to SPD right now, but look at this image, find in your SPD your workflow (EDIT) and see the below options