You'll need to include the module name as well ( "Contact.Contact_Application").
But I would recommend using the proxy classes for this. This can be done using the following code:
List<IMendixObject> NBapplicants = Core.retrieveXPathQueryEscaped(this.getContext(),
"//%s[%s = %s]",
Contact.entityname,
Contact.MemberNames.Contact_Application.toString(),
applicationID);
If you use the proxy classes for this any errors in the member names will show up as compile error instead of as runtime error. Let's say you would rename your contact entity or relationship you would not know your code was broken until you execute it. When using the proxy classes you would get a compile error because the proxy value no longer exists.