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                                                                EXHIBIT 10.6

                         SPACELINK INTERNATIONAL LLC
                        SPACELINK INTERNATIONAL, LTD.
                             SATCOMSOLUTIONS LLC



                               CLOSING LETTER
                               --------------



                              February 7, 2005



Engineered Support Systems, Inc.
201 Evans Lane
St. Louis, MO 63121-1126
Attn: Gary Gerhardt, Vice Chairman and CFO

         Re: Purchase Agreement dated December 9, 2004

Dear Sirs:

         Reference is made to the Purchase Agreement by and between
Engineered Support Systems, Inc. ("Buyer"), Spacelink International LLC (the
"Company"), Spacelink International, Ltd. ("Spacelink Ltd"), and
Satcomsolutions LLC ("Satcom;" and, together with Spacelink Ltd, the
"Sellers"), dated December 9, 2004 (the "Purchase Agreement"). Capitalized
terms used and not otherwise defined in this letter have the meanings given
to them in the Purchase Agreement. Unless otherwise specified in this
letter, references to Sections and Schedules shall mean Sections of and
Schedules to the Purchase Agreement.

         1. Closing and Effective Date. The Parties hereby agree that the
            --------------------------
Closing Payment and the Company Funded Obligations shall be paid, and the
certificates evidencing the ESSI Stock shall be delivered, on the date
hereof, but that for all other purposes the Closing shall be effective and
deemed to have taken place on and as of February 1, 2005. Without limiting
the previous sentence, all revenues generated and expenses incurred by the
Company after 12:01 a.m. on February 1, 2005 shall be for the account of
Buyer.

         2. No Material Adverse Change. The Sellers hereby represent and
            --------------------------
warrant to Buyer that since January 31, 2005 there has not occurred any
event, occurrence, development or state of circumstances or facts involving
the Company which has had or, in so far as can be reasonably foreseen, would
have a material adverse effect on the condition (financial or otherwise),
business, properties, liabilities or results of operations of the Company.



         Please acknowledge your agreement with this letter by signing in
the space indicated below.

                                        Sincerely,

                                        SPACELINK INTERNATIONAL LLC



                                        By: /s/ Laura Hoernig Rhodes
                                           --------------------------------
                                                Name:  Laura Hoernig Rhodes
                                                Title: Vice President


                                        SPACELINK INTERNATIONAL, LTD.



                                        By: /s/ Laura Hoernig Rhodes
                                           --------------------------------
                                                Name:  Laura Hoernig Rhodes
                                                Title: Vice President


                                        SATCOMSOLUTIONS LLC



                                        By: /s/ Laura Hoernig Rhodes
                                           --------------------------------
                                                Name:  Laura Hoernig Rhodes
                                                Title: Vice President


Acknowledged and Agreed to on
February 7, 2005:

ENGINEERED SUPPORT SYSTEMS, INC.



By: /s/ Gary Gerhardt
   ------------------------------------
Name:  Gary Gerhardt
Title: Vice Chairman of Administration and
       Chief Financial Officer




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