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Exhibit 8.1

|  | 12531 HIGH BLUFF DRIVE SUITE 100 SAN DIEGO, CALIFORNIA 92130-2040 TELEPHONE: 858.720.5100 FACSIMILE: 858.720.5125 WWW.MOFO.COM |  | MORRISON & FOERSTER LLP NEW YORK, SAN FRANCISCO, LOS ANGELES, PALO ALTO, SAN DIEGO, WASHINGTON, D.C. NORTHERN VIRGINIA, DENVER, SACRAMENTO, WALNUT CREEK TOKYO, LONDON, BEIJING, SHANGHAI, HONG KONG, SINGAPORE, BRUSSELS

November 24, 2008

Kratos Defense & Security Solutions, Inc.
Bridge Pointe Corporate Centre
4810 Eastgate Mall
San Diego, California 92121

Re:
  Registration Statement on Form S-4

Ladies and Gentlemen:

We have acted as counsel to Kratos Defense & Security Solutions, Inc., a Delaware corporation ("Parent"), in connection with the preparation and execution of the Agreement and Plan of Merger (the "Agreement") dated as of November 21, 2008, among Parent, Dakota Merger Sub, Inc., a Delaware corporation and a direct wholly-owned subsidiary of Parent ("Merger Sub"), and Digital Fusion, Inc., a Delaware corporation (the "Company"). Pursuant to the Agreement, Merger Sub will merge with and into the Company, with the Company surviving the merger (the "Merger"). The Merger is described in the Registration Statement on Form S-4 (the "Registration Statement") of Parent, which includes the Proxy Statement/Prospectus of the Company and Parent. Unless otherwise indicated, any capitalized terms used herein and not otherwise defined have the meaning ascribed to them in the Registration Statement.

In connection with this opinion, we have examined and are familiar with the Agreement, the Registration Statement, and such other presently existing documents, records and matters of law as we have deemed necessary or appropriate for purposes of our opinion. In addition, we have assumed (i) that the Merger will be consummated in the manner contemplated by the Registration Statement and in accordance with the Agreement, without waiver or modification of the material terms and conditions thereof, (ii) the truth and accuracy, on the date of the Agreement and on the date hereof, of the representations and warranties made by Parent and the Company in the Agreement, (iii) the truth and accuracy of the officer's certificates dated November 24, 2008 (the "Officer's Certificates") provided to us by Parent and the Company for use in preparing our opinion, and (iv) that any representation in an Officer's Certificate made "to the knowledge of" or similarly qualified is correct without such qualification.

The conclusion expressed herein represents our judgment of the proper treatment of certain aspects of the Merger under the income tax laws of the United States based upon the Internal Revenue Code of 1986, as amended (the "Code"), Treasury Regulations, rulings and other pronouncements of the Internal Revenue Service (the "IRS") currently in effect, and judicial decisions, all of which are subject to change, prospectively or retroactively. No assurance can be given that such changes will not take place, or that such changes would not affect the conclusion expressed herein. Furthermore, our opinion represents only our best judgment of how a court would conclude if presented with the issues

addressed herein and is not binding upon either the IRS or any court. Thus, no assurance can be given that a position taken in reliance on our opinion will not be challenged by the IRS or rejected by a court.

Our opinion relates solely to the tax consequences of the Merger under the federal income tax laws of the United States, and we express no opinion (and no opinion should be inferred) regarding the tax consequences of the Merger under the laws of any other jurisdiction. This opinion addresses only the specific issue set forth below, and does not address any other tax consequences that may result from the Merger or any other transaction (including any transaction undertaken in connection with the Merger).

No opinion is expressed as to any transaction other than the Merger as described in the Agreement or as to any transaction whatsoever, including the Merger, if all of the transactions described in the Agreement are not consummated in accordance with the terms of the Agreement and without waiver or breach of any material provision thereof, or if all the representations, warranties, statements and assumptions upon which we rely are not true and accurate at all relevant times. In the event any one of the statements, representations, warranties or assumptions upon which we rely to issue this opinion is incorrect, our opinion might be adversely affected and may not be relied upon.

Based upon and subject to the foregoing, we are of the opinion that the Merger will constitute a "reorganization" within the meaning of Section 368(a) of the Code, and each of Parent, Merger Sub and the Company will be "a party to a reorganization" within the meaning of Section 368(b) of the Code.

This opinion is being delivered to you solely for use in connection with the Registration Statement and may not be relied upon for any other purpose. Although you (and each of your employees, representatives, or other agents) may disclose to any and all persons, without limitation of any kind, the United States federal tax treatment and United States federal tax structure of the Merger and all materials of any kind that were provided to you by us relating to such tax treatment and tax structure, this opinion is intended solely for your benefit. You may not authorize any other person or entity to rely on this opinion, or otherwise make this opinion available for the benefit of any other person or entity, without our prior written consent.

We undertake no obligation to update this opinion, or to ascertain after the date hereof whether circumstances occurring after such date may affect the conclusions set forth herein. We express no opinion as to matters governed by any laws other than the Code, the Treasury Regulations, published administrative announcements and rulings of the IRS, and court decisions.

We hereby consent to the filing of this opinion as an exhibit to a Current Report on Form 8-K of the Company and to such Registration Statement. We also consent to the reference to our firm name wherever appearing in the Registration Statement. In giving this consent, we do not admit that we come within the category of persons whose consent is required under Section 7 of the Securities Act or the rules and regulations of the SEC thereunder, nor do we thereby admit that we are experts with respect to any part of the Registration Statement within the meaning of the term "experts" as used in the Securities Act or the rules and regulations of the SEC promulgated thereunder.

Very truly yours,

/s/ Morrison & Foerster LLP

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Exhibit 8.1