<PAGE>   1
                                                                 EXHIBIT 10.2.11


                                   FORM OF
                              ELEVENTH AMENDMENT TO
                            NOTE AGREEMENT AND WAIVER


          THIS ELEVENTH AMENDMENT TO NOTE AGREEMENT AND WAIVER ("ELEVENTH
AMENDMENT"), is made and entered into as of the 12th day of April, 2000, between
ORBITAL SCIENCES CORPORATION, a Delaware corporation (the "COMPANY"), and THE
NORTHWESTERN MUTUAL LIFE INSURANCE COMPANY, a Wisconsin corporation (the
"PURCHASER").


                                    RECITALS

          A.        The Purchaser is the holder of $13,333,333 12% Senior Notes
of the Company due June 14, 2001 (the "NOTES"). The Company and the Purchaser
are parties to that certain Note Agreement, dated as of June 1, 1995, the First
Amendment to Note Agreement dated as of June 30, 1995, the Second Amendment to
Note Agreement dated as of March 15, 1996, the Third Amendment to Note Agreement
dated as of July 31, 1996, the Fourth Amendment to Note Agreement dated as of
March 31, 1997, the Fifth Amendment to Note Agreement dated as of December 23,
1997, the Sixth Amendment to Note Agreement dated as of August 14, 1998, the
Seventh Amendment to Note Agreement dated as of May 27, 1999, the Eighth
Amendment to Note Agreement dated as of December 20, 1999, the Ninth Amendment
to Note Agreement dated as of January 31, 2000 and the Tenth Amendment to Note
Agreement and Extension of Waiver, dated as of February 22, 2000 (as amended,
supplemented or otherwise modified, the "NOTE AGREEMENT") whereby the Purchaser
purchased the Notes from the Company.

          B.        The Company and the Purchaser entered into that certain
Waiver Letter, dated as of October 31, 1999 (the "WAIVER LETTER"), whereby the
Purchaser waived certain Defaults or Events of Default arising or existing under
Sections 5.7, 5.8 and 5.9 of the Note Agreement until 11:59 p.m. C.S.T. on
December 31, 1999. Pursuant to that certain Letter Agreement, dated as of
December 23, 1999, between the Company and the Purchaser, the date and time of
termination of the Default Waiver Term (as defined in SECTION 1 of the Waiver
Letter) was extended to 5:00 p.m. C.S.T. on February 22, 2000. Pursuant to that
certain Tenth Amendment and Extension of Waiver dated as of February 22, 2000,
between the Company and the Purchaser, the date and time of termination of the
Default Waiver Term was further extended to 5:00 p.m. C.S.T. on April 30, 2000.
The Company and the Purchaser now desire to further extend the waivers of the
above-referenced covenants and waive defaults existing or arising under certain
other covenants in the Note Agreement on the terms set forth in this Eleventh
Amendment.

          C.        In addition, the Company and the Purchaser desire to amend
certain provisions of the Note Agreement as of April 12, 2000 (the "EFFECTIVE
DATE") in the respects, but only in the respects, set forth in this Eleventh
Amendment.

          D.        Capitalized terms used in this Eleventh Amendment have the
respective meanings ascribed thereto in the Note Agreement unless defined in
this Eleventh Amendment or the context otherwise requires.

          NOW, THEREFORE, upon full and complete satisfaction of the conditions
precedent to the effectiveness of this Eleventh Amendment set forth in SECTION 5
below, the Company and the Purchaser agree as follows:


<PAGE>   2

          SECTION 1. WAIVER

          Notwithstanding anything to the contrary set forth in the Note
Agreement, the Notes or any agreement or instrument relating to any of the
foregoing (collectively, the "NOTE DOCUMENTS"), the Purchaser waives any Default
or Event of Default existing or arising under (a) SECTION 5.6 of the Note
Agreement, (b) SECTION 5.7 of the Note Agreement, (c) SECTION 5.8 of the Note
Agreement, (d) SECTION 5.9 of the Note Agreement, or (e) SECTION 5.10 of the
Note Agreement (collectively, the "DEFAULT WAIVERS"); provided that the
effectiveness of the Default Waivers shall expire at 11:59 p.m. C.S.T. on
December 31, 2000 (the "DEFAULT WAIVER TERM"). In addition, the Purchaser waives
any Default or Event of Default under SECTION 5.17 of the Note Agreement
existing or arising on or prior to the Effective Date.

          SECTION 2. FUTURE AMENDMENT OF NOTE AGREEMENT

          2.1       The Company and the Purchaser shall use their respective
best efforts to enter into an Amended and Restated Note Agreement or an
Amendment to Note Agreement as the Purchaser shall deem appropriate (the
"AMENDMENT"), in either such case providing, inter alia, for (i) covenants that
will amend and restate those covenants set forth in SECTIONS 5.6, 5.7, 5.8, 5.9
and 5.10 of the Note Agreement, and (ii) such other terms and provisions as may
be considered necessary by the Purchaser.

          2.2       In connection with the execution and delivery of the
Amendment, the Company shall cause to be delivered to the Purchaser, (i) a
favorable written opinion of Hogan & Hartson L.L.P., counsel to the Company,
with respect to the due authorization, execution and delivery of the Amendment
and the enforceability of the same in accordance with its terms; and (ii) a copy
of the resolutions of the Board of Directors of the Company authorizing the
execution, delivery and performance by the Company of the Amendment certified by
its Secretary or Assistant Secretary.

          SECTION 3. AMENDMENTS

          From and after the Effective Date the Note Agreement shall be and
hereby is amended as follows:

          3.1.      SECTION 5.10 of the Note Agreement is hereby amended as
follows:

          (a)       Clause (i) is amended by deleting the period at the end
thereof and substituting in lieu thereof the text "; and".

          (b)       A new Clause (j) is added following Clause (i) as follows:

                    "(i)      liens on the assets of MacDonald, Dettwiler and
          Associates and its subsidiaries securing debt and other obligations of
          MacDonald, Dettwiler and Associates and such subsidiaries under the
          MDA Financing."

          3.2.      SECTION 5.13(c) of the Note Agreement is hereby amended as
follows:

          (a)       Clause (9) is hereby amended by substituting a semicolon for
the period and adding the word "or" at the end thereof.


                                     - 2 -
<PAGE>   3

          (b)       A new Clause (10) is added following Clause (9) as follows:

                    "(10)     the pledge by MacDonald, Dettwiler and Associates
          or any subsidiary thereof of the shares of any subsidiary of
          MacDonald, Dettwiler and Associates pursuant to the MDA Financing."

          3.3.      SECTION 8.1 of the Note Agreement is amended by adding the
following definitions in their proper alphabetical order:

          ""Covenant Restatement Date" - shall mean that date following the
Eleventh Amendment Effective Date upon which the Purchaser and the Company enter
into an Amendment to this Note Agreement, or an Amended and Restated Note
Agreement, in accordance with the terms and conditions set forth in SECTION 2 of
the Eleventh Amendment."

          ""Current Interest Rate"- shall mean the following:

          (a)       From the Eleventh Amendment Effective Date and thereafter,
12%;

          (b)       In the event that the Covenant Restatement Date shall not
have occurred on or prior to May 31, 2000, from June 1, 2000 and thereafter,
13%;

          (c)       In the event that the Covenant Restatement Date shall not
have occurred on or prior to August 31, 2000, from September 1, 2000 and
thereafter, 14%; and

          (d)       In the event that the Covenant Restatement Date shall not
have occurred on or prior to November 30, 2000, from December 1, 2000 and
thereafter, 15%."

          ""Eleventh Amendment" shall mean the Eleventh Amendment to Note
Agreement and Waiver between the Company and the Purchaser dated as of April 12,
2000."

          ""Eleventh Amendment Effective Date" shall mean April 12, 2000."

          ""MDA Financing" means the credit agreement to which MacDonald,
Dettwiler and Associates will become party providing for loans thereunder to be
used by MacDonald, Dettwiler and Associates for working capital purposes and
________________________; provided that the aggregate principal amount of debt
that may be incurred under such credit agreement (i) shall not exceed
$210,000,000 Canadian Dollars and (ii) shall be non-recourse to the Company."

          3.4.      The following paragraph shall be inserted immediately
following the first paragraph of the Notes and Exhibit A to the Note Agreement:

                    "Notwithstanding the foregoing, from and after the Eleventh
     Amendment Effective Date, (a) the 12% interest rate referenced in the
     foregoing paragraph shall be modified to equal the Current Interest Rate
     (as defined in the Note Agreement, as amended), and (b) the rate set forth
     in clause (b)(1) of the definition of "Overdue Rate" set forth above shall
     be modified to equal the Current Interest Rate as then in effect, plus 2%."

          SECTION 4. REPRESENTATIONS AND WARRANTIES OF THE COMPANY

          To induce the Purchaser to execute and deliver this Eleventh
Amendment, the Company represents and warrants to the Purchaser (which
representations will survive the execution and delivery of this Eleventh
Amendment) that:


                                     - 3 -
<PAGE>   4

          (a)       this Eleventh Amendment has been duly authorized, executed
and delivered by the Company and constitutes the legal, valid and binding
obligation of the Company, enforceable against it in accordance with its terms,
except as enforcement may be limited by bankruptcy, insolvency, reorganization,
moratorium or similar laws or equitable principles relating to fraudulent
conveyance or limiting creditors' rights generally;

          (b)       the Note Agreement, as modified by this Eleventh Amendment,
constitutes the legal, valid and binding obligation of the Company, enforceable
against it in accordance with its terms, except as enforcement may be limited by
bankruptcy, fraudulent conveyance, insolvency, reorganization, moratorium or
similar laws or equitable principles relating to or limiting creditors' rights
generally;

          (c)       the execution, delivery and performance by the Company of
this Eleventh Amendment (i) has been duly authorized by all requisite corporate
action and, if required, shareholder action, (ii) does not require the consent
or approval of any governmental or regulatory body or agency, and (iii) will not
(A) violate (1) any provision of law, statute, rule or regulation or its
certificate of incorporation or bylaws, (2) any order of any court or any rule,
regulation or order of any other agency or government binding upon it, or (3)
any material provision of any material indenture, agreement or other instrument
to which it is a party or by which its properties or assets are or may be bound,
or (B) result in a material breach or constitute (alone or with due notice or
lapse of time or both) a default under any indenture, agreement or other
instrument referred to in clause (iii)(A)(3) of this Section 4(c); and

          (d)       as of the date hereof and after giving effect to this
Eleventh Amendment, no Default or Event of Default has occurred which is
continuing.

          SECTION 5. CONDITIONS AND AGREEMENTS

          Upon fulfillment or receipt of all of the following, as the case may
be, this Eleventh Amendment will on the Effective Date become effective:

          (a)       executed counterparts of this Eleventh Amendment, duly
executed by the Company and the Purchaser, have been delivered to the Purchaser;

          (b)       the representations and warranties of the Company set forth
in Section 4 of this Eleventh Amendment will be true and correct on and with
respect to the date hereof;

          (c)       the Company has obtained any consents or approvals required
to be obtained from any holder or holders of any outstanding security of the
Company and any amendments of agreements pursuant to which any security may have
been issued which will be necessary to permit the consummation of the
transactions contemplated by this Eleventh Amendment; and

          (d)       a waiver fee in the amount of $33,333.33 by wire transfer in
immediately available funds to the account specified on Schedule 1 to the Note
Agreement.

          SECTION 6. MISCELLANEOUS

          6.1       This Eleventh Amendment will be construed in connection with
the Note Agreement, and except as modified by this Eleventh Amendment, all
terms, conditions and covenants contained in the Note Agreement and the Note are
hereby ratified and will be and remain in full force and effect.


<PAGE>   5

          6.2.      The descriptive headings of the various sections or parts of
this Eleventh Amendment are for convenience only and will not affect the meaning
or construction of any of the provisions hereof.

          6.3.      This Eleventh Amendment will be governed by and construed in
accordance with the internal laws of the State of Illinois.

          6.4.      This Eleventh Amendment may be executed in any number of
counterparts, each executed counterpart constituting an original, but all
together only one agreement.

          IN WITNESS WHEREOF, the Company and the Purchaser have caused this
Eleventh Amendment to be executed and delivered by their respective duly
authorized representatives.


                                     ORBITAL SCIENCES CORPORATION


                                     By:
                                         ---------------------------------
                                     Title:


                                     THE NORTHWESTERN MUTUAL LIFE
                                        INSURANCE COMPANY


                                     By:
                                         ---------------------------------

                                           Its Authorized Representative


                                     - 5 -