Exhibit 5.1

Goodwin Procter LLP

601 Marshall St.

Redwood City, CA 94063

goodwinlaw.com

+1 650 752 3100

September 15, 2025

Rocket Lab Corporation
3881 McGowen Street

Long Beach, CA 90808

Re:	Securities Registered under Registration Statement on Form S-3ASR

We have acted as counsel to you in connection with the Registration Statement on Form S-3ASR (File No. 333-285707) filed on March 11, 2025 by Rocket Lab USA, Inc. (the “Predecessor”) and Post-Effective Amendment No. 1 to such Registration Statement on Form S-3ASR filed on May 27, 2025 (as amended or supplemented, the “Registration Statement”) by Rocket Lab Corporation, a Delaware corporation (the “Company”), with the Securities and Exchange Commission (“Commission”) pursuant to the Securities Act of 1933, as amended (the “Securities Act”), which became effective upon its filing. The Company became the successor to the Predecessor on May 23, 2025 for purposes of Rule 414 under the Securities Act as a result of the merger of the Predecessor with the Company’s wholly-owned subsidiary, Rocket Lab Merger Sub, Inc., a Delaware corporation, with the Predecessor being the surviving corporation. Reference is made to our opinion letter dated March 11, 2025 and included as Exhibit 5.1 to the Registration Statement. We are delivering this supplemental opinion letter in connection with the prospectus supplement (the “Prospectus Supplement”) filed on September 15, 2025 by the Company with the Commission pursuant to Rule 424 under the Securities Act. The Prospectus Supplement relates to the offering by the Company of up to $750,000,000 in shares (the “Shares”) of Common Stock, $0.0001 par value per share (the “Common Stock”), covered by the Registration Statement. The Shares are being offered and sold by the sales agents named in, and pursuant to, a sales agreement among the Company and such sales agents.

We have reviewed such documents and made such examination of law as we have deemed appropriate to give the opinion set forth below. We have relied, without independent verification, on certificates of public officials and, as to matters of fact material to the opinion set forth below, on certificates of officers of the Company.

For purposes of the opinion set forth below, we have assumed that the Shares are issued for a price per share equal to or greater than the minimum price authorized by the Company’s board of directors prior to the date hereof (the “Minimum Price”) and that no event occurs that causes the number of authorized shares of Common Stock available for issuance by the Company to be less than the number of then unissued Shares that may be issued for the Minimum Price.

For purposes of the opinion set forth below, we refer to the following as “Future Approval and Issuance”: (a) the approval by the Company’s board of directors (or a duly authorized committee of the board of directors) of the issuance of the Shares (the “Approval”) and (b) the issuance of the Shares in accordance with the Approval and the receipt by the Company of the consideration (which shall not be less than the par value of such Shares) to be paid in accordance with the Approval.

The opinion set forth below is limited to the Delaware General Corporation Law.

Based on the foregoing, we are of the opinion that the Shares have been duly authorized and, upon Future Approval and Issuance, will be validly issued, fully paid and nonassessable.

This opinion is being furnished to you for submission to the Commission as an exhibit to the Company’s Current Report on Form 8-K relating to the Shares (the “Current Report”), which is incorporated by reference in the Registration Statement. We hereby consent to the filing of this opinion letter as an exhibit to the Current Report and its incorporation by reference and the reference to our firm in that report. In giving our consent, we do not admit that we are in the category of persons whose consent is required under Section 7 of the Securities Act or the rules and regulations thereunder.

Very truly yours,

/s/ Goodwin Procter LLP

GOODWIN PROCTER LLP

Rocket Lab Corp 8-K