Gemara

Shmuel: If a Shelamim was slaughtered before the Heichal doors were opened, it is Pasul - "U'Shchato Pesach Ohel Mo'ed", when it is open, not when it is locked.

Zevachim 55b: Clearly, if the doors are shut, this is like being locked.

Question: If a curtain covers the opening, is this considered open?

Answer (R. Zeira): It is only like an open doorway.

Question: If a tall obstruction (e.g. a mound or wood) is in front of the opening, what is the law?

Answer (Beraisa - R. Yosi b'Rebbi Yehudah): There were two openings in Beis ha'Chalifos (extensions of the Ulam to the sides; R. Yosi considers the Ulam to be like the Heichal) eight Amos tall. They permit eating Kodshei Kodoshim and slaughter of Kodshim Kalim in the entire Azarah.

Suggestion: In front of the openings were obstructions eight Amos tall, and still, they were considered open (to permit Shechitah)!

Rejection: No, there were no obstructions. The openings were eight Amos tall.

Question (Mishnah): All gates in the Azarah were 20 Amos tall and 10 Amos wide.

Answer: That does not apply to minor openings.

Eduyos 8:6 - R. Yehoshua: I have a tradition that we may offer Korbanos even without a Mikdash.

Tamid 3:7: They would not Shochet the Tamid until they heard the great gate open.

Toras Kohanim (on Vayikra 1:5): "(After Shechitah, Zerikas Dam Olah is on the Mizbe'ach) Asher Pesach Ohel Mo'ed" - but not when it (the Mishkan) is dissembled, or when the wind rolled the curtain;

R. Yosi ha'Gelili says, the verse teaches that the Kiyor does not interrupt between the Mizbe'ach and the Ohel Mo'ed. Rather, it is to the side.

Rishonim

Tosfos (29a DH Ela): We learn from Shelamim to other Korbanos that Shechitah before the Heichal doors were opened is Pasul. However, we do not learn to Melikah or Kemitzah.

Rashi (Zevachim Sof 61a) and Tosfos Menachos (95a DH Mar): Shechitah of Shelamim before the Heichal doors were opened is Pasul; we do not learn from Shelamim to other Korbanos

Rebuttal (Tosfos Zevachim 61a DH Kodem): They would not slaughter the morning Tamid until the doors were open. Surely this is learned from Shelamim. This is not merely mid'Rabanan. A Hekesh ("Zos ha'Torah la'Olah...ul'Zevach ha'Shelamim") equates all Korbanos to Shelamim, even b'Di'eved. We cannot say that the Pesul even regarding Shelamim is only mid'Rabanan, for it exempts Shechutei Chutz before the doors are opened.

Defense #1 (of Rashi - Malbim Vayikra 1:5 (44) DH v'Hinei): The first Tana in Toras Kohanim does not allow Shechitas Olah when the Mishkan is dissembled. This is like Tosfos. This Tana learns from Shelamim to all Korbanos. R. Yosi ha'Gelili holds like Rashi.

Defense #2 (Emek ha'Netziv Sifri Nasa p.141 b'Hagahah): Surely, "U'Shchato Pesach Ohel Mo'ed" requires the Heichal to be open from Shechitah through Zerikah, for the Gemara does not consider this a stringency of Shechitah. If we learn to all Korbanos, we would not need a verse to teach about Zerikas Olah! Rather, the Torah requires the Heichal to be open for Zerikas Olah, but not for prior Avodos. We delay Shechitah of the Tamid until the doors are open lest the blood congeal (if Zerikah may not be done right away), or lest amidst zeal the Kohen do Zerikah before they are opened.

Defense #3 (Tif'eres Yisrael Chomer ba'Kodesh (introduction to Kodshim) 4:10): We know that the Hekesh of all Korbanos to Shelamim does not apply to Tenufah, Nesachim and Lo Lishmah. Therefore, we can say that it also does not apply to Shechitah when the doors are open (see Pirush ha'Rosh Nedarim Sof 6b).

Question: R. Yehoshua permits Hakravah without a Mikdash. But we require the Heichal to be open!

Answer (Rashba Shevu'os 16a DH Ha): This requirement is only when there is a Heichal and doors.

R. Yakov Ilan (Hagahah to Rashba ibid. (72), Rav Kuk edition): One may not offer when the Mishkan was dissembled, for then the Kedushah lapsed. R. Yehoshua holds that the Kedushah of the Mikdash remains even after the Churban.

Note: According to this, the Toras Kohanim applies only to the era of the Mishkan, and it is unrelated to the law of Shechitah when the Heichal doors are open. This is unlike the Malbim and Emek ha'Netziv.

'Rashi' (Nazir 45 DH Tanu Rabanan): Only Shelamim requires the doors to be open because it is a gift.

Note: We find that also Olah is called a gift (Nazir 18b, Zevachim 7b).

Question (Tosfos Eruvin 2a DH Shelamim): Why must the Torah teach that Shechitah of Shelamim before the Heichal doors were opened is Pasul? "Ha'Olah" teaches that the morning Tamid must be the first Korban of the day, and they would not slaughter it until the doors were open!

Answers (Tosfos ibid.): "Ha'Olah" teaches that the first Haktarah of the day must be the morning Tamid. It need not be the first Shechitah. Alternatively, l'Chatchilah the morning Tamid must be first, but b'Di'eved it is Kosher (mid'Oraisa. A Tosefta disqualifies b'Di'eved; this is mid'Rabanan.) Alternatively, we need the verse to teach about if the doors were closed afterwards.

Question (Tosfos Yom Tov Tamid 3:7): What was Tosfos' question? We learn from Shelamim that Shechitah of the Tamid (and all Korbanos) must be when the doors are open!

Rambam (Hilchos Ma'aseh ha'Korbanos 5:5): If a Shelamim was slaughtered before the Heichal doors were opened, it is Pasul. It says "U'Shchato Pesach Ohel Mo'ed", when it is open.

Radvaz: Shmuel discusses before the doors were opened, but the same applies if they were opened and later closed, for we require "Pesach Ohel Mo'ed".

Question (Tosfos Yom Tov ibid.): Why does the Rambam teach only about Shelamim? Just like we learn from Shelamim to the Tamid, we learn to all Korbanos!

Answer #1 (Minchas Chinuch 115 DH v'Tzarich): The Rambam brought the very words of the Gemara. He explicitly rules (Temidim u'Musafim 6:1) that Shechitah of the Tamid is after the doors are open. He agrees that the same applies to all Korbanos.

Answer #2 (Sefas Emes Zevachim 55b DH Shelamim) - Question: Why is it Pasul b'Di'eved? In Kodshim, something is Me'akev only if two verses require it!

Answer: Regarding Shechutei Chutz it says "V'El Pesach Ohel Mo'ed Lo Hevi'o". This teaches that l'Chatchilah, the Heichal must be open during Shechitah of all Korbanos. Therefore, "U'Shchato Pesach Ohel Mo'ed" is Me'akev regarding Shelamim. The Rambam holds that for other Korbanos it is only l'Chatchilah. No Gemara disqualifies other Korbanos b'Di'eved. Tosfos learns that it is Me'akev for all Korbanos from "Zos ha'Torah". However, this teaches even about birds and Menachos, and Tosfos (29a DH Ela) does not require the Heichal doors to be open for Melikah or Kemitzah.

Tif'eres Yisrael (ibid.) learns like Sefas Emes, except that he says that "Ve'Hevi'um la'Shem El Pesach Ohel Mo'ed...v'Zovchu Zivchei Shelamim" (Vayikra 17:5) is a second verse regarding Shechitas Shelamim. Regarding other Korbanos the Torah mentions only Hava'ah (bringing the Korban to Pesach Ohel Mo'ed).

Rambam (ibid.): If the doors were shut, this is like being locked.

Question (Tosfos Zevachim 55b DH Govhah): How could the Gemara think that a tall obstruction is a problem? The entire Azarah is Kosher for Shechitas Kodshim Kalim, even in the east where the Mizbe'ach obstructs seeing the Heichal!

Answer (Tzon Kodoshim DH b'Emtza Devarav): The Torah taught that the entire Azarah is Kosher even though the Torah fixed a place for the Mizbe'ach which obstructs the view from parts of the Azarah. We have no source regarding obstructions the Torah did not require.

Question (Lechem Mishneh): The Gemara (Zevachim 55b) never settled the question about an obstruction. Why didn't the Rambam address this?

Answer (Ri Korkus): The Gemara concluded that an obstruction is not a problem. The Rambam disqualifies only when the doors are closed, therefore he did not need to discuss obstructions.

Rambam (ibid.): However, if a curtain covers the opening it does not disqualify.

Rashi (Zevachim 55b DH Hu): The curtain is considered open, for it is merely for privacy.

Rebuttal (Shitah Mekubetzes 7 and Tosfos DH Hu): The Gemara asked that perhaps it is not considered open. It cannot answer 'it is considered open' without a reason! Rather, the Gemara answers that the sole purpose of the curtain is for (privacy) when the doors need to be open, i.e. at the time of Shechitah.

Acharonim

Minchas Chinuch 141 (DH v'Din Zeh Hu): Surely, we are not concerned if the doors are closed if the Shechitah is in the Heichal.

Rebuttal (Mikdash David 27:2 DH v'Hinei, brought in Machon Yerushalayim edition of Minchas Chinuch, footnote 6): If so, one should be liable for Shechutei Chutz when the doors are closed, for the Korban would be Kosher in the Heichal!