Why do (Federation) starships need Bussard collectors in front of their warp nacelles?

Question

Ever since Zephram Cochrane's first human-built warp-capable vessel, the Phoenix, Human ships with warp drives have had Bussard collectors placed in front of their warp nacelles.

Why are the Bussard collectors so large and prominent, and why are they always placed in front of the nacelles (as opposed to in the front of the saucer, below the saucer, around/together with the deflector, under the bottom of the ship, etc.)?

Note: Recall that the nacelles contain the warp coils, into which the "drive plasma" resulting from the matter-antimatter reaction gets injected, to generate the warp field. Thus, supposedly, no fuel is used in the nacelles themselves.

Answer 1

tl;dr: The Bussard collectors are there for emergency situations, and it's complicated.

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Welcome to the magical and slightly convoluted world of starship propulsion technology!

Regular use of the warp nacelles

Spaceships of the United Federation of Planets are equipped with warp drive systems which uses a combination of two kinds of fuel:

• Deuterium (i.e. 1 proton + 1 neutron nucleus), stored in Deuterium cartridges.
• Anti-Hydrogen (i.e. 1 anti-Proton nucleus, a.k.a. Anti-Protium), stored in antimatter storage pods.

Both kinds of fuel are stocked in advance, and are not collected from the intersteller medium.

The fuel is used by first converting the Deuterium into some combination of gasses (?) ; and that gets intermixed with the Anti-Hydrogen in the ship's warp reactor core (which is not in the nacelles). This produces highly-energetic "drive plasma", which is transmitted via EPS conduits to the nacelles, where it is intermittently injected towards the warp coils. Energized by the plasma, the warp coils generate the warp field that makes the ship move faster than light.

                       annihilating      EPS                  -=|magic|=-
                         reaction     conduction
Deuterium + Anti-Hydrogen   ->   Plasma   ->  Plasma in drive coil -> Warp field

You will note that no fuel gets to the nacelles at all in this process. Thus, the Bussard collectors have no use for anything when the ship is at warp, and it shouldn't matter where they're placed, if at all. But - we're not done yet!

Impulse drive

There's also the way ships are driven when they're not at warp, or not warp capable - the Impulse Drive (a.k.a. Impulse Engine). It's actually quite similar to how warp driving works:

          fusion              EPS                    -=|magic|=- +
         reaction          conduction               plasma venting
Deuterium   ->   Weak plasma  ->  Plasma in drive coil    ->   Sub-warp-1 field

Why weak plasma? Since we get it from nuclear fusion, as opposed to a matter-antimatter reaction - a fusion reactor vs. the warp core reactor. This part is non-fictional physics: It's well known that the former is much less energetic than the latter, since in the fusion reaction you shave off a lot less of the mass involved.

As for drive coil used - that can be either a coil dedicated to impulse driving, or, in fact, a warp coil: Those can be used also for the weaker impulse-driving (see also the description of impulse drives at Ex Astris Scientia). And it is at this point in the discussion that Bussard collectors may finally come in.

Emergency + Bussard collector + coil = propulsion

Suppose your ship is in trouble, with one of the following happening:

• You're out of warp drive fuel
• Your warp core is damaged
• You've jettisoned your warp core
• Your EPS grid is damaged
• etc.

Now, according to the Federation Starship Datalink website, and specifically this page about warp engines:

When plasma is unable to reach the warp nacelles, the Bussard ramscoop assembly at the front of each nacelle is able to draw in low-grade galactic matter and use this as an energy source instead.

Well, we already know what it means to "use as energy source": Dump it into a reactor; and since we only scoop regular matter, not anti-matter, from the interstellar medium - it has to be a fusion reactor. This only gets you weak plasma, not the warp plasma you usually get from the warp core; but no matter: we noted the warp coils in the nacelles are usable also with the weaker plasma. So, you can cruise on home despite your emergency, albeit at a rather slow speed.

That is also the likely explanation of why the collector is part of the nacelle (although I am speculating here): The emergency situation making you use the collector in the first place is not being able to get plasma to the nacelle. Now, some of the reasons for that would still let you put the collector elsewhere and send the weak plasma over the EPS grid. But the more robust thing to do is have a collector right next to the warp coils: You're protected against EPS grid or other structural failures, you don't have to worry about saucer separation, and finally - each nacelle is now a self-contained impulse drive unit.

Note: If we look at the diagram for a Sovereign-class nacelle, we'll notice an element named a "fractionator". They is likely an Isotope fractionator, for choosing which collected isotopes actually get used in the fusion reaction. It's likely not for keeping just the Deuterium, though, since there's only 10^-5 as much Deuterium as there is Hydrogen, and there's not much Hydrogen... One would guess that Bussard-collected fuel is not as convenient as Deuterium.

Answer 2

I really think the answer is not as complicated as all that. Matt Jeffries designed the enterprise in 1964. He probably intended it to look like cool radar.

In my opinion, he probably never even thought of the warp nacelles as warp nacelles, but as Ordinary propulsion that shoots matter out the back. Okuda wasn't around then. My guess is that all the rest is retcon.

Answer 3

Bussard collectors are in front of the nacelles because that's the best place for them. Here's Ex Astris Scientia's definition of a Bussard collector:

Device usually affixed to the forward end of the warp nacelles that serves to collect interstellar hydrogen atoms for fuel replenishment, also referred to as Bussard ramscoop. The Bussard collector assembly consists of a set of coils which generate a magnetic field that channels the stray particles into the collector (generic). The Bussard collector is named for the 20th century physicist and mathematician Robert W. Bussard.

In 1960, Bussard proposed a theoretical method of space propulsion that generated a massive magnetic field to funnel hydrogen, then used thermonuclear fusion to propel itself. The Bussard collectors were clearly based on the Bussard ramjets, which you can read more about in this Wikipedia article.

Ramjets are also somewhat related; air that enters the intake gets compressed, mixes with fuel, gets ignited, and then is accelerated.

But what all these propulsion systems have in common is that they channel the fuel straight to the engines. It's just the most logical--you'd send the fuel through the engines, not through the cockpit/cabin/saucer. The warp nacelles are the engines, so that's where the collectors are needed, not anywhere else. Why would you channel the hydrogen through the saucer or engineering section (which contains living/working areas and lots of other things) when you can just feed it right to the engines?

Regarding your question about the size and prominence of the Bussard collectors: if you take a look at the Bussard ramjet article I linked, you'll see that the collecting portion looks pretty huge, so I'd say the Bussard collectors are actually pretty modest, comparatively.

You were also wondering if there's any conflict between the deflector dish and the Bussard collectors--but there isn't, as shown in this SFF Stack Exchange post. The deflection field can be manipulated to allow controlled amounts of matter to enter the collectors.