29

So we see Marty eventually travel back to 1885 using the Delorean that Doc hid in the mine, and we also see it being hit in the fuel tank by an arrow, hence crippling its ability to get itself up to 88 mph.

Both Doc and Marty are then seen trying various ways of getting the car up to the required 88mph, including using horses to pull it. Eventually they both have a lightbulb moment when presented with the locomotive and realise they might just be able to get it up to 88mph and have a chance at getting home.

However, Doc doesn't seem to ever consider something much simpler - rockets. He has gun powder, he has the ability to create packages to make the steam engines burn hotter, so why couldn't he have built a rocket propulsion sled?

It would have been much less risky than hijacking the locomotive - a siding car could have been borrowed or purchased, and they wouldn't have been beholden to any particular schedule that the train was on...

Why did they choose to do things this way?

  • 11
    First guess; they have limited funds and time. – Valorum Dec 16 '14 at 12:58
  • 3
    Speculating about alternative plots is offtopic here. – curiousdannii Dec 16 '14 at 13:22
  • 6
    @curiousdannii - Yes, but the root question (why did they need to hijack the train) is perfectly valid. – Valorum Dec 16 '14 at 13:47
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    Rocket nozzle design is tricky. en.wikipedia.org/wiki/Rocket_engine_nozzle I expect that building a nozzle capable of providing enough thrust and handling the temperatures involved for long enough would be well beyond the tech available in a small western town of the late 1800's. – Wayfaring Stranger Dec 16 '14 at 15:07
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    Additionally, gunpowder burns too fast to make decent rocket fuel. Unless you pelletize it properly, or modify its burn rate some other way, you're more likely to end up with a bomb than a useful large scale propulsion unit. Nitrocellulose might serve better, but like large quantities of nitrocellulose, or gunpowder, making ingredients would likely be hard to come by in a small western town. – Wayfaring Stranger Dec 16 '14 at 15:12
31

It seems to boil down to two factors; Time + Money.

Marty and the Doc urgently need to leave town. We know from the pictures of the gravestones that either Marty or the Doc are liable to be killed in the showdown with Biff the following day. As you can see from the slightly longer 'final draft script', the doc devises a solution that will allow them to get the hell out of Dodge before then :

MARTY : Uh, Clint Clint Eastwood.

BLACK BIFF : Well, Mr. Eastwood. I'll see you out on the street Friday the 13th, front of the saloon. And you'd better be there!

MARTY : Yeah, right. When? High noon?

BLACK BIFF : Noon? I do my kilin" before breakfast 8 o'clock!

enter image description here

later...

DOC : Now. in order to get a steam locomotive up to 88, we need a long stretch of track on a downhill grade, which still exists in 1985. That's here — Carson Spur, which goes off the main line at this switch track and out to Carson Ravine. We'll load the DeLorean onto the tracks here, on Carson Spur. The train leaves the station at 7:57 tomorrow morning...

MARTY : Whew. I'm glad it's before 8.

DOC : Indeed. So: we'll hijack — uh. borrow the train, uncouple the cars from the tender, throw the switch track, and then we'll butt the locomotive against the DeLorean. You'll get in the DeLorean while I run the steam engine, pushing the DeLorean along Carson Spur. I'll open up the throttle, putting her into a full highball, climb to the front of the locomotive, jump onto the DeLorean and get in, in time to reach 88 miles per hour, at which tine Mr. Fusion will kick on the flux capacitor, generating the traditional 1.21 jigowatts which will send us back to 1985.

As to why they don't build a rocket sled, the simple answer is that they're in a hurry. Renting a train is also out of the question because regardless of their success or failure their plan will destroy the train's firebox and possibly cause an explosion. That's an expensive and extremely dangerous proposal that most train companies would shy away from, even if they were offered a pile of cash (which the Doc doesn't have).

Stealing a train is both quicker and free.

  • 3
    Did the script really call him "Black Biff"? He's Buford "Mad Dog" Tannen in the movie. – phantom42 Dec 16 '14 at 15:06
  • 1
    Yup. There's a few changes (Seamus isn't married, for starters) but the overall structure is very similar. – Valorum Dec 16 '14 at 15:12
  • The question then becomes why did they need to leave immediately after defeating Tannen? Unless I'm recalling incorrectly, he was arrested immediately after attempting to kill Marty, and the gravestone then vanished from the photo before they left. With the primary motivation for hurrying out of the way, why continue with this admittedly risky, dangerous, illegal, and unethical plan? Why not scrap the plan and do something safer? – jpmc26 Dec 16 '14 at 19:17
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    @jpmc26 - Once the plan is in place, there seems little point in changing it. The hijacking itself is relatively painless and as far as the ethics are concerned, don't forget that Marty & Doc's entire relationship is based around criminality (bootlegging CDs, etc) – Valorum Dec 16 '14 at 19:40
  • 1
    Also, the Doc is very concerned about contaminating the timelines. The sooner they can be out of the past, the better. – Valorum Dec 16 '14 at 19:42
19

They were looking for a direct solution that got them (or just Marty) out in a timely fashion. I don't recall if the gravestone photo was the driver, or if it was the simple fact of Marty didn't want to grow older in the past.

I know they were trying multiple solutions. If I remember correctly, while they were trying alternate fuels and ended up blowing out a part with high punch whiskey, Doc said he could repair the car, it would just take months. I don't doubt he could figure out a high octane process in that amount of time. A rocket sled would need custom parts in the same way, some way to harness a longer burning fuel for momentum... time to build, time to test, time one of them didn't have. The train already HAD all that, and a speed potential that just needed to be pushed. They wouldn't need to make one, just take one.

  • 1
    Also, Doc had no problem building something completely new once given time (couple years at least) and reason (showing his new family the world (guessing)) to do so, along with a trip to the future to spruce it up with newer parts. Marty's time was the big factor. – Radhil Dec 16 '14 at 13:31
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    "I don't recall if the gravestone photo was the driver". They were trying to get out before the date on the tombstone as Doc/Marty was supposed to get shot. – phantom42 Dec 16 '14 at 14:24
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    @Radhil - What's not to work? The whole things seems essentially foolproof. Did you not see the model? – Valorum Dec 16 '14 at 14:34
  • 2
    Pssh, the model. It was crude, and not even to scale! – Aric TenEyck Dec 16 '14 at 15:29
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    @Moo It wasn't just the gravestone - they had the specific cause of death too. "Shot in the back over a matter of $80". And the uncertain nature of the changing events of history that may have predestined at least one of them to die. Remember - they don't have the benefit of hindsight that we do. They're the first pair to ever travel through time, and they have no idea what the rules are, if any, about causality and changing the course of history. – Zibbobz Dec 16 '14 at 16:12
6

The schedule you are referring to does not only included the train, but also the fact that one of them will be killed the following Monday: as they start making arrangements, Buford is gunning for Doc, but as events occur, his target shifts to Marty. This puts them on quite a tight deadline (pun very much intended), as it gives them 3-5 days to leave.

Building a rocket propulsion sled and making/buying enough gunpowder to generate enough energy to propel the DeLorean up to 88 MPH would most likely take longer than the few (3-5) days they have left.

All in all, I think that Time was in short supply and that the train was the simplest and most reliable option available within the deadline.

  • 300-400g seems remarkably low. Either your math is wrong or even if this is right, I don't think it's a very good explanation of why Doc Brown couldn't do it. 300-400g, 200 bullets worth is just not that much. – ThePopMachine Dec 16 '14 at 15:28
  • @ThePopMachine I agree, I think 300-400 grams is too little, but considering optimal conditions (frictionless vacuum and so forth) it might be possible. But, like I said, these conditions are impossible to reach: I was going for something like the ABSOLUTE minimum required. But you're right, reading my answer again I realise it's not clear just how understated I think 300-400 grams are: I'll revise and edit. – Gunnar Södergren Dec 16 '14 at 15:49
  • My point is, if you do the math, and it doesn't provide an explanation to the question, then why even give it as an answer? – ThePopMachine Dec 16 '14 at 15:51
  • @ThePopMachine Yes, I understood that, and like I said, I realize my emphasis was lacking and I will revise. I have now updated the answer to include only the time aspect. Thanks for your input. – Gunnar Södergren Dec 16 '14 at 16:00
2

One possibility is that the people who wrote the movie's script either did not consider it, or decided it was either not feasible or not dramatic enough. My guess, though, is that it has to do with consistency of speed. In the various iterations of Delorean impulse, the constant factor has been getting up to 88 MPH and then maintaining that speed for a few seconds while the transport process takes hold. Steam engines and internal combustion engines (and presumably fusion engines) are relatively easy to finetune to stay at a consistent speed. Trying to adjust the fuel of a rocket, and to do so without blowing yourself up, is much trickier.

  • 1
    You do not have to stay at exactly 88 mph., it is just the minimum. When he first time travels, Marty simply accelerates to 88 mph, at that moment the time jump is initiated, but he does not immediately stop accelerating (he crashes into the scarecrow, then hits the brakes but still crashes into the barn. – Lars Ebert Dec 16 '14 at 13:25
  • 1
    Also, at the end of BTTF2, the car is struck by lighting, spins on its axis at 88mph and jumps. There's no "consistency of speed" there. – Valorum Dec 16 '14 at 13:49
  • @Valorum, how exactly can the whole car "spin at 88 mph"? You can't measure rotational velocity of an entire object in mph.... – Wildcard May 26 '17 at 8:49
0

As Richard said I think the main issue was time, but I thought I'd add some calculations on how much gunpowder would actually be needed. The Tsiolkovsky rocket equation tells us that the change in velocity of the final payload is equal to the "effective exhaust velocity" (around 800 meters/second for a "black powder" rocket according to this) times the natural logarithm of the ratio of initial mass (including fuel) to final payload mass (with all fuel used up). Rearranging this equation, the ratio of initial to final mass would be e^(change in velocity / effective exhaust velocity). So with a change in velocity of 88 mph = 39 m/s, the ratio is e^(39/800) = 1.05, meaning the fuel must have a mass equal to 0.05 times the mass of the payload. A DeLorean with a full tank of gas has a mass of 1233 kg according to this, and the gas should weigh around 80 lb = 36 kg so let's say without gas it would be about 1200 kg. That means you'd need about 5% of that mass in gunpowder, or 60 kg (130 pounds). This might well be more gunpowder than local stores would actually stock at any given time, a poster here says that with a muzzleloader you can get around 70 shots per pound of gunpowder, enough for a fair number of hunting expeditions. Although this article indicates that the 36 barrels of gunpowder used by Guy Fawkes added up to about 2500 kg of gunpowder, or about 70 kg per barrel, so if there were some similar-sized barrels of the stuff around then it might be enough (not sure if those barrels were just used as supplies for cannons in the navy or if regular gun stores would have stocked them).

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