Why not fuel rockets externally during liftoff?
December 27, 2017 1:21 PM   Subscribe

Given the tyranny of the rocket equation, why not save mass by pumping fuel and oxidizer into the engines from an external source during liftoff?

For example, the first stage of the Saturn V burned for about 165 seconds, but 9 seconds of that (5.5% of the total fuel consumption during that stage) was spent just clearing the 106m gantry. Very roughly, if 5.5% of the first stage fuel and oxidizer were pumped in from an external source, that would save about 3.5% of the total mass of the rocket. The savings could be even greater if the external source could be maintained up to, say, 200m, at which point a Saturn V would still only be moving at a comparatively stately 33m/s or so.

So why not do that?

Note: I'm sure this is neither a novel idea nor a good one, but I'm having trouble finding an explanation of why it either wouldn't work or wouldn't help as much as I'm assuming.
posted by jedicus to Science & Nature (9 answers total)
Very roughly, if 5.5% of the first stage fuel and oxidizer were pumped in from an external source, that would save about 3.5% of the total mass of the rocket

It is only 3.5% of the total mass of the rocket before liftoff. 9 seconds in, it's gone and not really weighing anything. Compared to any sort of mechanism for during-liftoff fuel pumping, which is a source of both weight and failure.

I am certainly no expert but I think this winds up being pretty similar to the question "why not just have a lot more stages".
posted by aubilenon at 1:30 PM on December 27, 2017

The fuel being pumped in would still need to be lifted to the location of combustion (the engine). So you might displace some of the work to the pumping mechanism, but the work still has to be done. Also, what aubelinon said about failure points - there would be a lot more of them and the associated risk would be very high.

Now if they shot the rocket out of a giant cannon...
posted by under_petticoat_rule at 1:40 PM on December 27, 2017 [1 favorite]

Best answer: Any external tube/hose attached is going to have some weight, plus the fuel that's in the hose that that moment (for 9 seconds of fuel supply, this his is going to need carry a lot of volume) and it's going to be off cenetered. Additionally, what happens upon disconnect? There's a hose filled with rocket fuel now in the path of the rocket. What happens if the disconnect fails, will that affect the flight path of the rocket? Additionally, consider how far you want the main fuel source that's supplying the hose to be away from the ground effects of the rocket.

Is the hose fueling the rocket directly? Now you need the engine to cut over the fuel sources, or you need a whole new stage/engine. Instead is the hose refueling the primary fuel chamber? Now this now needs to be designed for additional inlets.

Just initially to me these issues seem like a larger complication than even the 3.5% weight savings might be worth. I'd initially imagine it may be less complicated to build an accelerated launching deck, and launch from there. And I say that thinking an acceelerated deck is an extremely impractical idea. On preview: under_petticoat_rule clearly has the better idea of a cannon! Way better than an accelerated launch pad!
posted by nobeagle at 1:47 PM on December 27, 2017 [3 favorites]

Yeah, for the reasons described above, an external propulsion mechanism makes a lot more sense than a ground-based tank for the rocket's own engines. The first thing that jumped out at me was safety issues: if you have a tank and hose on the launchpad, there'd better be some way of preventing the ridiculously hypergolic contents from being subjected to unsafe combustion from the rocket's own thrust. Spaceflight is mostly safe but as a few tragedies have shown, very small screwups quickly become lethal for everyone and you'd better have a really, really good reason for introducing new risk factors.

But even "a lot more sense than a ground-based tank" isn't a lot of sense.. Taken to its logical endpoint it basically becomes an extra stage for the rocket, jettisoned at a low altitude. A completely external launch mechanism (like the cannon idea, or some sort of electromagnetic launch system like EMALS) might not be a terrible idea but it would need to interface well with the rocket's own thrust capabilities so that after throwing the rocket up, its internal propulsion takes over smoothly.
posted by jackbishop at 3:01 PM on December 27, 2017

I think a disposable external tank would be better, but...

Reading about the Falcon Heavy, it was proposed with a system to allow the main engines to burn fuel carried in the tanks of the booster rockets until booster shutdown and separation. The idea was to get the main rocket + payload to as high an altitude as possible with full fuel tanks. That feature was dropped, or at least postponed.
posted by SemiSalt at 3:18 PM on December 27, 2017

You definitely wouldn't want to do anything like this with hypergolic fuels, which are lethal at the if-you-smell-it-you-are-already-dead level, but getting 9 seconds of liquid fuel/oxidizer (maybe just one or the other) opportunity (in the Saturn V example) using some kind of connector, and the pumps that feed it, which can keep pace with a rocket at the start of liftoff, while at the same time not risking an immediate catastrophic explosion in the tower which could blast debris into the rocket engine or first stage thanks to the proximity of, well, a thrusting rocket.

It does seem like this would be possible, if not safely possible, within the scope of existing technology, but it's not clear whether it would ever by worth the expense. It'd be easier to fuel a high-speed train that never stops moving, because at least you could make it not be accelerating through the fueling facility, whatever form that takes. (Snowpiercer, anyone?)
posted by Sunburnt at 3:30 PM on December 27, 2017 [1 favorite]

FWIW they gave a lot of thought to launching the space shuttles from somewhere high up in the Rocky Mountains. This would save a *LOT* of fuel (and/or allow a *LOT* more payload)--way more than your idea would.

Yet, in the end they decided not do implement the Rocky Mountain Launch Pad plan, regardless of the massive fuel savings/larger payload it would allow.

Why? Well, the Rocky Mountain Launch Pad has some up sides but it also has a lot of down sides. (I'll let you figure out what they are.) The down sides far outweigh the up sides.

Similarly with your idea, except that the up side you are talking about is like 3% less fuel whereas the Rocky Mountain plan was like 30% less fuel.

In short, you are always making tradeoffs in these types of situations and to make your idea practical and worthwhile it would probably have to save a lot more than 3%.
posted by flug at 9:25 PM on December 27, 2017

moving at a comparatively stately 33m/s

I will just point out that, though comparatively stately in comparison with orbital velocity, 33 meters/second is 74 miles per hour.

Pumping vast quantities of volatile fuel from a fixed object to one moving at 74 mph is not going to be a trivial task at all . . .

You'll notice that the fueling of rockets that does take place is a fairly complex process requiring some hours, a lot of equipment and personnel, and is the type of thing that often goes a bit awry and results in launch delays etc.

Now, we're somehow going to pump a significant fraction of that total fuel amount during a single 9-second period during which the receiver is accelerating from 0 to 74 mph and vibrating and rumbling around like crazy.

And . . . we get ONE CHANCE and one chance ONLY to get it exactly right.

And . . . that final 3% of fuel is absolutely mission critical (because otherwise, why are we even bringing it at all?) and if this procedure fails the entire mission will fail.

And . . . during that initial 9 seconds we are going to be burning 3% of this extremely volatile rocket fuel you are talking about from the bottom of the tank, while pumping in the extra 3% into the top of the tank (or something?). 9 seconds probably won't even be enough time for the draw on the tank to settle down enough for that 3% to be added safely--or at all.

In short: Extremely persnickety unnecessary mission critical process added to a launch sequence, where the design imperative is to make it as simple and foolproof as possible. And the alternative solution is 'just build the fuel tanks 3% bigger' which is dead simple and wouldn't add any new failure points to the process.

Complex, failure-prone solutions with marginal utility are going to lose out every time here.
posted by flug at 9:42 PM on December 27, 2017 [1 favorite]

Response by poster: FWIW they gave a lot of thought to launching the space shuttles from somewhere high up in the Rocky Mountains. This would save a *LOT* of fuel (and/or allow a *LOT* more payload)--way more than your idea would. ... the Rocky Mountain plan was like 30% less fuel.

I can find no evidence that such a plan was ever considered, and these calculations for launching from the top of Mt. Everest show an energy savings of just .2%. Here's another set of calculations coming to the same conclusion.
posted by jedicus at 6:40 AM on December 28, 2017 [1 favorite]

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