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Comment by vidarh | Hacker News Reader

It's quite amusing that they try to bill this as a space elevator, when it has almost nothing in common with what is typically considered a space elevator: It won't reach space; and it's envisioned as a building rather than a tether to a counter-weight. Seems like billing it as a space elevator is a pure PR move.

Also, is 30% of fuel costs really going to be worth it? According to [1] Elon Musk claimed fuel is only 0.3% of the cost of current rockets. Even if we assume SpaceX slashes the overall cost to 1/10th without touching fuel costs, that still only brings fuel up to 3%. 1% additional savings doesn't seem like it's going to do much to help finance a 20km tower.

Of course if single stage to orbit planes are viable from it, then that may make it more attractive, but it seems peculiar to focus on the fuel cost.

[1] http://www.space.com/21386-spacex-reusable-rockets-cost.html

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I think you're overestimating how much fuel could be saved by this. You need 7.8 km/s of delta-v expenditure in any event because that's orbital velocity. Generally a rocket uses up 10.5 km/s of delta v to reach orbit with the difference being atmospheric and gravity drag[1]. So you'e got almost 3 km/s of delta v you could theoretically save here. You'd probably lose most of the atmospheric component but there's still the thrust you need to counteract gravity for however long it takes you to reach orbital velocity. The lack of air means you can safely accelerate a little faster but you're still limited by the weight of your engines and the heavier structure that faster acceleration requires. I'd put the reduction in needed delta-v at closer to 5% with a fuel reduction around 12%.

[1]https://en.wikipedia.org/wiki/Gravity_drag

It's not my estimate. I'm going by the claim in the article.

EDIT: Which has now been replaced by the patent application, for the general confusion of everyone...

I'm the OP and the link to the patent [1] is what I originally submitted. Nothing has been changed.

[1] http://patft.uspto.gov/netacgi/nph-Parser?patentnumber=90858...

Ride the elevator out to geosync, and you'd be in orbit for free. Ride it past there and let go, and you'd end up flying away from the earth. Time it right and you could end up traveling to the moon or to mars for little to no fuel costs, and just have to brake at your destination.

Edit: My mistake, I didn't realize that this wasn't the normal concept of tether out past geosync.

[TFA isn't loading up for me]

3% to 30% is a 10x savings, because it's a ratio.

E.g., if it takes $1,000 of fuel to get a unit mass into orbit, then a 3% ratio means $33K, and a 30% ratio means $3K.

I am always skeptical of "massive upfront investment but lower per-unit costs," especially for space-launch. It's the way most of us learn about space, because sci-fi novelists talking about space have to explain what finally got man into space, and a massive infrastructure seems such a nice logical explanation. But Musk has shown that Plain Old Boring Rockets can be done much more efficiently than traditionally done. And that doesn't make a good sci-fi backstory.

I think you misunderstand me. Musk has stated fuel for current type launches is around 0.3%.

If we assume that SpaceX cuts the overall cost of launches to 1/10th mainly by reuse of the first stage, that is indeed a 10x savings, but not on fuel (in fact, SpaceX uses more fuel because they need to be able to slow the first stage).

0.3% of x is the same as 3% of x/10.

So my point was that even with SpaceX numbers, where fuel will (once they sort out reuse) make up substantially more than what it currently does (because they're aiming to massively slashed the non-fuel costs), a 30% cut in fuel costs will "best" case cut launch costs by about 1%.

If these guys are going to achieve substantial cost cuts it will need to be elsewhere unless Musk' has been massively misquoted or is totally wrong about the relative cost of fuel vs. the rest.

(I'm not going to bother being precise as all of these are handwavy estimates from people anyway).

30% less fuel means smaller, lighter rockets. Compound cost savings over all.

Exactly. Another way to say this is that this launch requires less Delta V. It's completely different than getting cheaper fuel of the same mass (which would be trivial).

smaller light rockets with a huge tower attached to it

That's a good point, but then it is an odd thing for them to care about the fuel savings vs. their claim of not needing the first stage and allowing for single stage to orbit with a plane type launcher, which is a far bigger deal in terms of cost savings.

E.g. consider that Musk a few years ago estimated the fuel and oxidizer costs for a Falcon 9 v1 at ca $200k per launch, with a list price for the launches at $54m to $59.5m. Cutting the cost of the rockets themselves on the other hand, whether by reuse or making them smaller/cheaper will matter much more.

I guess whether eliminating the first stage entirely vs. reuse will matter much will depend on how many times recovered stages can be reused and how much it will cost to prepare them for reuse, as the potential fuel savings are basically rounding errors.

When I read "...do much to help finance a 20k tower." I started laughing so hard (at the prospect of this thing) I broke my MacBook and had to buy a new one :/

I agree 100% that the economic viability of a "space elevator" like this is basically non-existent, even though it's a really fun thing to think about.

having 20 km high structure can dramatically cheapen launch of smaller non-live payloads using something like electromagnetic catapult as a replacement for the first stage - even just 40m long HARP gun was reaching 3.6 km/s.

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