Hacker News Reader

Comment by JumpCrisscross | Hacker News Reader

The tyranny of Tsiolkovsky's rocket equation [1] results from the need to burn fuel now to accelerate fuel for later. "Catapulting" sidesteps this issue by energizing on the ground.

In selling away the rocket equation you buy yourself drag. Lots of energy up front means lots of drag. Overcoming that drag means more energy and, hence, more drag. The middle child of this solution is tremendous G force.

You can't slingshot complex things into space. Pretty much just fuel and raw materials. That means you need in-space (a) refueling and (b) additive manufacturing capabilities. There are teams working on both problems, though they are presently in the domain of science versus engineering. Perhaps that will now be incentivized to change.

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

View on HN

Is it possible they spin the rocket in place then convert the rotation energy to linear with fins on the body acting as a propeller?

So just a quick calculation:

Assuming that the rocket has moment of inertia similar to a hollow cylinder with radius 1ft, conversion from rotation energy to linear is perfect, and final linear velocity is 5000 mph, the rocket would have to be spinning at

1/2 * I * w^2 = 1/2 * m * v^2

1/2 * (m * r^2) * w^2 = 1/2 * m * v^2

w = v / r = 7333 rad/s ~ 70,000 rpm

Maybe the payload of the rocket could be connected to the body of the rocket with magnetic bearings so that it isn't spinning, i.e. not experiencing the massive accelerations.

Assuming you can

1) accelerate fast enough to penetrate the atmosphere at high speeds (and remember, the faster you travel, the air resistance is the square of your velocity).

2) withstand the crazy max-q as soon as you leave the spiral (which I assume is vacuumed).

3) make the centrifugal forces reasonable during the acceleration

The article mentions acceleration to 5000mph, which would be enough if there was no atmosphere, but I highly doubt it will work with.

> In selling away the rocket equation you buy yourself drag. Lots of energy up front means lots of drag. Overcoming that drag means more energy and, hence, more drag. The middle child of this solution is tremendous G force.

Would it be possible and useful to do it with a 2 payloads system: one dumb piece of material first which can take lot of G followed closely by your "fragile" payload using its draft?

> followed closely by your "fragile" payload using its draft?

G-forces [1] are a consequence of acceleration. An accelerating object will experience g-forces in an atmosphere or in a vacuum. ("Drafting" [2] is an aerodynamic process by which a following object exploits a leading object's slipstream to reduce the former's drag.)

With a slingshot, the destructive force is the g-force inflicted by the acceleration. (With a rocket, the destructive forces are mostly vibrations and aerodynamic stress.)

Note that a slingshot doesn't require lots of Gs. One could use a super-long slingshot to achieve the necessary velocity. But super long is super expensive, so high Gs it is.

[1] https://en.wikipedia.org/wiki/G-force

[2] https://en.wikipedia.org/wiki/Drafting_(aerodynamics)

I can imagine a heavy dead weight accelerated in the slingshot. Once released, it drags a cable, carefully laid out at an angle, with the payload attached. That payload would be accelerated into line behind the weight over a brief period. Taking fewer Gs and only once.

Very long cables are a lot cheaper than very large slingshots?

Anyway just thinking out loud.

> Once released, it drags a cable, carefully laid out at an angle, with the payload attached

Unless your cable were elastic, this would not materially change the payload's acceleration. That said, you might find skyhooks [1][2] interesting.

[1] https://en.wikipedia.org/wiki/Skyhook_%28structure%29

[2] http://adventuretime.wikia.com/wiki/Skyhooks

Of course it would. Like a lever has different forces on the short end and the long end, the cable could be curved and lengthened to control exactly the rate of transfer of momentum between weight and payload.

You have to accelerate both pieces to the same speed, which implies (in a rotational launcher), the same G (acceleration). Maybe there's some configuration where the leading payload pushes air out of the way for the first few km (the 'real' payload drafting behind), but the blocking payload isn't given enough energy to reach orbit and falls short. Seems like the following payload would crash into the leading one though.

How much do you "gain" by launching from up hight (e.g. the Andes / Himalayas, as opposed to sea-level like in Florida)?

It saves you about 6% of fuel, assuming the launch location is on the equator, is suitable for roads and can handle a lot of noise / avelanches. Getting into space is the easy bit (energy-wise). Getting up to orbital speed takes much more fuel and energy.

The problem is that now you have to transport everything near and up the mountain, which costs a ton in logistics (getting a road up a mountain that can handle a entire assembled rocket isn't cheap, nor easy). Most launches have delta-v to spare because satellites are usually nowhere near the rocket's maximum payload weight.

tl;dr it can save 6% in a ideal situation, but the extra cost and logistical problems aren't worth it.

You seem to only be considering the fuel saving of the higher altitude. If we're considering a ground based mass driver launch, rather than a conventional rocket-only launch, then I think there would be a substantial practicality gain from firing into thinner air.

At the proposed 5000mph, air resistance at sea level would be tremendous. Greater atmospheric losses would entail either bringing more fuel to complete the orbit, or larger launch loop / greater acceleration force.

>tl;dr it can save 6% in a ideal situation, but the extra cost and logistical problems aren't worth it.

Unless you launch by means of a mass driver/rail gun [1] built into the side of Mt Everest with a piggy backed elevator for tourists. Tourists would arrive to a pressurized viewing area at the summit and have the option of donning an oxygen mask, going outside and posing for photos next to a cutout of Sir Hillary. I have never understood why serious rock climbing friends get so upset whenever I have mentioned this idea over the years.

1 https://en.wikipedia.org/wiki/Mass_driver

Skip Everest and go with Mt Kenya. It's equatorial location means that you are getting a nice delta-v boost, its shape is more suited to this purpose, and it is far more accessible.

I always thought they should just make stairs on Everest..

Reply on news.ycombinator.com