Hacker News Reader

Comment by ultramegachurch | Hacker News Reader

I suspect the deployed structure cannot handle the acceleration required for escape velocity. That also may require much more propellant. Then on top of that, we don’t have the capability for humans to service satellites other than the ISS. So this is all a moot point.

View on HN

Interesting though.

Does that really matter without air resistance?

Depending on how high you actually bring it. Like 500km away from earth is still an orbit (I think that's Hubble's orbit) but how much force do you need or will happen?

Rockets can easily accelerate with enough force to kill a human (cargo flights and unmanned flights use different launch profiles for this reason).

The less Gs you need to design a component for, the lighter/simpler it can be, so why unfurl early and add that extra mass and complexity to the design?

When rockets are firing lots of acceleration is applied that the delicate structures are only designed to handle when stowed. Think long arms on hinges. They can take acceleration in one axis, but not at 90 degrees to it.

I guess the acceleration needed to leave orbit might need so much more fuel if you need to do it slowly than doing it fast?

Otherwise it would just take longer.

Rocket engines could only be throttled down to certain power and manoeuvring thrusters would run out of propellant way before it reaches any orbit. Maybe something like electric propulsion can do it but it will take very long time to make it practical imho.

The parts of a ship that thrusters are directly attached to experience acceleration first, the other parts that are further out from the thrusters won't accelerate immediately and if the acceleration is too sudden or extreme could be damaged or break off entirely.

And to answer your question in the other response, many thrusters have a minimum thrust, and even that minimum may be too much for the parts when deployed.

By definition, as long as thrusters are firing, you are accelerating.

Acceleration is defined as the change in velocity, and velocity is defined as the change in position (w.r.t. some inertial reference frame). There will always be some bending when thrust is applied at one part of a body; nothing is perfectly rigid.

As an extreme example, imagine a stick one lightyear in length: if we ignite a rocket on one end, firing perpendicular to the stick's length, then the other end cannot start moving for at least a year.

We had the capability in 1993, and could of course develop it again.

A broken JWT could wait a few years in orbit.

That may be true, but designing a mission based off that hypothetical is a bad idea. The reality is we currently don’t have the capability for humans to service satellites, and developing that capability would probably take years and cost >$100 million. And NASA can’t just decide to take on that endeavor, it would require congress and months of political bickering. JWST was designed for what is currently feasible and practical.

> developing that capability would probably take years and cost >$100 million. And NASA can’t just decide to take on that endeavor

So what you're saying is, this could easily be funded by some billionaire, e.g. Jeff Bezos who already sells billions of dollars in Amazon stock per year to fund Blue Origin?

Not saying this should be done privately, but if funding is the problem, that problem can be solved.

Space travel is less expensive than most people think, it just isn't very high up on our list of priorities.

“Maybe Jeff Bezos could fund this” is not a good parameter to design a mission around.

$100M is 1% of what JWST cost.

Looks like a reasonable repair cost (and only if) it turns out to be broken.

It’s still not that simple, unfortunately. Ironically, there are too many single pint failures. Maybe JWST broke in a way that can’t be repaired. Maybe congress doesn’t approve the repair mission. Maybe the repair mission would actually cost $1 billion. Maybe the repair mission fails. Now imagine you’re the mission designer. You could trade increased complexity for some small chance of a repair mission maybe being possible. Or you you could decrease complexity and just accept that repair won’t be possible. The answer becomes pretty clear.

Referring to hubble? The JWT is designed to observe from the Lagrange point in permanent shadow of the earth. It won’t work from earth orbit. First parking it in orbit, and then restarting the engine after unfolding comes with a whole new set of risks and tradeoffs.

What about an unmanned mission to fix it? And maybe to refuel it.

Incredibly unlikely. First, it would have to fail in a way that’s possible to fix. We don’t have robots that can replace screws, solder joints, and polish mirrors in space. Then we’d have to design a brand new spacecraft and mission. That would take years, lots of money, and political will. NASA would likely cut its losses, document the lessons learned, and try again.

I was watching a documentary about James Webb and they claimed that NASA has plans of using unmanned robots to potentially refuel it but no mention of fixing.

But who repairs the repairers?

Reply on news.ycombinator.com