Hacker News Reader

Comment by nordsieck | Hacker News Reader

The big one off the top of my head:

Dream Chaser flies in a fairing for aerodynamic reasons. In order to fly crew (so that the vehicle could have a working launch abort system), they'd need to figure out how to fly without a fairing.

View on HN

From some quick reading it seems like the crewed Dream Chaser is intended to fly without a fairing. The cargo version's ability to fold the wings and fly within a fairing seems to be for 1) compliance with NASAs CRS-2 requirements, 2) wide compatibility with existing boosters that weren't designed for the forces that flying without a fairing would create.

Could be spin from Sierra but that's what they were saying to the press as of 2015 when they announced the cargo variant.

https://spacenews.com/sierra-nevada-hopes-dream-chaser-finds...

The problem with the wings is they generate lift during launch and that will screw up the rocket, thus the fairing. I thought there was no known solution for that.

It seems like you could mitigate that by designing the wing such that when mounted the angle of attack is 0 and thus no lift is generated. Obviously the wing would still have an effect when the booster changes orientation or in cases of high winds. I'm not remotely qualified to calculate the scale of those forces but I don't see why any of that would be a guaranteed showstopper given a booster with enough thrust vectoring capability.

Not any different than shuttle or energia. Lift can be accounted for. It’s not as easy as launching in a fairing, but it’s certainly not impossible.

Fold the wings up?

Seems like Starship fins and Falcon 9 grid fins did that trick as well.

Naive question: Planes and helicopters do not have the ability to safely eject passengers mid-flight. We largely accept these conditions as a risk of those modes of travel. Why is LES/LAS a unique requirement for space shuttles?

> Planes and helicopters do not have the ability to safely eject passengers mid-flight. We largely accept these conditions as a risk of those modes of travel. Why is LES/LAS a unique requirement for space shuttles?

That's a fair point, although my understanding is that parachute systems for small planes are becoming more common.

My view is that flight rate is the fundamental issue at hand. Airplanes and helicoptors fly many orders of magnitude more than these capsules, which means we know they are many orders of magnitude more reliable.

They've also generally been through a long process of refinement - the original airplanes were extremely dangerous compared to modern variants.

Additionally, aircraft can afford to have a lot higher margin of safety baked in to them. Because of how high gravity is on Earth and the nature of the Rocket Equation[1], it's just not possible to have a lot of margin in rockets of capsules. They need to be extremely svelt to launch at all.

And lastly, we have experience with human spacecraft without an LES/LAS - it was the Space Shuttle. And it killed 14 people - easily the most dangerous spacecraft ever created. No one has any desire to build on that particular legacy.

---

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

Also worth noting that test pilots of experimental aircraft generally wear parachutes, at least for higher risk tests. This includes tests of commercial aircraft.

> which means we know they are many orders of magnitude more reliable

No, it means we have orders of magnitude more reliability data. Same result, different point.

We have enough data to know that rockets are not as reliable as airplanes.

1) Commercial planes and helicopters are orders of magnitude safer than space flight. https://usafacts.org/articles/is-flying-safer-than-driving/

2) Both planes and helicopters have an ability to glide (or autogyro) to a relatively safe landing in the event of most failures. A spacecraft can also do that with wings or parachutes, but only if it gets far away from its exploding booster fast enough to survive.

3) Many military planes do have the ability to safely eject passengers.

4) astronauts dying live on stream is a really bad look.

Planes and helicopters do not frequently fail by exploding, but rather things like engines failing. An engine failure, even if it’s the only engine in a given airplane or helicopter, does not automatically involve a deadly crash. Airplanes can glide, frequently for very long distances, and helicopters can use the air moving across the rotors to effectively “glide” down. It’s not always possible, but they do have inherent redundancies that rockets necessarily do not.

Helicopters are actually even safer than fixed-wing airplanes this way: as long as the rotor system is working properly (i.e., it's just an engine failure), the pilot can autorotate and land on any nearby spot of clear land large enough for the helicopter, and can achieve a safe landing with no or minimal damage. In fact, helicopter pilots frequently practice this maneuver while in flight training.

Airplanes need some kind of runway, by contrast, and this isn't usually available within the glide distance. So they can "land", but it frequently won't be a pretty landing.

The main dangers with helicopter engine failures are 1) it happens too close to the ground, so there's no time to react and enter a safe autorotation, and 2) the pilot is too slow to react (low-inertia rotor systems are more dangerous this way, so helicopters like Robinsons are worse).

Planes and helicopters burn, though.

https://www.smithsonianmag.com/smithsonian-institution/freak...

Here's an attempt to develop a jet fuel that wouldn't burn:

https://www.youtube.com/watch?v=Y33N0raKZBo

You could go without an LES if you can convince NASA that you have sufficient contingencies to not need it (and, of course, you can do almost anything you want on a private flight). SpaceX has been entertaining the idea of not using an escape system on Starship and instead proving its safety through sheer number of flights. Although there also is just a general consensus that the flip maneuver would probably make that necessary even if they had an LES.

Startship will not have an abort capability either.

Neither did the Shuttle, unfortunately.

Unlike the Shuttle there will be a lot more uncrewed testing of Starship, so hopefully it will be more reliable.

I think rockets in this design space have frequently been closer to prototype quality rather than commercially deployment quality.

Those other systems have other redundancies and safety mechanisms.

Airline passengers are in the dime-a-dozen category of expendable. Investments in astronauts are significantly higher. You really want them to be re-usable more so than the rockets

planes/helicopters have a fuel source that is orders of magnitude less volatile, and are also able to safely land without power

What fuels do you mean in particular? If you’re talking about hypergolic propellants, those aren’t commonly used as a main propellant on manned rockets. Most planes and helicopters use kerosene, which is also the primary fuel used by the launch stages of manned rockets.

Can't they blow the fairing as part of launch abort?

Soyuz launches with the crew capsule under fairung and even stacked under the orbital module - it still has a LES[0], that pulls off the whole fairing away & then drops the capsule once clear of the rocket.

A bit complicated but was already used in emergency and worked.

[0] https://en.wikipedia.org/wiki/Soyuz_abort_modes#Jettisonable...

> Can't they blow the fairing as part of launch abort?

I'm not a rocket scientist, so I don't really know the answer. But I have some questions:

* How reliable will the "blow the fairing" system be? If it's only used in emergencies (instead of the regular fairing separation mechanism) than it'll suffer from the same problem as emergency generators - they're rarely tested and fail very often when needed.

* How easy is it to get the fairing out of the way once it's opened? Normally, regular aerodynamic forces slough the fairing halves off, but an LES/LAS doesn't have time for that - it's escaping a potentially exploding rocket. And those fairings will act like sails - huge surface area:volume ratio means they're just not going to move fast.

* What happens if DreamChaser hits one of the fairings on the wing? Would it damage it enough that it'd have trouble landing? Is it enough to cause it to foul the escape trajectory? Or even put it in a spin?

It seems like a lot of work to get it to work with or without a fairing.

That's one more thing to go wrong. In a LES scenario something(s) has gone wrong in an unrecoverable way. Very few systems can be relied upon to exist let alone work correctly. A LES on a crewed capsule is supposed to be able to pull itself from the vehicle all under its own power. It can't assume explosive fasteners on the fairing are functional or actual all function correctly.

You don't want the LES to activate, seem to be working correctly, and then blast the crew into a fairing panel that did not fully separate. The crew doesn't have time to roll down the window and kick it loose.

Reply on news.ycombinator.com