It's more than that - this isn't just hippies, it's every major power on the planet. To fly Orion you have to put hundreds of nuclear explosives in orbit and pinky-swear that you're not going to use them to obliterate your enemies. Nobody is going to accept that.

Orion's a fun paper rocket but will never be more. Engineers and they "yay nukes!" crowd will drag it out every couple of months for a repost, though.

To reiterate berntb's point: Every nation who has the ability to do this already has ICBMs that can deliver nukes anywhere on the planet in 20 minutes. So what difference does this make?

Maybe one day we can launch Orion from Mars.

From a strategic point of view, what's the difference between building on Mars and building it in orbit? Obviously there is a difference because the hostile faction has to seize control of it and move it to Earth, but once they do that, the calculation becomes the same. Do you commit to obliterating them the second they start moving it to Earth?

What's the difference to building it in orbit? Building it in earth orbit you're dropping it from under 500km and your assembly makes 10~20 orbits a day, you can deliver it quickly at pretty much any time.

The former allows countermeasure, the latter not really.

Er… yes?

> Sure, you can try to take it out during the journey, but that may be complicated by it being the highest g ship in existence for the Mars-Earth distance.

That's not really a complication, it's still taking months to make the trip, and while it might be "the highest g ship in existence" that's linear acceleration, it's not a nimble ship performing a tailside in space to avoid projectiles.

> Once/if it arrives, you have the same strategic problems you have if you build it in orbit.

With the pretty damn big difference that you've had several months head start knowing the think was incoming at high speed instead of it just dropping of your head more or less instantly.

If it's coasting along it's significantly increasing transit time and thus available window to consider and prepare counter-measures.

> it's in principle quite possible to turn it and fire bombs in the direction of any interceptor

Sure but that means less payload to deliver and even more time for the target to prepare counter-measures.

> and the thick pusher plate makes it actually pretty durable from some aspects.

No matter how thick your pusher plates are they're not going to stand up to the kind of kinetic impact involved.

Sorry, I didn't understand why you were talking about bothering to do that and then crashing it into the atmosphere.

If you want to argue that crashing into the atmosphere would be an inevitable result of the trip, okay, but I don't understand what else it would have to do with my initial comment.

That's not what I'm arguing FFS, I'm saying that'd be the result of going with the fastest possible trip, which would still leave the target significant time to set up countermeasures.

Here's the rest of the paragraph of my original answer since you apparently stopped at the first period:

> If you want to actually deliver your payload in a useful manner you'll need twice that at the very least. 2+ months is a long lead time, and the engine you'll need will make the craft impossible to miss.

The point is that no matter how you slice it, "the difference between building on Mars and building it in orbit" is the target gets a warning several months in advance in the former case, a few minutes or seconds in the latter. One gives ample opportunity to set up and deploy countermeasures, the other not so much.

[deleted some pointless back and forth]

That's quite the understatement, the galileo probe was half heatshield by mass. And it took a 230g hit to the face.

[Edit: The bombs would be directed, throwing light weight molecules up against the pusher plate.]

Also, there is no direct lack of ways to distribute nuclear devices to capitals today, for the countries able to build an Orion. Compare that with the possibility to launch enough material to e.g. start on getting an industrial infrastructure going outside the atmosphere.

I haven't seen any real details of directed bomb design discussed. Thanks for the pointer to that book.

Interstellar flight is a problem after we have gotten to a reasonable cost/kg for both launching from the Earth and for interplanetary travel.

I doubt there will be a pressing need for getting close to even 100 km/s in the inner solar system for decades. (Yes yes, "640K is enough for anyone". :-) )

(Consider: An AU is 150 million km. 100 km/second means 8.64 million km a day; 2 AUs in 35 days! Then you have to lower the speed, even more energy.)

Launching an Orion from the ground is probably not realistic after the 1960s either. It would have the problem with bombs and EMP killing satellites. I've seen suggestions to coast through the sensitive zone without using bombs, but haven't seen any discussion about how many Gs it would mean...

Quoting https://en.wikipedia.org/wiki/Solar_Probe_Plus :

> Solar Probe Plus or Solar Probe+, previously NASA Solar Probe, is a planned robotic spacecraft to probe the outer corona of the Sun. ... As the probe passes around the Sun, it will achieve a velocity of up to 200 km/s (120 mi/s) at that time making it the fastest manmade object ever, almost three times faster than the current record holder, Juno. ... Launch date: July 31, 2018 (planned).

Too bad nuclear tech is basically too regulated for any non governmental organization to experiment inexpensively, not to mention the other nontechnical issues.

You could launch one from the far side of the Moon, but if you have that kind of space presence there are better engine designs. Some have been posted elsewhere in this thread.

So this is like being concerned how you will spend all that money, once you make it :-)

There's a lot of discussion on interestellar travel, but "we" have almost frozen solar system travel. Yes, there are very nice probes out there doing important work, but manned vehicles are limited to keep ISS's lights on.

The "impossible" tech needed to reach the stars will most probably be discovered or refined while conquering the solar system.

Big sailing ships were developed along four centuries of sailing. The analogy may not be perfect, but I think it's eye opening.

Basically you turn the vehicle around and use the propulsion to slow you down. It would use a heap of fuel to do this but it was possible.

Edit, also, great find of this by the poster.

The chart from the paper was pretty convincing. I'll see if I can dig it up. Making the explosions more efficient can only get you so far. I'd prefer to be optimistic about it, but the math was showing that even with an ideal ability to convert mass into energy, it would require too much fuel, to put it mildly.

Later studies indicate that the top cruise velocity that can theoretically be achieved are a few percent of the speed of light (0.08-0.1c).[17] An atomic (fission) Orion can achieve perhaps 9%-11% of the speed of light. A nuclear pulse drive starship powered by Fusion-antimatter catalyzed nuclear pulse propulsion units would be similarly in the 10% range and pure Matter-antimatter annihilation rockets would be theoretically capable of obtaining a velocity between 50% to 80% of the speed of light. In each case saving fuel for slowing down halves the max. speed. The concept of using a magnetic sail to decelerate the spacecraft as it approaches its destination has been discussed as an alternative to using propellant, this would allow the ship to travel near the maximum theoretical velocity.[18]

https://en.wikipedia.org/wiki/Project_Orion_(nuclear_propuls...

===

2.3. Deceleration: the canonical issue

While accelerating to a sizable fraction of the speed of light is a daunting task, deceleration is a yet more challenging task. The reason is that no realistic scenario to provide the gravy-train approach to either momentum transfer or pellet energy transfer at distances of 4+ light years seems within known physics. We are therefore challenged with either sending sufficient numbers of pellet-sheath elements so that the spacecraft can decelerate with on-board energy, or to come up with some other innovative concept.

2.3.1. Fueling at departure point

One option is to provide all necessary propellant at the departure location. If the specific power and specific impulse is sufficiently high, this is a potential approach. However, for our MMO concept, it is not particularly feasible. For example, our specific power is certainly high enough, but our specific impulse is dictated by the pellet-sheath combination, which limits the Isp to 128,955 s. While this may seem like a prodigious specific impulse, the ratio of spacecraft speed .1 c to g * Isp is 23. This means that the deceleration propellant must be e^23 times the spacecraft mass, which is an unworkable solution.

2.3.2. Mag Sail deceleration

In 2003 both Andrews and Lenard postulated using a large superconducting ring to intercept charge particles in interstellar space to slow the spacecraft down from high speeds. Additionally, the solar wind emanating from a star system provides an additional source of charged particles that can interact with the magnetic field. Deceleration can actually begin a sizable distance from the target star system. The following two charts indicate deceleration, velocity and time as a function of distance from the target star system. In this case the first phase of the deceleration starts at 21600 AU with a twoturn superconducting carbon nano tube reinforced loop. This loop captures the charged interstellar medium and deflects it to decelerate the spacecraft. This initial hoop size is 500 km in radius and carries 1,000,000 A of current. The spacecraft decelerates from .1 c to 6300 km/s by the time the spacecraft reaches 5000 AU. This will be quite a light show, so if there are any intelligent life forms with an observing system, they should be able to see the arrival.

[1] https://books.google.dk/books?id=A_1oCAAAQBAJ&pg=PA451&lpg=P...