Salvage One flight plan - would it even work?

[JonnyBGoode]

Back in 1979 there was a shortlived (1 1/2 season) TV show called "Salvage 1." The plot was that a curmudgeonly rocket scientist (Andy Griffith) built a home-made rocket in his junk yard, with the intent of going to the moon and salvaging discarded Apollo hardware.

The flight plan was based on an idea called the "Trans-Linear Vector Principle." Under this principle, the rocket lifted off for the moon under slow but constant acceleration. Although the initial velocity seemed much too slow to ever reach the moon, the steady acceleration soon brought the Vulture to great speeds once in space. Acceleration continued until they reached the halfway point between the Earth and Moon, at which time the ship began to decelerate ending in a smooth touchdown on the lunar surface. Total time to reach the moon was a mere 24 hours compared to three days for the Apollo missions. The trip back home followed the same procedure. The advantages of the Trans-Linear Vector Principle were many:

• Two days round trip to the moon and back.
• A single stage rocket was all that was needed for the flight, reducing complexity.
• No mid-flight orbiting of either the Earth or the moon. It was a direct ascent and descent flight.
• No re-entry heating since the trip through the atmosphere was a slow, gentle descent.
• No weightlessness since the ship was always in a state of acceleration or deceleration.

Now knowing what I've learned from Orbiter over the years, I know that the principle of constant acceleration craft does work - in space. For instance, ion drive propulsion. But I seriously doubt that it would work for escaping Earth's gravitational field. The fuel expenditures I think would be enormous for any sort of chemical rocket (even for the fictional "mono-hydrazine" unobtanium fuel they had in the show). I'm assuming I'm correct, yes?

 

[Izack]

Flaw 1: Acceleration from the engine must be greater than 1G to leave the Earth's surface. I think that alone qualifies as a showstopper.
Flaw 2: No way in heck the Vulture could carry enough fuel to be an SSTO of any type, let along go to the Moon and back under constant thrust.

 

[jedidia]

I know that the principle of constant acceleration craft does work - in space.

When word is about constant acceleration, it usually means acceleration around a few mili-G, a few centi-G at max (depending on the mass of the craft, but an ion engine usually doesn't have the thrust to push its own weight at a few centi-Gs, left alone any satelite it is attached to).

Of course any kind of takeoff and landing from or on any body whatsoever, baring small asteroids, is completely out of the question. You'd need to bring a lander along, and you'll have to leave most of the craft in Orbit on return (not too bad an idea actually: leave the engine and most of the structure up there and re-enter with a capsule).

The second problem is that such an engine indeed doesn't have enough thrust to go on a "Trans-Linear Vector" (what kind of technobabble is this? it's just a straight line, for crying out loud! :lol, instead you'd spiral out to the moons orbit. Not recommended for manned missions, because you spend too much time crossing the VanAlen Belt.

Now lets suppose we had an engine that could pull one and a half G (it needs that much for getting off of earth) and enough Isp to keep the thrusters running at at least one G for 2 days. I won't do the math right now, but as far as my expierence goes and assuming the mass of the whole rocket is something around a hundred tons, we're talking about a few gigawatts at least. Very hot for a backyard rocket. The backyard it launches from certainly wouldn't be there anymore...

Then there's another trouble with the "Trans-Linear Vector principle"... straight lines are all nice, but you still have to catch up to the orbital speed of the moon. Not really a problem for such a powerfull engine for sure, but it won't be a straight line...

 

[Mandella]

Oh come on guys!!!

The ship was only "low" thrust in that it didn't pull the five or six Gs people were used to hearing about from sitting on top of a Saturn V or the like. I don't remember what it's top end was from the show, but it could easily pull the G and a half or so necessary to pull up off the planet, then drop down to a standard G while cruising.

And yes, such an engine could make it to the moon in (again, without crunching the numbers I'm in a hurry gotta put the ham in the oven in about five minutes) less than a day. Eight hours comes to mind, in fact, but that might be mismemory.

Of course, the rocket capable of that would have to be ..something really special.. but accepting the handwaved unobtanium the numbers work just fine.

 

[T.Neo]

When word is about constant acceleration, it usually means acceleration around a few mili-G, a few centi-G at max

Unless you're on Orbiter-Forum, where there are regular mad science discussions stemming from a certain deranged armchair rocket scientist.

Nevertheless, your point is sound; there is a big difference between ion drives and whatnot, and nuclear or chemical drives, just in terms of thrust. A solid-core NTR doesn't nearly have the ISP of an ion drive, so it must be used more-or-less like a chemical rocket.

Considering the calculations that I did in this thread, a spacecraft able to travel to the Moon in 2 hours, or even 4 hours, requires a large spacecraft with a scary thrust power. It also shows, that at those Delta Vs, matching velocity with the Moon is not a major problem compared to the actual acceleration and deceleration burn; lunar orbital velocity is only around 1km/s.

However, this is 24 hours, not 4... so the acceleration is going to be lower.

The following equation is from the Atomic Rockets Torchship page.

A = (4 * D) / T^2

A = Acceleration.
D = Distance.
T= Time.

If my calculation is correct, that means you'd need to accelerate at 0.206 m/s^2 for a 24-hour brachistochrone to the Moon. That is around 2 centi-Gs.

Also from the Atomic Rockets Torchship page;

transitDeltaV = 2 * sqrt[ D * A ]

A = Acceleration.
D = Distance.
transitDeltaV= Delta V requirement.

If my math is correct, for a 24 hour brachistochrone to the Moon that is 17 800 m/s. Adding lunar orbital velocity, that is 18 800 m/s.

A solid core NTR with an exhaust velocity of around 8000 m/s could handle that with a mass ratio of 10.2. An enclosed gas core with an exhaust velocity of around 20 400 m/s could handle that with a mass ratio of 2.51. And an open gas core with an exhaust velocity of 50 000 m/s could handle it with a mass ratio of 1.45.

Obviously there is going to be needed Dv for launch from Earth and landing on the Moon as well. Launch from Earth is maybe 9500 m/s, and lunar orbit insertion 800 m/s, whereas landing on the Moon is maybe 1500 m/s. This gives a total of 30 600 m/s.

The solid core would need to have a mass ratio of 45 for that Dv, so obviously that is impossible. The enclosed gas core would need a mass ratio of 4.48, and the open gas core a mass ratio of 1.84. And that obviously, does not include the return trip. Better ship fuel to the Moon beforehand, or something. Enclosed gas core would need a mass ratio of 20 (which is pushing plausibility to the limit) and an open gas core would need a mass ratio of 3.4.

Where a curmudgeonly rocket scientist, presumably working unofficially and on his own, gets a nuclear rocket, and/or a nuclear rocket that spews radioactivity, is anybody's guess.

The good news is the acceleration to the Moon; for that, you only need a thrust of a few tens of kilonewtons, with thrust powers ranging from 200-250 megawatts to move a 10 ton spacecraft, which is quite good. I suspect however that the spacecraft would mass a bit more than that, but not be a 100 ton + behemoth.

But you are going to have to have high accelerations on launch... if you accelerate at only a small amount over one G, you are going to spend too much time launching, and waste too much fuel to take off. You'll run into bad problems due to gravity losses and whatnot.

 

[JonnyBGoode]

But you are going to have to have high accelerations on launch... if you accelerate at only a small amount over one G, you are going to spend too much time launching, and waste too much fuel to take off. You'll run into bad problems due to gravity losses and whatnot.

That was my main thought, actually.

 

[jedidia]

Unless you're on Orbiter-Forum, where there are regular mad science discussions stemming from a certain deranged armchair rocket scientist.

Oh well, we're all just armchair rocketeers here, so everyone's in good company

If my math is correct, for a 24 hour brachistochrone to the Moon that is 17 800 m/s. Adding lunar orbital velocity, that is 18 800 m/s.

The problem is, of course, that 0.2 m/s^2 aren't enough acceleration to make a brachistochrone orbit to the moon. Gravity doesn't just stop at LEO, and a rocket with that thrust won't be able to break free if it just starts to burn orbit outward. The only way you have with that low a thust is extending your APR, resulting in a spiral orbit, nowhere near brachistochrone...

 

[Ghostrider]

Oh well, we're all just armchair rocketeers here, so everyone's in good company

The logical step up is to attach rockets to the armchair...

 

[jedidia]

To be honest, I don't even HAVE an armchair. It's a mundane chair for me, but attaching rockets to it might still be a feasible idea. My wife might complain about the hole in the ceiling, though...

 

[T.Neo]

The problem is, of course, that 0.2 m/s^2 aren't enough acceleration to make a brachistochrone orbit to the moon. Gravity doesn't just stop at LEO, and a rocket with that thrust won't be able to break free if it just starts to burn orbit outward. The only way you have with that low a thust is extending your APR, resulting in a spiral orbit, nowhere near brachistochrone...

Not quite... initially you are going to be spiraling out, but eventually the APA starts to rise much faster than the PeA.

But it isn't 100% a brachistochrone, so the dynamics will obviously be different.

 

[Izack]

To be honest, I don't even HAVE an armchair. It's a mundane chair for me, but attaching rockets to it might still be a feasible idea. My wife might complain about the hole in the ceiling, though...

Scenes from Mythbusters are coming to mind...

 

[dougkeenan]

His son directed Apollo 13. I reckon he had access to secret NASA technology.

 

[Ghostrider]

It's a mundane chair for me, but attaching rockets to it might still be a feasible idea.

Might be a winner for IKEA. Buy the ATLAS armchair before December the 31, get 4 TITAN rocket boosters for it free. SATURN pillow included.

 

[statickid]

didn't read the whole forum so sorry if this is a repeat, but you should look at the Tintin "explorers on the moon" rocket that is available (i think its on the DGIV site) as it had a very similar mission profile with the constant acceleration, direct path, and direct landing. The mission would be nearly identical