Technology in search of a mission?

Oldfart1939 · 2017-08-04 08:06:44

In an article posted today on the website spacenews.com, entitled "The uncertain future of solar electric propulsion," many of the NASA proposed missions are semi-critically examined;

Here's the link to the article: http://spacenews.com/the-uncertain-futu … ropulsion/

The most telling remark was the penultimate paragraph.

Unfortunately, SEP seems to be one of those technologies a small group within NASA is using to obstruct a Mars Direct, or Mars Semi-Direct mission by insisting that their technology is mission-critical and must be included. SEP was to play a major role in the ARD Mission, Asteroid Re-Direct. That mission was a non-starter from the beginning, as it was overly complicated and was a poor use of the SLS, which itself is moving forward at a glacially incremental rate. I'm lumping SEP in a group with several other technologies which were included in the ill-fated "90 Day Plan" proposed by the Bush I administration, thus making it unaffordable.

My position on going to Mars is simply the brute force method, using to best of our abilities the currently available technology of chemical propulsion. I detest the NASA proposed "Deep Space Gateway," in cislunar space. Just another makey-workee project that gets in the way of progress but which ensures employment for a branch of NASA. Is NASA so risk averse that they cannot step up and say "LET'S GO TO MARS?"

Last edited by Oldfart1939 (2017-08-04 08:08:17)

GW Johnson · 2017-08-04 09:54:21

Electric propulsion as we currently know it, whether solar or nuclear powered, is not useful for pushing people around. It could be useful for pushing nonliving cargo around.

The problem for manned flight with any form of electric propulsion as the main means is two-fold: (1) very long travel times because you get mere newtons of thrust out of multiple tons of propulsion equipment, and (2) spending months exposed to high radiation while spiralling out through the Van Allen belts.

Nonliving cargo doesn't care about long flight times, unless there is an exterior mission requirement demanding a faster flight. It might be subject to radiation damage. Some is, some isn't.

The kind of Mars mission I roughed out and posted over at "exrocketman" saved a bundle of thrown mass and a bundle of cash by sending equipment and propellant ahead to Mars orbit by solar electric propulsion, at the cost of calendar time (well over a year one-way to Mars). This included the landers and their propellants, and the return propellants for the crew.

I sent it to orbit, NOT the surface! That's where I differ with every variation I have ever seen on Zubrin's plan, and with what Musk proposed in Guadalajara. You verify it arrived before you ever send the crew by much faster chemical rocket means (I'd prefer to use nuclear if it existed, but it doesn't). Then you base your surface excursions from this equipment and propellant depot you placed in orbit. This resembles the old 1950's ideas more than any other ideas.

The way to use electric on manned missions is complicated. You have to do your departure and arrival maneuvers the high-thrust way in order not to add months upon months to your flight time. But, you can thrust electrically during the interplanetary cruise, accelerating the first half, and decelerating the second half.

That is how you speed up a min-energy Hohmann transfer to Mars from its nominal 8.5 months, to something at (or perhaps under) 6 months, without burning chemical rocket propellant. It is questionable how compatible this idea is with spinning your ship for artificial gravity. Of the two, I consider artificial gravity the more critical requirement.

GW

Last edited by GW Johnson (2017-08-04 09:55:28)

SpaceNut · 2017-08-04 16:33:32

I agree that the spin creation of AG is the big technology to be built and tested followed by radiation mitigation.

elderflower · 2017-08-07 09:10:18

Provided the thrust is aligned with the spin axis it would be possible to accelerate/decelerate a spinning assemblage without running into spin stability problems. You would need to despin in order to adjust the direction of the thrust vector. This would be difficult if you use reaction wheels due to conservation of angular momentum about the original spin axis so I think it would have to be done with thrusters.

RobertDyck · 2017-08-07 10:19:37

Actually, no. With a tethered vehicle spinning in the same plane as the transfer orbit, simply wait until the spacecraft is aligned with the direction you want to thrust. Then apply thrust, which means pulling on the tether. That will pull the counter weight. No reaction wheels, no problems with angular momentum.

Notice a Frisbee can fly a curved path. And as it flies, it can change direction due to lift. But the spinning disk remains spinning in the same plane as when you threw it. That's the idea. Spin stabilizes orientation; course corrections will not change spin orientation, just direction of flight.

elderflower · 2017-08-07 11:40:30

Yes, you could apply intermittent thrust in the plane of rotation (which is always perpendicular to the axis) but the angle of that plane will vary relative to the travel vector over the duration of the voyage as it is fixed in galactic coordinates, not solar system coordinates. I still think that trying to apply thrust at an angle less than 90deg to the axis may produce irregular rotation. You will need to make corrections during transfer.

Last edited by elderflower (2017-08-07 12:20:23)

GW Johnson · 2017-08-08 09:41:18

Well, it's fairly obvious that a vehicle spinning for artificial gravity must spin up after departure and spin down before arrival. It's also fairly obvious that, like most other interplanetary flights, there will be at least one midcourse correction. The question is: can that be done without spinning down before, and spinning back up after, the correction?

In theory, you can, even with a cable-connected system, by carefully timing small bursts of thrust. In practice: this has never ever been done. It is not a technology ready-to-fly. So, you either commit to making it ready to fly by the time you want to fly, betting that it really will work. Or, you commit to spinning down, then back up for the midcourse correction.

Note that there is less risk to doing course correction small thrusts on a rigid spinning object, than a cable-connected system. Abandoning the cable idea in favor of rigid structures is one way to raise the odds on your bet that you can make this adjust-while-spinning technology actually ready-to-fly.

However, maximal prudence says if your flight design depends on a critical technology yet to be made flight-ready, then it is unlikely in the extreme you will ever actually fly. So go with what you know already works, and only that. Which means, you must spin down to make your midcourse, then spin back up afterwards.

So how does that affect your design? Which is easier to do reliably? Spin up and spin down operations with a cable-connected assembly, or a rigid assembly? What do we have more experience with? The answer is on display in any tire shop, by the way: rigid assemblies.

So, if you go with a rigid assembly, how do we do it? Do we build a giant wheel, or something similar, which is going to fall in the range of building Battlestar Galactica? Do we use trusses to connect things like we did for ISS, which add to inert weight, so that the rocket equation pushes you into building Battlestar Galactica?

Or, do we use the things we already have to have as our "truss"? Things like cargo containers, propellant tanks, and some but not all of our habitation space? THAT is where my spinning baton idea comes from: answering those otherwise unanswerable-because-of-costs-and-risks questions.

GW

Last edited by GW Johnson (2017-08-08 09:48:05)

RobertDyck · 2017-08-08 11:25:23

GW Johnson wrote:

In theory, you can, even with a cable-connected system, by carefully timing small bursts of thrust. In practice: this has never ever been done. It is not a technology ready-to-fly.

Respectfully, sir, on this point I disagree. Before X-1 flew, breaking the sound barrier had never been done. Well, it had been during World War 2 by fighters in a dive, but never on purpose, and never in level flight. Before Sputnik, nothing had ever entered orbit. Before Yuri Gagarin, a man in space had never been done. Actually, Russians had flown dogs on suborbital flights. Everything has a first. And this won't be the first time America spun a spacecraft with astronauts in space using a tether: Gemini 8. They had a problem that one thruster fired uncontrolled, but I believe modern electronics are far more reliable. Transitioning from 1G on the ground, to high-G during assent, to zero-G on orbit, to partial-G during orbital manoeuvres? It's done all the time for satellites, Shuttle, Dragon v1, Cygnus, etc.

Yes, this technology has to be developed. I have said we could do that with a Soyuz spacecraft carrying astronauts tethered to a Progress cargo ship. After Progress had off-loaded its cargo to ISS, and been filled with garbage for disposal. These vehicles will be in space anyway. With any test you have to prepare for success, but you also have to prepare for failure. If something goes wrong and you have to sacrifice one craft to save the other, the second being an cargo ship at end-of-mission filled with garbage makes that decision a no-brainer. Or you could do this with Dragon v2 tethered to Dragon v1. Or CST-100 Starliner and Cygnus. Or Dragon v2 and Cygnus, etc.

Some people in NASA are obsessed with the Moon. I would like to go straight to Mars, but even Dr. Zubrin came up with a variant in 1990 for the Moon. I've posted a modified version of Mars Direct for the Moon, using a Mars Direct habitat as a permanent Moon base. I don't think a Mars Direct ERV can be adapted for the Moon, so I suggested an Apollo architecture. You could land the hab unmanned, send crew via the Apollo-style system. The hab could demonstrate artificial gravity during transit to the Moon. The same flight profile it would use for transit to Mars.

GW Johnson · 2017-08-08 13:13:54

Well, cannon shells had been flying supersonic since pointy projectiles were introduced in the late 19th century. The WW2 vintage aircraft diving to transonic speeds nearly always resulted in fatal crashes. The key to supersonic aircraft was identified by the X-1 flights, and indirectly by its lack in the D-558-2 Skyrocket(***): the all-moving horizontal tail.

There is always a first time, and always some experience and development is required before application of a new technique. The all-moving tail was identified by the X-1 in 1947. Surprisingly enough, it was in production on the F-86 Sabre only 3 years later, and baseline on the F-100 Super Sabre by 1955. Usually, it takes longer than that.

As for Gemini-8 that was no tether, that was Gemini hard-docked to an Agena. The spin was accidental, and nearly killed the crew and destroyed the spacecrafts before Neil Armstrong got it under control with most of his re-entry attitude budget.

The tether has NEVER EVER been tested as a spinning item. There's a few years' work there before you risk a crew outside of LEO with it. How long do you want to delay going to Mars, when there is a perfectly acceptable way to sidestep the issue of the missing technology?

That being said, I wholeheartedly agree that tether spinning should be tested, starting now. The sooner this is done, the sooner my objections go away. I'm all for that. But (and here's the rub) NOBODY is actually doing it. And it's not like "they" don't know about it.

"They" don't really want to go, that much is clear. It is the likes of a Musk and tiny handful of others that wants to go. Not any government agencies, or we would have gone 20+ years ago, when we first could go with good assurance of getting the crew back.

GW

(***) I say that about the Skyrocket because it did not have an all-moving horizontal tail. Scotty Crossfield, like Bill Bridgeman before him, flew those supersonic missions on the pitch trim tab. They could push the yoke through its entire travel with no effect whatsoever on aircraft pitch. Flying the profile on the pitch trim tab was quite dangerous because it had so little authority, albeit infinitely better than no authority at all. Bridgeman as a company test pilot took it to Mach 1.98 before Douglas Aircraft turned it over to USN and Crossfield, who finally broke Mach 2 in it.

The X-1 started out flying as a conventional tail, killing one pilot (I think it was Slick Goodwin). With Chuck Yeager, they tried the all-moving tail option that had been built in, and it worked, and Yeager busted Mach 1 without dying. He eventually took it to Mach 2.5 as the -E version. The same all-moving tail went into the X-2 quite successfully. It broke Mach 3, although they killed another pilot (Mel Apt) before they enlarged the vertical fin. As originally built, it had insufficient natural yaw stability in the thin air at 70 kft.

Last edited by GW Johnson (2017-08-08 13:29:40)

RobertDyck · 2017-08-08 13:30:02

GW Johnson wrote:

As for Gemini-8 that was no tether, that was Gemini hard-docked to an Agena. The spin was accidental, and nearly killed the crew and destroyed the spacecrafts before Neil Armstrong got it under control with most of his re-entry attitude budget.

You're right. My bad. It was Gemini 11.

Wikipedia: Gemini 11

Astronauts Charles "Pete" Conrad, Jr. and Richard F. Gordon Jr. performed the first-ever direct-ascent (first orbit) rendezvous with an Agena Target Vehicle, docking with it one hour and thirty-four minutes after launch; used the Agena rocket engine to achieve a world record high-apogee earth orbit; and created a small amount of artificial gravity by spinning the two spacecraft connected by a tether.

Last edited by RobertDyck (2017-08-08 13:31:50)

Oldfart1939 · 2017-08-08 14:12:08

As my initial comment here--I really like the concept of a rigid "baton" structure for providing AG for the astronauts. As GW mentioned about the Battlestar Galactica size spaceships, the ITS is getting close to that size, isn't it?

But--until we've even tried out the tether system--it's only a conceptual wet dream. The one line that keeps popping into my head from all my professional training: Theory guides, but EXPERIMENT DECIDES.

RobertDyck · 2017-08-08 14:30:03

Oldfart1939 wrote:

Theory guides, but EXPERIMENT DECIDES.

Do the [explitive] experiment already!

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#1 2017-08-04 08:06:44

Technology in search of a mission?

#2 2017-08-04 09:54:21

Re: Technology in search of a mission?

#3 2017-08-04 16:33:32

Re: Technology in search of a mission?

#4 2017-08-07 09:10:18

Re: Technology in search of a mission?

#5 2017-08-07 10:19:37

Re: Technology in search of a mission?

#6 2017-08-07 11:40:30

Re: Technology in search of a mission?

#7 2017-08-08 09:41:18

Re: Technology in search of a mission?

#8 2017-08-08 11:25:23

Re: Technology in search of a mission?

#9 2017-08-08 13:13:54

Re: Technology in search of a mission?

#10 2017-08-08 13:30:02

Re: Technology in search of a mission?

#11 2017-08-08 14:12:08

Re: Technology in search of a mission?

#12 2017-08-08 14:30:03

Re: Technology in search of a mission?

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