Efficient space transportation

[Roger Arnold]

With all the discussion here recently about space elevators, launch loop, single stage vs multi stage, laser propulsion, and a new generation of nuclear propulsion, I guess it's time for me to emerge from hibernation and chime in. 

These are all issues that were kicked around a lot when I was a young engineer at Boeing in the 1970's (BAC, not BCAC). I wasn't in Space Systems Division, but I knew a lot of the folks there. I used to eat lunch in the Kent cafeteria with Gordon Woodcock and his crew. I even got to kibitz on the reference SPS design that Boeing did for NASA.

As far as transport to LEO is concerned, the first thing to say is good luck beating what's projected for Starship, with full reusability of both booster and orbital stage. It may take longer than Musk would like to get there, but SpaceX is committed to a reusability target of hundreds of launches per vehicle, and rapid turn-around times of hours, not days or weeks. If they can get there -- and I see no fundamental reason why they can't -- then the cost of an orbital launch will become not much more than the cost of propellant. That means the cost of payload to orbit could eventually approach $20(?) per kg. (I don't actually believe that figure, even though it's been mooted by Musk. But $50 I can believe.)

Can the cost of transport to orbit be reduced below that figure by reducing the cost of propellant consumed? There are three potential approaches: (1) increase the ISP of the engines, so that less reaction mass is required to achieve a given delta V; (2) reduce the specific cost of the propellant mix; or (3) reduce the delta V required to reach orbit. 

Regarding the first approach, the chemical reactions and high expansion ratios in the best rocket engines are quite efficient at converting chemical potential energy in the fuel and oxidizer into kinetic energy. Figure maybe 80% for a rocket engine in sea level atmosphere and 90% for operation above the atmosphere. Not a lot of room for improvement there. So the only real options for higher ISP are air-breathing engines and / or an external energy source. The latter is what boost phase microwave or laser propulsion is all about.

Air breathing engines add a lot of mass and complexity to the booster. They're useful in the initial part of the boost phase, but after that they're mostly dead weight. They waste rocket propellant accelerating to booster separation velocity. The weight penalty isn't limited to the engines themselves; they add enough weight to the booster that it isn't feasible to use a rocket-driven return to launch site and vertical landing. They dictate a winged reentry, U-turn in the atmosphere, and powered cruise back to base. 

When the weight penalty of wings and air-breathing engine are factored in, you find that it just isn't practical to include them in the main booster. The only feasible possibility is a 2.5-stage system. Vertical takeoff, but early separation of a pair of air-breathing winged boosters at ~ 60,000 feet and ~ mach 2 on a sloped trajectory. The winged boosters are the ".5" stage. Before separation, they cross-feed fuel to the main booster. 

The main booster is firing in parallel from the time of liftoff, but by design, its thrust isn't quite enough for unassisted launch. At the time of side booster staging, most of the main booster thrust is horizontal. Lateral acceleration goes from just under one gee at side booster staging to perhaps 3 gees at main booster cutoff and separation. The main booster can be light enough that, like SpaceX boosters, it takes only a modest propellant reserve at separation to cancel and partially reverse downrange velocity. The booster returns on a mostly ballistic trajectory for a powered vertical landing at the launch site. 

You can think of the system I just described as an optimized Falcon Heavy. It uses air breathing winged flyback vehicles as the side boosters in place of modified Falcon 9 booster stages. Their separation velocity would be lower. But the propellant cross-feed to the main booster stage should result in a similar or slightly greater payload capability to orbit. The airbreathing engines would allow it to deliver that payload at a savings in total fuel and oxidizer. I'd estimate the savings at perhaps 25%. Whether that's enough to justify the development cost and risk is arguable. It's certainly secondary to high reusability and rapid turnaround.

The other option for increasing ISP is external energy input: laser propulsion. I think that may be feasible, but I doubt that the cost of payload to orbit would beat what Elon's Starship is projected to deliver. The only thing that might drive us in that direction is if the amount of water vapor, CO2, and unburned methane added to the upper atmosphere by frequent Starship launches were to prove unacceptable.

There may be a feasible way to reduce the cost of propellants to place payload in orbit. It's counterintuitive, but engine thrust can be increased and fuel and oxidizer consumption reduced by injecting inert mass (water or liquid argon being likely candidates) into the rocket combustion chamber. (Or maybe into the throat and expansion bell?) You'd be deliberately lowering the initial ISP, but increasing the impulse per gigawatt of engine power. Carrying the inert reaction mass increases the vehicle's total liftoff weight, but it doesn't have to be carried for very long. It only pays off in the early boost phase. By design, it would be expended in the first few hundred mps of acceleration. If the tankage for the inert reaction mass is carried in drop-off side tanks -- or short range fly-back side boosters -- then the weight and fuel load of the main booster can be reduced. 

The use of flyback side boosters -- whether as straight air-breathing boosters or as tankage for thrust augmenting inert reaction mass, or (most likely) as some combination -- might reduce the cost of propellant per ton of payload to LEO by as much as 25%. That's small change until full reusability has driven down the cost of payload to orbit so far that a 25% savings in propellant cost becomes significant. 

That brings us to the 3rd approach for reducing the cost of payload to orbit: reducing the delta V required to reach orbit. How is that possible? Funny you should ask. 

As it happens, Don Kingsbury and I wrote about that in a 2-part "science fact" article that was  published in the November and December 1979 issues of Analog magazine. We called it "the Spaceport". A few of you on this list already know about it, but most probably don't. Don repeatedly encouraged me to write a serious journal article about the concept that could be published in JBIS or as an AIAA paper, but I never did. It was a busy time in my software engineering career, and the truth is that I never took the concept all that seriously.

I don't mean by that that I had any doubts about the technical soundness of the concept. It was the issue of system economics that stopped me. The orbiting Spaceport (or LEOport) would by any measure have been an enormous -- and enormously costly -- piece of space infrastructure. I didn't see any way it could be economically justified until fully reusable launch systems had brought down the cost of payload to orbit and created a sufficient space transportation market to justify the effort. 

I wrote the article to serve as a kind of "existence proof" that very low cost transportation to orbit was possible using technologies already available -- provided we were willing to invest enough in the appropriate infrastructure. We didn't need to wait for the kind of super high strength cables required for space elevators, or magic hypersonic space plane technology that could fly payloads to near-orbital velocity in the upper atmosphere. All we needed was a long horizontal mass driver / mass catcher in low orbit. It meant that a transport vehicle would no longer need to achieve orbital velocity on its own. It only had to achieve orbital altitude, which is vastly easier. Of course, it had to get there on a very precise trajectory at a very precise time. It would be shooting into the barrel of a 400 km long orbiting "cannon", operating as an electromagnetic mass catcher. But that was a just matter of precise measurement and control. Laser interferometry and late 1970's era real time computing capabilities actually made it relatively easy.

In the 43 years since the Spaceport article was published in Analog, a lot has changed. I've rethought many aspects of the design. If and when I manage to publish a revised version, it will bear scant resemblance to the original -- beyond the general concept of an orbiting "mass catcher". The original version that Don and I wrote about in 1979 was strongly influenced by Gerard O'Neils design for the lunar mass driver. There are quite a few reasons for changing that. I won't go into most of those here, but I do want to comment on one issue in particular.

Keith Lofstrom is one of the individuals on this list who definitely knows about the Spaceport concept. He was an avid Analog reader, and read the article as soon as it came out. He looked up my address and phone number, called me up, and arranged to drive up for a visit. (He was living in Portland, and I lived on Vashon Island near Seattle, so it was only about a 3-hour drive.) He had just recently invented his Launch Loop concept, so we had a lot to talk about. It was fun! None of which is relevant to the issue I want to talk about here. It's just background. 

Keith recently commented here that he thought I had abandoned the orbiting Spaceport concept after realizing that long horizontal structures in orbit are unstable. Given any deviation from the metastable horizontal position, they would want to rotate away from horizontal to align with the gravity gradient. With a structure 400 km long, that would send the tip into the lower atmosphere, quickly bringing down the whole structure in a slow motion mega-disaster all along the path of its orbit. Sorry, Keith; that's true of a passive structure, but I was aware of the issue from the start. The Spaceport was always going to be an active structure, consisting of many segments joined by linear actuators. It could shorten or lengthen to tweak its rotation rate to maintain dynamic stability. It could also bend slightly to accommodate lateral loads from passage of an arriving or returning vehicle. It would be more like a living snake than a rigid tube.

My latest conceptual design iteration actually has four independent mechanisms that are each sufficient to maintain dynamic stability of the structure in orbit. The primary mechanism is control of the linear actuators between segments. It's a pretty robust mechanism, as there are thousands of linear actuators involved, and hundreds of them could fail without impacting the integrity of the overall control system. Corruption of the control software would be the only real risk. But just in case ..

The secondary mechanism is dual flywheels in each segment of the trusswork structure of the Spaceport. Their primary role is for energy storage, but they can also be used to apply torque and increase or decrease the angular momentum / speed of rotation of the structure.

The third mechanism is control of the spin rate of the space station modules that would be distributed along the length of the Spaceport. It's an emergency mechanism that does the same thing as controlling the axis of the paired energy storage flywheels. Invoking it has the undesirable side-effect of varying the artificial gravity in the space station modules involved. However, small changes should still allow for enough control authority to maintain dynamic stability if the first two mechanisms somehow crapped out.

The fourth mechanism is station keeping thrusters in each segment. Never expected to be needed, but present and tested regularly in the spirit of good aerospace engineering "just in case".

The fifth and failsafe mechanism: explosive decoupling of all segments of the Spaceport. If activated, the structure becomes unusable, but all of its segments remain in stable orbit. They could probably be reassembled, once the engineers understood the nature of the sabotage or hostile attack that had triggered it.

That's more than enough for now. If people are interested, they can try pestering me into finishing and.publishing my magnum opus on "The Spaceport 2.0". 

[Jay Lewis]

That's interesting.  You said  "It would be more like a living snake than a rigid tube." which is an interesting visual.  Where would the first one go?

[Tim Cash]

Jay,

Transformer space solar powersats?  That can transform and move to avoid obstacles, maintain orbit, etc?  An almost living AI structure that beams power to the Earth?

If I did not believe something like this is possible, I would say you have been bingeing far too many Netflix movies!

If you have not yet seen Glass Onion, just let me say it appears to me to be a parody of a certain rich, manic space launch enthusiast we all know and love to tears.

I would like to see flexibility and autonomy mixed, not all one-sided but an appropriate mix of the two.

With continuous improvements in the main system components on SSP, all we need is a strategy that maximizes reliability of each Powersat for 30 years, or whatever the payback period needs to be.

By the way, I respect, highly respect the living area of a snake, and leave it alone whenever possible.

If I lived in Australia, I most likely could not afford life insurance against poisonous creatures, the cost would be prohibitive.

What about insurance against getting irradiated by a Powersat?  Does such a product exist yet?  If not, is there a market?  Can one make a living off the irrational fears of others?  Of course, you can!

My own computations suggest there really is not a problem from the fundamental or the harmonics of a Powersat, but I do not know everything.

I hope we spend more money trying to solve vexing technical problems in 2023 than trying to and achieving killing each other.

Tim Cash

cash...@gmail.com

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Tim Cash

cash...@gmail.com

[Jay Lewis]

Hi Tim, 

I'm intrigued by the idea of offering powersat insurance. Presumably the concern is higher rates of leukemia so in a way its just cancer insurance? Cover all cancers and price accordingly?

Jay

[John K. Strickland, Jr.]

It seems physically impossible to get cancer from SPS beam radiation, as the frequency is LOWER and the wavelength is LONGER than visible light.

“Light” with shorter wavelengths has higher energy photons, so “light” with longer wavelengths has lower energy photons.

High energy photons are needed to disrupt molecules in living tissue.

Real simple – visible light does not cause cancer, UV does.

 

John S

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[Tim Cash]

John et al,

I have made studies and lectures on both LTE and 5G cell phone frequencies and the assumed maximum power levels, and I have been told by experts that in certain high density population spots along the East coast, that E-Field levels are in the hundreds, of volts per meter.

Wow!

DO NOT MAKE A CALL WITH THE PHONE AT YOUR HEAD, ever, AS IT IS DANGEROUS to your health.

Why take such a chance when we can use earphones?

There are likely far higher E-Field levels from our cell phone use than there will be from Powersats, according to my own research on the topic.

However, there are a few experts in the IEEE who do raise the danger of cell phone/5G/6G radiation as we race to bring about instant communications everywhere, incredibly stupid and obtuse, as we need to slow down and ponder what we are doing as a civilization before jumping off the cliff en masse like Lemmings.

I am not against such technologies, I am just saying we need to think before we act, as we all know Telcos are driven by GREED and PROFIT.

Think before we act!

Tim Cash

cash...@gmail.com

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Tim Cash

cash...@gmail.com

[Jay Lewis]

On Thursday, January 12, 2023 at 9:54:11 AM UTC-7 Tim Cash wrote:

DO NOT MAKE A CALL WITH THE PHONE AT YOUR HEAD, ever, AS IT IS DANGEROUS to your health.

There are likely far higher E-Field levels from our cell phone use than there will be from Powersats, according to my own research on the topic.

If this is true then extraverts with on the phones most of the day should stand out vs the introverts posting and texting with an occasional, short, call.

5G is still somewhat new so when should we do this comparison?

How does the danger compare to the use?  Are people living near towers at greater risk even if they never make a call?  Are there nodes where 2 toward overlap and make a higher power zone?  For Powersats how does it compare?  1/10th?  Can there also be hot spots?  Could side lobes overheat neighboring commsats?

Jay

 

[Tim Cash]

Jay,

Look up IEEE James Lin, one of the best experts on the dangers of overuse of cell phones.

I assumed the typical use of cell phone on your person, not at the head, and computed E-Field at various distances from a human user, also from use at the head.  I attach my 2019 lecture and Matlab code predictions on the topic for your reference.  As far as any device for either communications and/or wireless power beaming, I advise actual E-Field measurements, and not trusting anyone's predictions.  The measurements represent one sample taken at one time for TRUTH.  I believe our civilization has went off the cliff on the use of personal communications devices (cell phones) for the purpose of convenience, and has dispatched common sense to the nether regions.  I am also guilty as charged, I carry my cell phone everywhere, and am hooked on it's use and convenience.  There is no going backwards.  We need to learn how to conduct conversations once again, instead of being glued to our cell phone. The cell phone is more addictive than any known drug or device, analog or digital, that I have ever known or used.

Tim Cash
cash...@gmail.com

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[Jay Lewis]

I assumed the typical use of cell phone on your person, not at the head, and computed E-Field at various distances from a human user, also from use at the head.  I attach my 2019 lecture and Matlab code predictions on the topic for your reference.  As far as any device for either communications and/or wireless power beaming, I advise actual E-Field measurements, and not trusting anyone's predictions.  The measurements represent one sample taken at one time for TRUTH.  I believe our civilization has went off the cliff on the use of personal communications devices (cell phones) for the purpose of convenience, and has dispatched common sense to the nether regions.  I am also guilty as charged, I carry my cell phone everywhere, and am hooked on it's use and convenience.  There is no going backwards.  We need to learn how to conduct conversations once again, instead of being glued to our cell phone. The cell phone is more addictive than any known drug or device, analog or digital, that I have ever known or used.

Tim Cash
cash...@gmail.com

Thanks Tim, yes every company has armies of behavior experts hacking our built in defaults to maximize use.  Future generations may be as shocked that we gave these to kids and used without constraint.  The physical risks may be smaller than the mental risks.  

[Erinn van Wynsberghe]

Hi all,

This article offers more specifics about CalTech's recent SSP in-space test:

https://oilprice.com/Alternative-Energy/Solar-Energy/SpaceX-Rocket-Sends-Solar-Power-Prototype-Into-Orbit.html

Cheers,

Erinn

Erinn van Wynsberghe
(mobile device)
VanWyn Inc.
eri...@vanwyn.com