In my talk at Space Access in April on the “Impedance Matching” panel, I raised the issue of how to completely decouple atmospheric vehicles from pure in-space ones. That is, right now, all paths to LEO seem to go through a launch pad, even coming back from some place else (e.g., the moon or deep space). This is because it’s difficult and expensive to circularize there from places less deep in the well. It’s difficult to do aerobraking safely and reliably in a single pass, and multiple passes means that the maneuver can take a long time, which can be a problem for crewed vehicles. And of course, this doesn’t even address the issue of getting into the right orbital plane. But until we can fix this, we’ll always have the ugly and inelegant situation of having to come all the way back the the earth’s surface from any beyond-LEO destination, and have to spend resources relifting crew for each trip, and make a true transportation node in LEO (i.e., one that can be reached from any destination, either from the surface or in space) impractical.
Anyway, I’d like to see what kinds of ideas get kicked around in comments here, perhaps with the hope of doing a presentation at the SSI conference in October.
[Afternoon update]
Circularizing propulsively is of course an option, but it’s hard to see how it’s a cost-effective one, until propellant in space is really cheap. Assuming that one doesn’t aerobrake at all, it takes just as much delta-V to get into LEO as it does to leave it, and it would require an improbably large vehicle if the departing vehicle has to carry enough propellant to recircularize on the way back. Which, of course, again demonstrates the value of depots. With one at L-1, perhaps supplied from either the lunar surface or an asteroid, it might make sense to fuel up there for the circularization in LEO. It all comes back of course, to the point that I made in my essay last year — that reusability implies gas stations, and that it’s impractical without them. As Jon Goff demonstrated with his amusing “We don’t need no stinkin’ depots” slide at Space Access, which was a picture of his car with extra gas tanks for driving cross country, the more often you can fuel on a trip, the smaller your vehicle can be and, due to the exponential nature of the rocket equation, that goes in spades for space vehicles.
[Bumped]
Repeated passes also have the added disadvantage of multiple passes through the van Allens. Maybe you can avoid the upper one, but not the lower one. This is not a problem for cargo or a recently evacuated crew ship, but is is undesirable for crew. If the ship has a storm shelter, as it may need to have anyway, that might be less of a problem.
It may be useful to reflect a bit on precisely why we want to return to LEO. It is certainly desirable, but it would be good to enumerate some reasons.
The biggest reason (as I said) is to not have to waste money having to relift a crew if they’re going to go on another trip, though in practice, this probably isn’t an issue in the near term, because it’s unlikely that they want to head out again without at least a visit earthside.
But maybe I’m making a bigger deal of it than it really is. It just bothers me aesthetically. I was probably too influenced by 2001.
The other reason is that a vehicle that has to come back all the way to the earth surface is going to be less efficient in space (e.g., Orion having to carry chutes and heat shield all the way to the moon and back). A pure-space vehicle will be both lighter and more spacious.
A pure-space vehicle will be both lighter and more spacious.
True, but you probably would still want to have the ability to return directly to Earth in an emergency. Of course, that capability can be provided by a separate escape capsule, but in that case the deep space vehicle could also have its “home port” based at L1/L2, with no need to return to LEO.
Another advantage might be that servicing in LEO could be cheaper than servicing at L1/L2 once you had cheap return to LEO.
It would also be useful for cargo (including orbit raising from LEO->GEO), but we can probably already do that with existing technology, perhaps augmented with evaporative cooling.
It could help make export of lunox to LEO more economical / less uneconomical.
I like the idea of return to LEO too, but I cannot yet make a compelling case for it. Coolth is certainly somewhat of a reason (as it is for space tourism), but that alone is probably not compelling.
For cargo, SEP with self-annealing solar panels (near term technology I believe) would be an alternative to aerobraking. Maybe even for tourist cruises, with sufficient radiation shielding.
Return to GEO or perhaps MEO is another possibility, and with fewer radiation problems, but deorbiting from there, while fast, is expensive and hard on your TPS. A circumlunar cruise powered by SEP to and from GEO and with chemical propulsion from Earth to GEO and back could be wonderful. The long duration would not be a disadvantage the journey itself being the goal, provided you make it back in one piece.
Well, I’m envisioning an era of abundant transportation to/from LEO, and it seems silly that a deep-spacer wouldn’t ride home on a regular flight, but perhaps it’s just not practical, and doesn’t really save money, to stop at LEO first.
Heh, another advantage could be that it would facilitate efforts to make a law that says NASA cannot build launch vehicles or vehicles capable of taking off or returning from the Earth ever again. It would have to restrict itself to deep space vehicles.
I think it would be useful to consider a hybrid aerobraking approach, where some small to moderate amount of propulsive assisted trimming is used to reduce the effect of variations in atmospheric density, which I believe is the single largest uncertainty associated with aerobraking. I know that all aerobrake approaches use pure propulsive trimming to adjust the final orbit; I am thinking of the old idea of running an engine into the shock to modulate drag.
What are the ground rules? I assume you’re ruling out chemical rocket braking for delta-vee reasons. Are you limiting the scope to chemical propulsion?
What about Buzz Aldrin’s cycler idea? It doesn’t solve the aerobraking problem but does reduce the amount of mass that must return to Earth (or to LEO). I haven’t heard anyone talk about cyclers for quite a while, except for Buzz and then only in the context of Mars. Of course, Buzz isn’t much interested in the Moon these days. Do lunar cyclers make sense, or is there some sort of fatal flaw that causes people to ignore them?
I’m not ruling out anything.
LEO is the obvious place for space base camp, as such there is also a need to get large quantities of extra terrestrial resources to LEO (preferably sooner rather than later so the processing and manufacturing technologies can start being developed) – this is not just a problem for people carrying missions.
Tethers/rotovators are one obvious option, but not necessarily short term.
Propellant depots with cheap propellant greatly reduce the need for aerobraking. Earth and Lunar LOX/LH2 is probably the most likely short term prospect, with depots, etc. Can also scrape Earth’s atmosphere for volatiles using an electromagnetic tether to make up ISP.
A solar or nuclear power high ISP rocket is the other main prospect I can think of. I have been wondering a bit about thorium reactors of late (suggested as a solution to Earth’s energy problems). They scale down well, fuel is cheap and inert for launch, reasonable power to weight ratios possible, very fuel efficient, more user friendly, etc.
A more ideal aerobraking maneuver might actually capture some of Earth’s atmosphere (ram air collection shield), and accelerate it to orbital speed. Volatiles collected could be used as reaction mass to circularize the orbit with the rest added to depots. Could do this by towing a large inflatable sock (large area for reradiating heat), though hopefully more integral solutions might be possible. Doing the aerobraking higher up with a much larger shield might enable greater control.
Might it be possible to aerobrake at high altitude by maintaining a very high electric charge on the vehicle? This would impart momentum to charged species in the surrounding atmosphere.
HEO, GEO, or Earth-Moon L4/L5 stations seem the most likely future end-points for interplanetary travel to/from Earth. Likely with additional stations in LEO and dedicated transport (e.g. reusable shuttles from L5 to LEO and LEO to Earth).
Eventually, when there’s enough off-Earth infrastructure returning all the way to the Earth’s surface won’t seem so necessary.
Jon Goff had an interesting series of posts on MHD Aerobraking and Aerocapture here:
http://selenianboondocks.com/2010/02/mhd-aerobraking-and-thermal-protection-part-iii-aerobraking-and-aerocapture/
It looks like there are many useful combinations of aerobraking, propulsion, MHD, and tethers possible for getting back to LEO from deep space. It is impossible right now to say which one of these will work out best. I’m currently most intrigued by the possibilities of MHD. It looks like it could be very useful for reducing what are currently ferocious thermal loads during reentry.
On a different subject Cyclers of various types have not been getting the love they probably out to have. A cycling space station flying a free return trajectory between the Earth and Moon looks like a good, reasonably near-term destination for space tourism. Eventually, cyclers between Earth and Mars could have their place. All of these would need shuttles for the trip between Earth and cycler.
What about a catcher’s mitt? That would be a separate high thrust ship that matches velocity with incoming ships to slow them down?
Throw up enough livable bubbles in orbit and the risk of an irrecoverably catastrophic emergency occurring before rescue rapidly drops below that of barbecuing during reentry. And let’s face it, for most destinations of interest even in the Earth sphere, “emergency escape to the planet” just doesn’t make sense as an option.
You want to place some payload at orbital velocity at some altitude. How can this be anything more than an exercise in deciding amongst launch from the surface, hook and tow from orbit, or something in between? Hell, if you know that much, the problem should be straightforward enough to automate search for the region of most favorable configuration, right?
Tom D beat me to the punch. I don’t have enough background in MHD yet to know for sure if the MHD aerobraking concept will really close, and how many passes it would take to do the aerobraking with, but it’s a cool technology with impressive potential. One of these days I’m going to put together a project to fly one of those as a secondary payload on a GTO mission……
~Jon
Are the problems of rendezvous and deorbit as difficult as achieving orbit in the first place?
Slightly related to aerobraking/aerocapture, and to the point about cheaply replenishing orbital propellant.
Demetriades, S.T., “A novel system for space flight using propulsive fluid accumulator”
Abstract:
Demetriades did a few more papers on PROFAC and MHD. Can’t find the original paper online, but would love to know more about this. Bisbos has a lay summary with pictures.
Have only the crew cycle between LEO and EML1/L2.
EML1/L2 can be the hub that major components cycle to and from, a moon lander, a NEO/Asteroid spacecraft, a Mars spacecraft. What constraints this applies I do not know.
Such a crew cycler can be designed for minimum mass for its purpose, transporting a small amount of astronauts for a short trip between the LEO hub and the EML1/L2 hub. Think Soyuz orbital module, or Orbital’s Cygnus cargo vehicle for example. Or Gemini, and that even had the burden of rentry. Or a small inflatable pod the size of the small interior needed. Purposefully minimalistic and spartan.
Address the cost of LEO propulsive circularization through a minimizing of the components it applies to, crew, and minimalizing that in turn.
Is there a tether solution?
Even if tethers can’t be used for crew (and maybe they could be), having the ability to throw around propellant cheaply will help enormously. The same would be true for self-annealing SEP and for different but similar reasons.
Photonic Laser Braking. I’d figure you could target the vessel on its inbound trajectory from quite a ways out and slow it down gradually.
It seems to me that the simplest solution is to rely on fuel depots for everything.
the judicious placing of fuel depots means you can get anywhere in the Earth/Moon/Mars area with steps of 5kps or less, just refueling your ship at each point along the way.
It works equally well coming back to LEO, so why invent exotic methods of LEO capture when fuel depots give a simple answer.
Sure there is the problem of cost.
Initially the cost of fuel being transferred around the place is going to be expensive. I take it this is the primary reason for considering alternatives for the L1 to LEO return leg of any trip.
However once you have the fuel depots in place and can go anywhere then the ability to find other sources of fuel becomes possible, and urgent, whether the source be Lunar, NEOs or Deimos.
Personally I would think in the interests of simplicity it would be better to swallow the high cost of providing fuel for the L1 to LEO trip in the short term rather than waste effort and resources looking for an alternative solution knowing the price od fuel will eventually fall.
Photonic Laser Braking.
High energy orbits like GEO or L1/L2 might be ideal for that.
Like in-orbit refueling, aerocapture should be a part of the next stage of space tech R&D. A test vehicle could be built out of a minimum Centaur stage with TPS from Orion, for example, mountable at various positions to test the efficacy of using partial rocket braking. Refuel in orbit, set on an excentric orbital path, bring it back for capture tests. Look for ways to judge atmospheric conditions; MHD plasma measurement perhaps, even if you are not using magnetic TPS. I like the idea of using boil-off from the H2 tank to cool the TPS and perhaps provide attitude control duriing the atmospheric pass. If we are using magnetic TPS, the boil-off may be usefull to cool the magnets.
Decouple crew return from that of the rest of the vehicle. You could do that by using a Soyuz-style reentry module just for the crew and aerocapture/aerobrake the rest. That eliminates the need for extra radiation shielding due to repeated passes through the van Allen belts and gets the crew down quickly using proven technology. If it takes a few weeks to fully circularize the orbit of the habitation module, so what? Once it’s safely in the proper orbit, do the necessary inspections and maintenance then refuel it for the next trip. Launch the crew in a new reentry module, lather, rinse, and repeat.
Rotating electrodynamic tethers hold the promise of being able to achieve altitude, plane and other orbital element change in LEO, from rougly 150km to 2000km altitude, with no expenditure of propellant. See Pearson’s work on EDDE (ElectroDynamic Debris Eliminator) and related work for background.
So a big decoupling that could take place is this: whatever the launch site is, launch all rockets due east into a low LEO orbit. The spinning electrodynamic tether tug can then take it from there to any LEO orbit, with no expenditure of propellant.
Decouple crew return from that of the rest of the vehicle.
In that case you could simply have the main vehicle return to a Lagrange point instead.
So decelerating into LEO is essentially the same thing as accelerating to LEO: the same rocket equation applies. The way we solve that problem is by either increasing deltaV or decreasing mass.
So far everyone has been trying to decelerate a capsule with stuff and people in it. But that includes systems meant to keep the people in it alive for extended periods of time. Why do you need all of that mass when you’re decelerating in order to re-enter or dock with a station?
Reduce mass and use propulsive deceleration by reducing the “capsule” down to one person per unit and only what’s necessary to keep them alive for the amount of deceleration/capture time. Aha! Its already been invented:
http://defensetech.org/2006/11/20/deadlies-nominee-inflatable-space-pod/
In that case you could simply have the main vehicle make a course correction (and perhaps pick up more passengers), return to the vicinity of the Moon, and then loop back again — why decelerate to park in a Lagrange point and waste time and fuel? It’s called an Aldrin cycler, as several comments above this one have pointed out.
And the rest of it is called an Aldrin cycler.
It’s difficult to do aerobraking safely and reliably in a single pass,
Why? Mars has a highly variable atmospheric density and it changes rapidly due to its low overall density and low heat capacity. The Earth does not have this issue and a nice laser sounder can map out the density in the path of the returning vehicle in almost real time.
In the 1980’s NASA did not think that this was a show stopper, why do you think this now? We have far better abilities today to know the exact orbit of the incoming vehicle than we did in the 80’s, and far better than at Mars. You could do a perigee lowering burn and as soon as you get below about 100,000 km you could more than likely dial in a window less than 100 meters across using fast processing and GPS for navigation.
This is especially the case if the intersection velocities are not that high such as a return from EML-1.
Please explain why this is such a show stopper.
The other reason is that a vehicle that has to come back all the way to the earth surface is going to be less efficient in space (e.g., Orion having to carry chutes and heat shield all the way to the moon and back).
Hmm. How about a facility at EML1 or a lunar base that fabricates single-use re-entry heat shields from lunar material? At least you wouldn’t have to carry the mass of the heat shield all the way up from Earth’s surface.
Don’t mind me, just clutching at straws …
help me out here, why dont we do the obvious? just figure out how to do aerobraking?
how much r&d could it possibly take? we do all this other impossible to comprehend orbital stuff. why not fly some test dragons and figure out aerobraking.
Seems to me you all are talking about 2 or 3 “stages” to the return vehicle. The Aerobrake and the Crew section with a possible emergency crew return vehicle. What if they were all modular.
The Aerobrake is big and could fit different types of vehicles behind it with some sort of standard adapter. Vehicles would have to meet some sort of center of mass definition. The aerobrake would bleed off a bunch of the energy of the combined vehicle and then a tug would later catch it and return it to its starting point (say L1). The vehicle would then use chemical propulsion to finish its “descent” to LEO/MEO. If the cargo, the people were headed earthside then the vehicle could split yet again. This would save the aerobrake for future use and allow alternate and probably slower transportation of the earobrake back to it’s starting point.
The larger vehicle would never leave space (or enter deep into the atmosphere) and would be reused for it’s useful lifetime. Return to earth could be accomplished but at a much lower initial velocity as a result of the aerobraking phase. If an abort were nescessary, the abort scenarios might require multiple dips into the atmosphere to bleed energy.
Some EDDE (ElectroDynamic Debris Eliminator) references:
http://www.star-tech-inc.com/id27.html
The article from The Bent of Tau Beta Pi is a nice overview.
Are there sufficient economies of scale to ever justify a Heavy Lift solution to some high orbit?
Once on-orbit maufacturing, and in situ resource usage is in full gear, and you have a steady stream of people and rare materials coming up, perhaps a one-way stream of heavy lifters would be optimal?
The materials in the lifter would be repurposed (can you embed volatiles in structural materials?), and lunar slag could be used for heat shields. Expensive electronics/rocket motors could be returned.
Any operations in LEO/MEO would ‘lower’ from the higher orbits (with the exception of microsatellites and special/secret projects).
Or is it that ground-to-orbit costs are always going to justify any infrastructure that limits how high the rocket needs to travel? Assuming of course good solutions to the sort of issues raised in this post.
I’m thinking of supertankers on the ocean. It’s an awful analogy, but if you develop the technology (automated return flying boosters, reusable rocket motors, repurposed fuel tanks), and there’s going to be a heavy traffic flow, would bigger be better?
Although, I am particular to the SKYLON project for just being darn cool. That is definitely a LEO only platform – giving its reliance on atmosphere.
I like the idea of a cycler and decoupling the transport of e.g. crew from the machinery that gets them different places. I’m flying to Boston this weekend, but that doesn’t mean I’m struggling to figure out how to land a 767 in the parking lot of my hotel. I take the jet to the airport, then transfer to a car, then park the car in the lot and walk to my room. Different machines for different phases of the trip, optimized for the specific requirements of the phase — no one-vehicle solution required, or wanted.
A bit OT but for Earth to LEO and using the aircraft industry as a rough guide one might assume say a fleet of a thousand vehicles* with an average payload of say 5 ton flying an average of four times a day (economically favored high capacity rates, to a station/depot and back). That works out at some 20,000 ton per day – not going to be able to justify heavy lift anytime soon, would need to be in the 100,000 ton/day plus market range, especially if one is to achieve significant vehicle variety (necessary for ample competition) and production numbers (necessary for low cost).
* The number of large aircraft and ships in the world is in the tens of thousands, so the one thousand fleet size I suggested might be on the low side, though perhaps if there are only a small number of orbital ports.
For got to add perspective – 20,000 ton a day which I suggest is still far too low to justify heavy lift is roughly ten thousand times current launch rates.
No answer to my question?
Why is single pass aerobraking beyond our ability today?
I can not remember but presumably it is theoretically possible to do a single pass dive and then lifting aerobraking maneuver that results in a ballistic trajectory that ends in a circular orbit of the desired altitude – with in limits. Is this worth the effort compared to a circularization burn? Having some control over lift would presumably allow a single pass maneuver to be actively managed – much more robust.
“Why is single pass aerobraking beyond our ability today?”
I tend to agree with Dennis, that this shouldn’t be all that difficult. A lot of work has been done on both aerobraking and aerodynamic-assisted plane change (separately and as one problem). The main challenge is achieving low ballistic coefficient (or modestly L/D) and low mass, but not so much in control of such a vehicle.
The last article on the subject I read was years ago, and had a ballute-type device couple with a GN2 cold-gas thruster serving as both retro and coolant. Even if one were to use an ablator, however, I doubt if the mass would ever be as high as a pure retrorocket.
But the control issue, I think, is rather specious.
I meant to write “modestly high L/D.” By that I mean on the order of 1.0.
I can not remember but presumably it is theoretically possible to do a single pass dive and then lifting aerobraking maneuver that results in a ballistic trajectory that ends in a circular orbit of the desired altitude
By its very nature this is not what happens in an aerobrake maneuver. Lets say we start at L1.
First we do a perigee lowering burn and plane change at the same time (from L1 plane change dV is small).
Then we dive into the atmosphere at the right height to remove the apogee orbital energy and put it to a number that we want, say 51.6 degrees at 340 km (ISS), then at the apogee of the orbit, you do a short burn to lift the perigee out of the atmosphere to the ISS orbital altitude. You can mess around with the burn to get your line of nodes correct as well.
Then rendezvous with the station.
You are never going to get a circular orbit out of an aerobrake by the very nature of the activity.
The only reason for a “heavy” heat shield (and the parachutes) is if you have to get back to Earth quickly. Otherwise, VASIMIR, fuel depots and reasonable materials technology for aerobraking (per Wingo) seem to answer most questions.
Some papers on the subject
http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19890004083_1989004083.pdf
This is just the beginning.
We all need to get smarter on this subject.
Note: Rand I had several links to papers at http://ntrs.nasa.gov but your spam filter kills it.
Go to ntrs.gov and then do a search on Aerobrake. Lots of really good papers and studies there.
You are never going to get a circular orbit out of an aerobrake by the very nature of the activity.
Yes sorry, brainfart. Admittedly I did do a quick numeric model to reacquaint myself.