GEO Orbital Debris Mitigation Paper Excerpts

Posted on April 7, 2022 by Kirk Sorensen

Back in 2006, I helped a high-school student (Daniel Rodrigues) who was interested in momentum-exchange tethers to write a paper for a high-school class about a concept for a tether that would remove spent geosynchronous satellites from their orbits quickly, putting them into an elliptical orbit with a perigee that would intersect the atmosphere. He recently recovered the paper and sent it to me at my request, and I am publishing some of the more relevant sections of it here, with minor edits and occasional expansions for explanation.

Background on Momentum Exchange Tethers

The Momentum Exchange Tether is a concept originally pioneered by Hans Moravec in 1977[1]. This orbital facility, essentially a rotating cable in orbit of the planet, had the ability to touch the surface of the planet every 20 minutes, and lift payloads into orbit. Carroll, in 1991, evolved this design into a totally in-space system able to lift payloads from sub-orbital trajectories and toss them into higher orbits [2]. The Momentum Exchange Tether system was refined further in 1998 by Bangham, Lorenzini, and Vestal, who designed the system to transfer payloads from LEO to Geostationary Transfer Orbit (GTO), and who concluded that the system should be composed of two separate facilities: one at an altitude of 2019 kilometers, and another at 25048 kilometers [3]. In 1999, Hoyt and Uphoff reestablished the one tether design due to simplicity concerns, but retained the LEO to GTO configuration[4]. Since this study, the Momentum Exchange tether has been refined by Hoyt once again in 2000[5], by Sorensen et. al. in 2003[6], and finally Hoyt once more, also in 2003[7]. As of now, the standard Momentum exchange tether is situated in a GTO, rotating so that its angular velocity and orbital velocity, when combined, equal the orbital velocity of it’s payload in LEO. At the tether’s perigee, it rendezvouses with the payload, rotates 180 degrees, and releases the payload. This adds momentum to the payload, but takes it away from the facility, consequently lowering its orbit. However, ballast at one end of the tether disallows for a significant drop in altitude. The station is then reboosted, and is then ready for another payload. The largest portion of the system, the cabling, is composed of a series of interlocking primary and secondary lines, a design known as the Hoytether [19].

The Problem with Orbital Debris

This investigation aims to apply the momentum exchange concept to the deorbiting of unwanted satellites (otherwise known as orbital debris, or simply debris). Orbital debris consists of inactive spacecraft, spent rocket stages, spacecraft fragments, and other miscellaneous objects [8]. Objects in the .01 to 1 cm size category can cause significant system damage, and objects larger than a centimeter can conceivably be catastrophic [8]. Additionally, spacecraft can only be shielded against debris up to 1 cm, due to mass practicalities [8]. Therefore, it can be concluded that orbital debris is a considerable threat to spacecraft.

Orbital debris can be especially troublesome in Geosynchronous Earth Orbit (or GEO), a very valuable region in space. At GEO altitude (35,678 km above the Earth’s surface) satellites orbit the Earth at the same rate as the Earth rotates. This means that the satellites stay over a fixed point on the planet, which as obvious commercial value for communication satellites. However, this region in space is rapidly filling with debris. As more satellites are put into GEO, the amount of debris only increases. As of January, 2005 there were 153 tracked, uncontrolled objects in GEO, or about 14% of all known objects in GEO [9]. These uncontrolled objects will only endanger the, on average, 21.1 satellites that will be launched annually into GEO from now into the near future, which is a number certain to increase in time [10].

Objective Statement

A momentum exchange tether will be designed that is capable of capturing uncontrolled satellites and orbital debris of moderate dimensions in Geostationary Earth Orbit (GEO) with a maximum mass of 5400 kilograms (encompassing 83% of satellites expected to be launched, into 2013)[10], and propelling the debris to a negative perigee altitude, ensuring the destruction of the debris. The momentum exchange tether will also be capable of capturing satellites in Geostationary Transfer Orbit (GTO), and slinging the payload into Geostationary Earth Orbit, losing momentum gained as a consequence of deorbiting debris.

Initial Calculations

The design of the tether system began with the analysis of the change in velocity required to remove a satellite from GEO into a deorbit ellipse intersecting with Earth’s atmosphere before perigee. The perigee of this ellipse was selected to be at an altitude of -100 km. This was done for several reasons: First, it ensured the destruction of the debris with a moderate margin for error, and second, it allowed for some flexibility in the coming design stages. The delta-v was set up between the initial orbit of the debris, GEO (circular equatorial orbit with an altitude of 35786 kilometers) and the target ellipse, which intersected with the Earth’s atmoshere. An infinitely massive ballast was also assumed in these initial, preliminary calculations, in order to simplify the center of mass variables. To calculate the speed of a satellite in GEO, the following equation was used:

This equation solves for the tangential velocity of an orbiting body (v), taking into account the Gravitational Parameter (mu) and the radius of the orbit (r). Afterward, the specific mechanical energy of the target ellipse was calculated. The specific mechanical energy (epsilon) is essentially a value that gives the energy per unit of mass of an orbiting body. The equation for this value is as follows:

where “a” is the semi-major axis. From the specific mechanical energy, tangential velocity of an orbiting body can be calculated using an alternate from of the equation for specific mechanical energy:

This equation was used to calculate the tangential velocity of the payload at the apogee of the target ellipse (after release from the tether). When the velocity of the payload in GEO is subtracted from this value, a new value of -1.5017 km/sec is calculated. This means that a vector with a magnitude of 1.5017 km/sec must be applied to the payload, in the opposite direction of the path of the payload, which accounts for the negative value.

Tip Velocity and Rendezvous

Once it was calculated how much the debris needed to be slowed down by (1.5017 km/sec), the actual tether facility could be designed. The first step in this process was to determine how the tether would perform its intended task. In this particular configuration, the tether would be situated below its payload. The tether’s center of mass (CM), about which it rotated, would be traveling in the same direction as the payload, albeit slower. The tip of the tether configured to capture the debris would also rotate in the direction the debris was traveling. Capture would occur with the payload directly above the tether’s CM, when the tether reached its apogee. The tether would then release the debris one-half of a rotation later, decelerating the debris into the above-discussed deorbit ellipse. Possibly the most critical value in this scenario would be the tether’s tip velocity, which is the velocity the tether would impose onto its payload. As mentioned previously, the debris needed to be slowed by 1.5017 km/sec. But because the tether imparts this velocity onto the payload in two ways (this will be elaborated upon in a moment), this value was divided by two, which gave the value of 0.75085 km/sec as a tip velocity.

Rendezvous between the payload and tether tip would not be possible if the velocities were not matched [6]. Therefore to find the speed of the tether’s CM, the tip velocity was subtracted from the velocity of the debris (3.074 km/sec). Therefore, the combination of the tether’s CM vector and the tip vector would equal the vector of the debris. In this scenario, the tether’s CM was calculated to be traveling at 2.32315 km/sec, at apogee. From the debris’ point of view, because vectors along the x-axis would be matched, the tip of the tether would be approaching from below it, along the y-axis. When the payload is released, 180 degrees later, the tip vector is once again added to the tether’s CM vector. However the tip vector is not 0.75085 km/sec, but is now -0.75085 km/sec. Therefore, the payload is released at the desired velocity of 1.5017 km/sec, and enters the deorbit ellipse. Also, it is now clear why the tip velocity was equal to the desired velocity divided by two: the other half of the velocity to be imposed on the payload came from the CM vector. This allowed for a capture with a 0 km/sec relative velocity when the tip was rotating with the debris and the CM, and allowed for the desired velocity of 1.5017 to be attained once the CM and tip were traveling in opposite directions. It is important to note that because an infinitely massive ballast mass was assumed in these initial calculations, actual velocities before capture would be different. However, these variables would change commensurately, retaining the 0 km/sec relative velocity.

Ballast

The next component that must be analyzed is the ballast mass. The ballast serves as a mass that stores momentum, allowing for smaller changes in altitude after release [7]. The simplest and most efficient way to fix a ballast mass to a tether station is to utilize the rocket the station was launched in, which lowers initial launch cost by preventing the launch of additional mass to be used as ballast[7]. In this study, the Ariane 5-ECA launch vehicle was chosen as the most suitable rocket. Its high capacity (10,500 kg[14]) allows for massive satellites such as the tether station to be launched, and it has the capability to launch into a GTO [15]. The upper stage on the vehicle, the ESC-A, has a dry mass of 4540 kg [15], which is therefore the mass of the ballast.

Capture Mechanism

On the opposite side of the station is the capture device. The purpose of this device is to physically anchor the payload (in this case, orbital debris) to the tether itself, once position and velocity is matched. The design of the device must incorporate some margin of error. A mass of 200kg was estimated for this device, which will be capable of securing payloads of at least 1 meter long in any dimension. This is because debris in GEO that is tracked must be at least a meter wide to be traced, due to limitations in radar technology [10].

Concept of Operations

The facility will be launched in an Ariane 5-ECA rocket equipped with an ESC-A upper stage, into its orbit. The tether can then be fully deployed, and spun up to an angular velocity of 0.01882 radians/sec. This value will remain constant. The system is now ready for operation. The angular velocity of the system gives the tip a tangential velocity of 1.0921 km/sec. Added to the CM velocity of 1.9819 km/sec characteristic to the system’s current orbit will yield a value of 3.074 km/sec, identical to the orbital velocity of debris in GEO. After capture of debris, the CM of the tether shifts, toward the tip. This slows the tip velocity to 0.75085 km/sec, while accelerating the CM velocity to 2.32315 km/sec. This is because the tangential velocity of the point where the CM was shifting to is added to the old CM orbital velocity. This acceleration causes the altitude of the facility to increase, and is the first illustration of momentum exchange. After half a rotation, the debris is released. Once again, the CM shifts up, and accelerates, increasing its velocity. This also increases the station’s altitude, just as the previous maneuver. After release, the debris enters its deorbit ellipse. The velocity of the debris is now 1.5723 km/sec, at a radial distance (from the Earth’s center) of 42066.2 km. The debris under these circumstances would travel in an ellipse with a perigee altitude of -67.5 km. This means that the debris would enter the Earth’s atmosphere before its minimum altitude, allowing for a significant margin of error.

After release, the tether would be in a significantly higher state of energy, and therefore a higher orbit. The tether must be brought back to an altitude where it can perform its task of deorbiting unwanted space junk. There is a nearly “free,” and almost instantaneous way to bring down the facility. This would involve capturing a functional satellite on its way to GEO in a GTO intersecting with the fully boosted tether facility. The new satellite would interact with the tether in the exactly opposite way as the debris, as long as it has the same mass as the debris that was deorbited. If not, on board thrusters could complete the slowing of the facility to its original orbit. But inserting a functional satellite into GEO would save a significant amount of propellant, and would eliminate the need for an entire upper stage on the new satellite, saving more money. On a 5400 kg satellite, almost 2000 kg is used as on-board propellant and thrusters. This would mean a reduction in launch costs from Earth into GTO, which is typically about $10,000 per pound[16].

Of course, sending debris crashing through Earth’s atmosphere raises the question of safety. However, reentry of space debris is a very common occurrence. In the past 40 years, there have been over 16,000 known re-entries of cataloged space objects, without significant damage or injury[17]. This is due to both the fact that most if not all of the debris disintegrates in the atmosphere, and the fact that any remnants of the doomed spacecraft have a very low probability of impacting populated areas[18]. Regardless of the unlikeliness of a ground impact in a populated area, the decision to deorbit any particular non-functional spacecraft will have to be made on a case-by-case basis. Large objects have been known to survive reentry in past, and the reentry of any spacecraft containing radioactive substances is out of the question[18].

Conclusions

This investigation has outlined the basic configuration of a momentum exchange tether in GTO capable of both deorbiting debris, and putting new satellites in GEO. This tether will increase lifetimes of satellites in GEO by reducing the threat of debris, while reducing the cost of launching new satellites. Orbital debris is accumulating rapidly, and a solution such as the momentum exchange tether needs to be considered. GEO is a valuable natural resource that needs to be conserved, just as any other. The combination of a less hazardous environment in space with lower launch costs is certain to stimulate the development of a stronger space infrastructure, undeniably helping humanity expand its horizons.

References

1.Hans Moravec, A Non-Synchronous Orbital Skyhook, AI Lab, Computer Science Dept., Stanford University, Stanford, Ca. 94305

2.Carroll, Preliminary Design of a 1 km/sec Tether Transport Facility, March 1991, Tether Applications Final Report on NASA Contract NASW-4461 with NASA/HQ
3.Bangham, Lorenzini, Vestal, Tether Transportation System Study, NASA TP-1998-206959, 1998

4.Hoyt, Uphoff, Cislunar Tether Transportation System. AIAA 99-2690, 1999

5.Hoyt, Design and Simulation of a Tether Boost Facility for LEO-GTO Transport, Tethers Unlimited, Inc., Seattle, WA, AIAA 2000-3866, 2000

6.Sorensen et. al., Momentum eXchange Electrodynamic Reboost (MXER) Tether Technology Assessment Group Final Report, NASA Marshall Space Flight Center, 2003

7.Hoyt, Slostad, Frank, A Modular Momentum-Exchange/Electrodynamic-Reboost Tether System Architecture, AIAA-2003-5214, 2003

8.Interagency Report on Orbital Debris, The National Science and Technology Council, Committee on Transportation Research and Development, 1995

9.Serraller, Classification of Geosynchronous Objects Issue 7, European Space Agency, January 2005

10.2004 Commercial Space Transportation Forecasts, Federal Aviation Administration’s Associate Administrator for Commercial Space Transportation and the Commercial Space Transportation Advisory Committee, May 2004

11.Pro Fiber Zylon, Toyobo Co., LTD., 2001

12.Personal Correspondence with Kirk Sorensen, In-Space Propulsion Technology Projects Office, NASA Marshall Space Flight Center, June 22, 2005

13.Sorensen, Conceptual Design and Analysis of an MXER Tether Boost Station, Propulsion Research Center, NASA Marshall Space Flight Center, AL, AIAA 2001-3915, 2001

14.Technical Information Ariane 5, Arianespace International Affairs and Corporate Communications, Arianespace, 1999

15. Ariane 5 Users Manual Issue 4 Revision 0, Arianespace, Courcouronnes, France, November 2004

16. Futron Corp., Space Transportation Costs: Trends in Price Per Pound to Orbit 1990-2000, Sep. 6th, 2002

17.Technical Report on Space Debris, United Nations Scientific and Technical Subcommittee (STSC), New York, ISBN 92-1-100813-1, 1999

18. R. P. Patera, and W. H. Ailor. The Realities of Reentry Disposal, A98-43901 12-12, Spaceflight Mechanics 1998: Proceedings of the AAS/AIAA Space Flight Mechanics Meeting, Monterey, California, February 9–11, 1998

19.Foward, Hoyt, Failsafe Multiline Hoytether Lifetimes, 31st Joint Propulsion Conference and Exhibit, AIAA 95-2890, 1995

20.Orbital Debris: A Technical Assessment, National Academy Press, 1995

21.Grun et. al., Collisional Balance of the Meteoritic Complex, Icarus, 1985

22.Cour-Palais, B. G., Meteoroid Environment Model-1969, NASA SP-8013, 1969
​

Posted in Orbital Debris Remediation/Mitigation, Orbital Dynamics, Space Tethers | 1 Comment

L4 Opportunity

Posted on February 4, 2022 by johnhare

https://www.space.com/earth-extra-moon-trojan-asteroid-2020-xl5-discovery

An Asteroid 1.2 km across has been discovered at the Earth Sun L4 region. This is the second asteroid discovered in one of the Lagrange volumes of the Earth Sun with the other one about a third of that size. It is suggested that it is possible that there are more, quantity unknown, bodies in those regions. They are very hard to locate from Earth due to the distance and sun angle so actual number, makeup, and size in each L region is mostly speculation.

I think it might be possible that this is an ideal target for in situ resource utilization. The advances in cubesat capabilities open many new possibilities. A lander could test out a number of resource extraction techniques for immediate use. Regolith shielding? Steam rocket with whatever volatiles are located? Sintered structure in vacuum and low gravity? Low tech metal extrusion for structural components? Low tech film Solar sails?

The purpose would be to use it as a base to locate other materials in the region with a relatively small scope. And go visit them if found using local resources. Prospecting and exploring and evaluating. If there does turn out to be a large number of bodies in the L4 region, DeltaV per visit should be very low. This one is a C type, dark and carbon. There might just be enough variation in different bodies in the area to make it a resource rich destination.

How simple and cheap could a probe be and still investigate the possibilities??

Posted in Uncategorized | 5 Comments

Random Thoughts: A Joint International Debris Remediation Effort

Posted on October 22, 2021 by Jonathan Goff
Galactic trash orbiting Earth
We Don’t Want This to Happen (Credit: Getty)

As humanity’s ambitions in space increase, one of the biggest potential risks to to those ambitions is the dangers caused by the debris we’ve left on orbit over the first half century of space development. Since Sputnik’s launch in October of 1957, we’ve left over 18,000 pieces of debris on orbit that are larger than 10cm, and millions of pieces too small to track but big enough to destroy a satellite. One of the biggest threats comes from large satellites and more especially rocket bodies that were left on orbit during the early days of space development before countries properly realized the risks that leaving debris on orbit could pose. At last year’s IAC conference, a joint group of researchers from 11 countries published a list of the 50 most dangerous pieces of space debris on-orbit, and the vast majority of those 50 were rocket bodies, with most of them being ones launched by the now no-longer existent USSR.

Why Should We Care About Large Debris?

Derelict rocket bodies and large satellites have been known to breakup on-orbit, sometimes due to pressure vessels or batteries failing, and those explosions can create hundreds or thousands of new pieces of debris1. More importantly, derelict space objects can’t dodge, so while we can track them, there’s not much we can currently do if two school bus-sized derelict space objects are on a collision path at relative velocities several times faster than the fastest bullet2.

As Joe Carrol once put it, the best way to avoid creating “BBs” (untrackable but lethal space debris) and “hubcaps” (barely trackable lethal debris) is to get rid of the derelict school buses. If we want to see a world with multiple commercial LEO facilities, propellant depots, space settlements, and especially things like large fleets of Starship-sized spacecraft heading out to Mars or other destinations, we can’t allow the Low-Earth Orbit environment to become a shooting gallery of space debris.

What’s Standing in the Way of Solving this Problem?

Almost all of the most dangerous space debris up there was launched by governments as part of civil or military space missions. As such, it makes sense that governments should pay to clean up the environmental mess their activities created. While there are now early efforts in Europe and Japan to begin tackling Active Debris Removal of dangerous space debris launched by Europe and Japan, that still leaves most of the most dangerous pieces of space debris currently unaddressed.

As I understand it from conversations with my friends in the Space Law community3, a big part of the problem with addressing many of the most dangerous pieces of space debris is that as part of the Outer Space Treaty, which governs the actions of all major spacefaring nations, you’re not allowed to interfere with objects launched by other countries without their permission. Unlike on the oceans, there is currently no space equivalent of salvage law, or “flotsam and jetsam”. Once an object is launched by an actor within a state, that state retains responsible for those objects indefinitely. However, as I understand it, those states are only liable for damages if the object deorbits and damages something or hurts someone on the ground — if it damages something in space, holding them accountable would be really hard unless you could prove that they violated the accepted standards of care at the time they last had control of the object. So, as I understand it, most of the most dangerous space debris is owned by a country that can’t realistically pay to clean it up, can’t be deorbited by someone else without the original country’s express permission4, and space liability law won’t actually hold the owner responsible if that debris creates a ton of new debris, so they don’t have any incentive to clean things up, other than wanting to be a good citizen.

Additionally, the US government has been very reticent to fund developments in Active Debris Removal, because of the fear that adversaries might see these technologies as dual-use technology5, causing them to invest in similar technologies that could be used against the US.

Anyhow, at a World Economic Forum meeting on the topic of space debris and space traffic management, someone floated an idea for a potential way to solve these problems. I can’t remember who suggested the idea, to give proper credit, but I wanted to run with it, and try to bake it a little more6. The idea is what if we propose to clean up the most dangerous pieces of large debris via a joint international cleanup effort?

Concept for a Joint International Debris Remediation Effort

While we’re on the cusp of having the technology to solve the problem of large space object active debris removal (ADR), the policy and international relations issues are the bigger unsolved pieces of the puzzle. So the idea is, what if we find a way to jointly tackle the problem in a way that incentivizes all of the key players to act, and removes some of their bigger concerns about acting?

What I’m Proposing: A joint effort, involving at least the US and Russia, but possibly also China, Japan, and Europe, to capture and recycle7, and/or controllably dispose of the most dangerous 100-200 pieces of space debris.

Key Elements of the Concept:

  • Most of the funding for the effort would need to come from the wealthier spacefaring nations. We have the wealth to do this, and stand the most to lose from inaction.
  • To make it worth Russia’s while to participate, there should be a way for them to economically benefit from this effort — my suggestion would be to have them involved at least with the on-orbit recycling part of the effort. Helping them build up new and useful space economic capabilities and industries is a good incentive for cooperation. Though in theory, the funding states could maybe insist on these being joint ventures between companies in their countries and Russian companies, as a way to share in the upside.
  • If China also wants to participate in this a mutually-acceptable portion of the project could be done via Chinese companies, or joint ventures with Chinese companies.
  • It’s a good idea to have dissimilarly redundant RPO sensors, especially for something as dangerous as capturing and detumbling a spinning rocket stage. So, it might be possible to have both commercial-grade US and Russian (and Chinese?) RPO sensors on the vehicle, with the feeds from both of them being live streamed during RPO events for enhanced transparency8
  • Having the most sensitive operations, like the RPO and detumbling/grappling efforts, lead by commercial companies in more neutral countries might also help. Companies from the US, Russia, or others could still develop technologies for use on these missions, but have them operated by someone that is as innocuous as possible to both US and Russia. Conveniently, two of the main companies working on active debris remediation are a Swiss company, Clearspace9, and the Singaporean/Japanese/US/UK/Israeli company Astroscale.

There are probably plenty of other elements here, but I wanted to get the idea out there. A joint US/Russia or US/Russia/China mission like this could be a way to break the logjam on cleaning up the space environment before the situation degrades further, while also helping greatly accelerate development in advanced on-orbit operations that could help enable not only in-space recycling, but also in-space construction and eventually asteroid mining.

What do you all think?

Posted in Business, Commercial Space, International Space Collaboration, International Space Competition, Orbital Debris Remediation/Mitigation, Random Thoughts, Satellite Servicing | 3 Comments

Suborbital Balloot

Posted on August 1, 2021 by johnhare

Watched a bit of the two suborbital flights, not that much though, certainly not with the interest I would have had roughly a decade and a half ago. I had thought back then that the technical route to routine space travel would be through incremental advancements from suborbital to orbital to cislunar to other planets and asteroids. Looks like I was wrong again.

One thing that stood out to me was the people trying to play in microgravity with limited room in which to do it. Much more limited in volume and more clutter than on the ZeroGee flights. Looked to me like time was limited enough that there was just time to glance out the windows and play half a round of ZeeGee catch.

A balloot in the spirit of the inflatable space structures might help here if some problems of fast deployment, rapid access, and safety could be addressed. A 5-10 meter diameter inflatable without seats and other clutter would give considerably more play room than either SS2 or NS. Fast deployment might be a bit of a challenge as there wouldn’t be time for a leisurely five minute deployment and check out.

I don’t see how the idea could be adapted to SS2, but it does seem that a VTVL ship could have a fast clamshell hatch in the nose that could allow people in a fairly tight cabin access to a large play area within seconds of reaching vacuum and engine cut off. So a ship that would have had six people might fly with a dozen, each of which would have a lot more room to play and experiment.

With a large balloot starting deceleration at much higher altitude, it seems possible that a ship could fly much higher without subjecting the participants to excessive gee loads. Every extra Mach number gives on the order of another minute of play time in microgravity. On the way back, the participants get back to their seats while the hatch closes. The balloot remains deployed until landing as a drag device incidentally reducing terminal velocity considerably, possibly enough to make a failure of engine relight survivable, if painful.

This seems to me an idea that, even if feasible, is very late to the party. It looks like orbital tourism is likely to become fully operational in the same timeframe as the two suborbital contenders. The contrast may well cut into the desirability of the short pop ups. One interesting factoid though is that the people that were insisting that orbital was 64 times as hard as suborbital have to be wondering what the two companies could have accomplished with 64 times the investment.

Posted in Uncategorized | 9 Comments

Layover Lunar Lander

Posted on June 19, 2021 by johnhare

A lunar lander concept I haven’t seen elsewhere is a layover concept. A tall ship lands on its’ tail the way the several suborbital hoppers and the Falcon9 has been doing for several years now. After landing and checking the area to one side, a small thruster tips it over on two legs. Before reaching the surface, some landing rockets in the nose fire to bring velocity to zero as the nose lands on a couple more much smaller legs.

If the lander is 100 meters tall, the terminal velocity that must be braked before the nose landing is under 20 m/s. Stopping 100 tons just at the surface would require between 400 and 700 kg of propellant depending on the Isp in use. If feasible, seems like a fair trade off as opposed to raising and lowering people, supplies, and equipment 100 meters for the duration of the mission.

The lower legs on the pivot side would need to be able to handle loads from two directions, though the legs at the nose could be quite light. One alternative leg option is to have the lower pivoting legs articulate up under the body of the vehicle to split the loading better and to reduce braking thrust requirements a bit more.

The tip over landing should be quite simple as there would be no question of where the vehicle is in relation to the ground at any given time. Of course smaller vehicles would have even less trouble and use less propellant. A 20 meter high vehicle with a 10 ton payload would require only 20-30 kg of braking propellant.

Posted in Uncategorized | 24 Comments

Unorthodox Reusable Lunar Landers Concepts

Posted on June 12, 2021 by Jonathan Goff

[Author’s Note: Back in summer of 2019, shortly after Mike Pence announced the goal of having NASA return astronauts to the lunar surface for the first time in over half a century, I had the idea of doing a blog post about the benefits of fully-reusable lunar landers, and then going over a few of my favorite unorthodox reusable lunar lander configurations. I got side-tracked at the time by my entrepreneurial day job1, and by the time I freed up from that, Altius was busy supporting one of the three Artemis HLS teams, so I felt a blog post like this might be impolitic. Now that we’re no longer actively supporting that team, and given that I no longer have running a startup as an excuse for not blogging, I wanted to finish fleshing out these ideas and at least get them out there for discussion.]

While traditionally most lunar landers and lander conceptual designs have been fully-expendable, many people, including NASA have begun to see some of the benefits of reusable lunar lander systems. Some of those benefits as I see them include:

  • Lowers the cost of sending hardware to the Moon: This is the obvious one that people get about reusability, is that so long as it’s done right, it can significantly lower the per mission cost, especially if the lander can be reused many times over its lifetime.
  • Makes the program easier to throttle up or down: One of the big challenges the Augustine Commission noticed for NASA Human Spaceflight missions was that they could rarely afford to both develop new capabilities while operating previous ones. With a fully-reusable lander system, especially one designed to not require a huge standing arm to support it, much of the cost of a given mission could be the marginal cost of launching new crew/cargo/propellant, which means it’s easier to throttle down temporarily without losing the capability.
  • More amenable to non-critical-path international participation: With reusable landers, once they’re launched, incremental missions mostly require refueling, reprovisioning, and a crew swapout. Government space agencies typically don’t want to spend money outside of their country–they typically try to find a way where they can handle things through barter and no-exchange-of-funds agreements. For instance, in exchange for getting some of the ISS crew slots, Japan and ESA both developed cargo vehicles to bring some of the cargo that ISS needed. So they could spend their space agency money locally, and use that contribution to get ISS astronaut slots without having to develop their own crew launch capabilities2. Reusable lunar landers provide an easy barter option for international participation in NASA lunar missions — launching propellant and/or cargo. The nice thing is that if done wisely (say using a Low-Orbit HSF Depot in LEO), this might require minimal development cost for foreign countries, while allowing them to usefully contribute, but in a way where they’re not on the critical path, and they can throttle up or down their involvement as desired.
  • Creates near-term demand for lunar ISRU: Once you have reusable lunar landers, the vast majority of the mass needed per mission is propellant. Being able to source that locally could significantly reduce the cost of missions3, and could increase the capability of landers by enabling them to be refueled both in orbit and on the surface4. With reusable landers, you have established demand at an established price point, which makes closing the business case for lunar ISRU easier, so long as you can truly extract it cheaper than shipping it from Earth5.
  • Enables a much more ambitious exploration program: This should be obvious, but once you have reusable landers, you have tons more flexibility for doing things beyond simple flags-and-footprints missions. Things like lunar search and rescue, doing suborbital sorties from a bigger outpost to explore areas of economic or scientific interest, etc. become more feasible.

Anyhow, if you’re reading this blog, I’m probably preaching to the choir here, but I wanted to lay out some of my thoughts on why reusable lunar landers matter.

Unorthodox Reusable Lander Concepts

Given the desirability of fully-reusable lunar landers, it’s sad that most of the best-known reusable lander concepts have used a very similar landing configuration — tall skinny landers with the crew and cargo mounted on top.

Traditional Reusable Lander Concepts: One Long Elevator Ride for a Man, One Griant Crane for Cargo… (credits: SpaceX, Chesley Bonestell x2, Lockheed Martin)

In this series I wanted to highlight a few other potential fully-reusable lander configurations worth considering, some thoughts on variations on the themes, and their pros and cons. Only maybe one of these configurations is one I could claim to have invented, but I thought it would be worth highlighting some other good ideas that may or may not be as well known, especially to younger space engineers or enthusiasts.

In the following parts I’d like to discuss the following reusable lunar lander configurations:

  1. Bottom-loader SSTOs
  2. Horizontal-landing SSTOs
  3. Un-Crasher TSTOs

I may think of and add additional configurations later if time permits. But next up: Bottom-loader SSTO landers.

Posted in ISRU, Lunar Commerce, Lunar Exploration and Development, Propellant Depots, Reusable Lunar Landers | 8 Comments

Goff Family 2021 Summer Sabbatical Part 1: Utah Trip

Posted on June 1, 2021 by Jonathan Goff

Okay, so for the first leg of my sabbatical, we went on a trip to Utah to visit my family. It had been two years since the last time we had seen any of my family in person. Our last trip had been to go be there for my “little” brother’s wedding in the Salt Lake temple. A lot has happened since then, including Jeremy’s wife Emily graduating from BYU, my older brother Adam graduating from Western Governor’s, and Jeremy and Emily having their first baby.

So as early as last December, when it was clear that effective COVID vaccines were soon to be available, we had been notionally planning on going out sometime in May or June. Once Tiff and I had our shots scheduled, we decided to plan something for end of May. Serendipitously, by the time we went to leave, Tiff, Jonny, and I were fully vaccinated, and Jamie and Peter had at least been able to get their first shots. At the time we planned this trip, we had no idea that I was going to be taking the full-time position at Voyager and transitioning out of my Altius role. Once we decided to do that, we set my last day at Altius as the day before we wanted to leave for Utah, and then worked extra hard to make that happen.

I ended up working late my last day at Altius, so we ended up taking an extra day (the 21st) to finish packing and take care of passport things. It turns out that in our part of Colorado, all of the places you can apply for a passport require an advanced appointment to be scheduled, and all of them were booked over a month in advance. With expedited passports taking 4-6wks, and us wanting to leave on our Canada trip by the second week of July, we couldn’t wait that long. Fortunately, Utah didn’t have as much of a backlog, so I scheduled a passport meeting at the civic center for my parent’s town for our third day there. And then of course made it look like on Twitter that the whole Utah trip was some last-second idea we decided to do on a lark as a way to get our passport applications submitted, because silliness.

We had a great time out there, participating in the baby blessing for Jeremy and Emily’s little girl, Victoria, visiting with my family and all of my siblings (and Tiff’s little brother), getting to see Jenny and Alan’s new place they’re remodeling, visiting and getting longsword lessons from my long-time friend Danny Farnsworth (and then later catching dinner with him and his girls), catching lunch and playing pool with Tiff’s “fake adopted sister” and her family, visiting BYU and getting a tour of the new engineering building1, getting to “hike the Y” with Jonny, hiking Bridal Veil falls with the family, a visit to the cemetery on our way out of town, and a side trip to Arches National Park on our way home.

We also got to stay in scenic Parachute, CO because the Eastbound I-70 was closed due to a huge accident in Glenwood Canyon. All the hotels in the town where the road closure started (Rifle) were completely full by the time we got there around midnight, but fortunately we only had to backtrack to Parachute. And we were able to make it home Saturday afternoon.

Anyhow, here are a few pictures from favorite parts of the trip:

Mountain Scenery on our way out to Utah
A great picture Ki caught of Jonny holding Baby Victoria2 after her baby blessing
Someone lost their first mouth stone at the pool party we had at our hotel
Peter and Jamie looking at a sculpture outside the BYU Museum of Art
BYU still doesn’t have an Aerospace Engineering degree3, but between their rocket club, their spacecraft club, and the robotics and UAV groups, they’re heading in a good direction.
Patrick Walton giving us a tour of the new engineering building at BYU. This lab was where his startup, CareWeather built their first satellites
Patrick also showed us the ground stations for BYU’s spacecraft club that they used for their PICSats. For CareWeather’s cubesats they switched to using an Iridium transceiver and had much better success.
On our second-to-last day in Utah, Jonny wanted to “hike the Y” with me. The Y trail is about 2.2mi round trip, and has over 1000ft of elevation change. I think the last time I made this hike was as a freshman at BYU, back when I was Jonny’s age.
The first few sections were pretty brutal, but ultimately we both made it all the way to the top!
Here’s a panoramic view from the top of the Y, looking out over Utah Valley.
That evening, we went to Bridal Veil falls with my mom, my little sister Mary, and Tiff and the boys. Tiff, Jamie, and Pip decided to hike all the way up to the first falls.
A view of the upper falls, with Jamie and Pip
Me, Jonny, and my youngest sister, Mary sitting around the firepit in my parent’s “Secret Garden”
Another view of their Secret Garden
A trip to Utah wouldn’t be complete without a visit to Jarom’s grave. Tiff had the good idea of having us sing a hymn while we were there, so we sang Families Can Be Together Forever. Very apropos.
On our way home, we went to Arches National Park for the first time as a family. It was a lot of fun. Here were the youngest three at the North Window arch.
And here are all the boys, looking silly as usual, in front of the South Window
I think the Double Arch was my favorite one of the park. Interestingly while we were here, I met a Filipino family from Cavite4, and then we ran into a family from our latter-day saint Ward here in Lafayette, CO who were also out on vacation. Small world!
The boys, striking a pose at the Double Arch
Peter and Jamie at the upper viewpoint for the Delicate Arch. We didn’t have enough time (or water!!) to do the hike out to the arch itself.
That night at Olive Garden in Grand Junction, Pip got to have spaghetti with a meatball that was almost the size of his head. If you’re wondering how I did with my calorie budget on this vacation, I believe the best description is the line from Alien “Nuke it from orbit, it’s the only way to be sure!”
With westbound I-70 blocked due to a huge accident up Glenwood Canyon, we had to backtrack to find a hotel. Fortunately we got the last room at the Grand Vista Hotel in Parachute, CO, and didn’t have to drive an extra hour back to Grand Junction. It was a bit crowded, but with the rollaways, we made it work.

Anyhow, that was the first leg of our adventure. We’re now back in Colorado, starting to pack up so we can move out of the house we’ve been renting for the last 8yrs. Tomorrow morning the dumpsters arrive, and Friday we get several of U-haul’s equivalence of the PODS mobile storage units. Wish us luck!

Posted in Family, Friends, Jonny Bloggin, Pip Bloggin | 2 Comments

Transitions and Summer Sabbatical

Posted on May 31, 2021 by Jonathan Goff

I imagine most of you who read Selenian Boondocks probably also follow me on Facebook and/or Twitter. But for those of you who don’t, as of May 21st, I decided to move up to a full-time position at my parent company, Voyager Space Holdings, and transitioned out of my role as President and CEO of Altius Space Machines1.

This will be an interesting change, basically moving into more of an intrapreneurial role rather than the entrepreneurial role I’ve been in for most of my career. As the VP of On-Orbit Servicing at Voyager, I’ll be providing leadership for their whole On-Orbit Servicing portfolio, including helping guide Voyager investments/acquisitions in the OOS space, developing and implementing a go-to-market and technology development strategy for Voyager OOS capabilities, and generally leading all things OOS at Voyager.

As part of this transition, I’m taking a 3-month sabbatical to recharge before jumping into my new role2. As part of my “unplugging” efforts, I’ve signed off of social media, unplugged from things work related, and am planning on using this blog as my primary means of communicating with people about my family’s misadventures this summer.

High level plans for the sabbatical include:

  1. A trip to Utah to visit family for the first time in two years (last week).
  2. Packing up and moving out of our current rental (most of June).
  3. Some camping stuff (late June/early July).
  4. A roadtrip to Canada3, the US Pacific Northwest, and possibly Alaska4 (mid July/early August).
  5. Looking for a new house to buy or rent5 in the Denver North area, and getting resettled (August).

My first week back at Voyager should be just in time for Space Symposium in Colorado Springs this August. Between now and then, I’ll mostly be blogging about our travels, but I may do a space blog post or two (not work related) that had been sitting on the backburner due to running a bootstrapped aerospace startup.

Oh, and if you want to comment on the blog posts, please comment here instead of on Twitter. I’m trying to avoid getting sucked into Twitter comments while I’m gone, and will only be on long enough to post a link6.

Posted in Administrivia | 2 Comments

The last few years

Posted on May 28, 2021 by johnhare

john hare

I noticed the other day that it had been over a year since I posted, and at least double that since posting on a space topic. I’ve had a bit going on for distraction, and haven’t had anything new to add in the way of space related concepts.

A few years ago with the recession going away, we started trying to expand the construction company back out to where it was pre-recession. Went through a couple of dozen people trying to expand with no success. Most couldn’t pass a drug test, or didn’t have transportation, or were unreliable, etc etc. Focusing on the company tends to restrict hobbies like blogging about space. I have my core crew and myself with no new people.

Hired an intern (paid) from Florida Polytechnic two years ago for the summer. His job was to work on remote controls for various small equipment we use. If I could not find the people to do the hard work, possibly make the work itself easier. He accomplished nothing useful in three months. Literally nothing. I have been slowly learning to get this done myself. It would be far more efficient if I could hire someone that understands the field, but have failed to find them.

We had a slowdown during the Covid lockdowns last year followed by a slammed schedule from last May through April this year. Slowing down some now with material shortages and difficulties with building departments permitting and inspections.

I got married in February this year and have had other things on my mind. We are buying a lot and will build over the next year mostly with cash. Granddaughter graduated University of Georgia this year with a degree in Biochemical Engineering. (don’t know what that means but she gets a good job right out of school) Other things going on locally.

Noticed a comment on another site that suggested that many of the concepts discussed here have been made obsolete by the success of SpaceX. While I will acknowledge that some of the concepts that made sense in the first place (many didn’t) may be less important, I still believe there is room for improvement all the time. SpaceX may be on the way to becoming the Walmart of space launch, but I believe there are many companies that are well poised to become the Dollar Stores and Discount Auto Parts of space. Those companies may benefit from anything that gives a competitive advantage. One of them may also pass on the outside when we are looking at the infield. As an aside, I am impressed with the progress SpaceX has made, while being very uncomfortable with the “hand it to Elon” meme going around.

I am inclined to restrict my limited blogging to things that are either new, of where I have some experimental evidence on a previous idea. I did an experiment a few years back that seems to support a method of doing the Thrust Augmented Nozzles that Jon wrote up only quite low tech. I have an idea of an inexpensive way to focus sunlight better over long (GEO to surface) distances and will run an experiment or two before discussing. And of course my various pump concepts still fascinate me, but would bore anyone else unless I produce some experimental proofs. Same with compensating nozzles.

Posted in Uncategorized | Leave a comment

Payload fraction derivation for vehicle with split delta-V (case #1)

Posted on March 14, 2021 by Kirk Sorensen

Consider a vehicle that undertakes a first $\Delta v$, then picks up a payload and undertakes a second $\Delta v$. It carries the propellant for both maneuvers, but only on the second maneuver does it have the added mass of the payload. This situation might be representative of:

1. a one-way lunar vehicle, arriving in low Earth orbit fully fueled but with no payload. It executes a trans-lunar injection burn and then a lunar orbit insertion burn, all with no payload. It then picks up a payload in lunar orbit and descends to the surface with the payload and lands with essentially all its propellant expended.

2. a reusable lunar lander, based on the surface of the Moon, fully fueled by lunar propellant but lacking any payload, which ascends to a lunar orbit and recovers a payload in that orbit, then descends to the surface, landing with nearly all its propellant expended.

3. a space tug that departs for geosynchronous orbit and recovers a satellite, then returns with it to a low-Earth orbit for repair.

First, define the mass conditions at the beginning and end of $\Delta v_1$:

(1)   \begin{equation*} \eta_1 \equiv \exp(\Delta v_1/v_e) = \frac{m_\text{vehicle} + m_\text{prop1} + m_\text{prop2}}{m_\text{vehicle} + m_\text{prop2}} \end{equation*}

Alternatively, and just as importantly, the conditions bracketing $\Delta v_1$ can be described in terms of an initial mass:

    \begin{displaymath} \eta_1 = \frac{m_\text{initial}}{m_\text{initial} - m_\text{prop1}} \end{displaymath}

This expression can be conveniently rearranged to yield the propellant mass consumed by the vehicle in $\Delta v_1$:

    \begin{displaymath} m_\text{prop1} = m_\text{initial} \left(1 - \dfrac{1}{\eta_1}\right) \end{displaymath}

In a similar manner, we define the mass conditions at the beginning and end of $\Delta v_2$:

(2)   \begin{equation*} \eta_2 \equiv \exp(\Delta v_2/v_e) = \frac{m_\text{vehicle} + m_\text{payload} + m_\text{prop2}}{m_\text{vehicle} + m_\text{payload}} \end{equation*}

We can also express the conditions bracketing $\Delta v_2$ in another way, in terms of initial mass:

    \begin{displaymath} \eta_2 = \frac{m_\text{initial} + m_\text{payload} - m_\text{prop1}}{m_\text{initial} + m_\text{payload} - m_\text{prop}} \end{displaymath}

Yet another approach will later yield a useful relationship:

    \begin{displaymath} \eta_2 = \frac{m_\text{initial}/\eta_1 + m_\text{payload}}{m_\text{vehicle} + m_\text{payload}} \end{displaymath}

    \begin{displaymath} \eta_2\left(m_\text{vehicle} + m_\text{payload}\right) = m_\text{initial}/\eta_1 + m_\text{payload} \end{displaymath}

    \begin{displaymath} \eta_2 m_\text{vehicle} + \eta_2 m_\text{payload} = m_\text{initial}/\eta_1 + m_\text{payload} \end{displaymath}

    \begin{displaymath} \eta_2 m_\text{vehicle} + (\eta_2 - 1) m_\text{payload} = m_\text{initial}/\eta_1 \end{displaymath}

    \begin{displaymath} m_\text{initial} = \eta_1\eta_2 m_\text{vehicle} + \eta_1(\eta_2 - 1) m_\text{payload} \end{displaymath}

The propellant mass consumed by the vehicle in $\Delta v_2$ can also be expressed in a manner analogous to prop1:

    \begin{displaymath} m_\text{prop2} = \left(1 - \dfrac{1}{\eta_2}\right)\left(\dfrac{m_\text{initial}}{\eta_1} + m_\text{payload}\right) \end{displaymath}

    \begin{displaymath} m_\text{prop2} = m_\text{initial}\left(\dfrac{1}{\eta_1} - \dfrac{1}{\eta_1\eta_2}\right) + m_\text{payload}\left(1 - \dfrac{1}{\eta_2}\right) \end{displaymath}

Now we are positioned to calculate the total propellant load:

    \begin{displaymath} m_\text{prop} = m_\text{prop1} + m_\text{prop2} \end{displaymath}

substituting the definitions for prop1 and prop2

    \begin{displaymath} m_\text{prop} = m_\text{initial} \left(1 - \dfrac{1}{\eta_1}\right) + m_\text{initial}\left(\dfrac{1}{\eta_1} - \dfrac{1}{\eta_1\eta_2}\right) + m_\text{payload}\left(1 - \dfrac{1}{\eta_2}\right) \end{displaymath}

collecting terms and simplifying

    \begin{displaymath} m_\text{prop} = m_\text{initial} \left(1 - \dfrac{1}{\eta_1} + \dfrac{1}{\eta_1} - \dfrac{1}{\eta_1\eta_2}\right) + m_\text{payload}\left(1 - \dfrac{1}{\eta_2}\right) \end{displaymath}

(3)   \begin{equation*} m_\text{prop} = m_\text{initial} \left(1 - \dfrac{1}{\eta_1\eta_2}\right) + m_\text{payload}\left(1 - \dfrac{1}{\eta_2}\right) \end{equation*}

now let us define the vehicle’s “dry” mass entirely in terms of initial-mass-sensitive ($\phi$), propellant-mass-sensitive ($\lambda$), and payload-mass-sensitive ($\epsilon$) mass terms. This is a substantial simplification, but it should do for now.

    \begin{displaymath} m_\text{vehicle} = \phi m_\text{initial} + \lambda m_\text{prop} + \epsilon m_\text{payload} \end{displaymath}

    \begin{displaymath} m_\text{initial} = \eta_1(\eta_2 - 1)m_\text{payload} + \eta_1\eta_2 m_\text{vehicle} \end{displaymath}

substituting the definition of the vehicle’s mass in

    \begin{displaymath} m_\text{initial} = \eta_1(\eta_2 - 1)m_\text{payload} + \eta_1\eta_2(\phi m_\text{initial} + \lambda m_\text{prop} + \epsilon m_\text{payload}) \end{displaymath}

we collect terms related to the initial mass on the left hand side

(4)   \begin{equation*} m_\text{initial}(1 - \phi\eta_1\eta_2) = m_\text{payload}(\eta_1(\eta_2 - 1) + \epsilon\eta_1\eta_2) + \lambda\eta_1\eta_2 m_\text{prop} \end{equation*}

Take equation(3) and multiply it through by $\lambda\eta_1\eta_2$:

    \begin{displaymath} \lambda\eta_1\eta_2 m_\text{prop} = \lambda\eta_1\eta_2 m_\text{initial} \left(1 - \dfrac{1}{\eta_1\eta_2}\right) + \lambda\eta_1\eta_2 m_\text{payload}\left(1 - \dfrac{1}{\eta_2}\right) \end{displaymath}

    \begin{displaymath} \lambda\eta_1\eta_2 m_\text{prop} = m_\text{initial} \left(\lambda\eta_1\eta_2 - \dfrac{\lambda\eta_1\eta_2}{\eta_1\eta_2}\right) + \lambda\eta_1\eta_2m_\text{payload}\left(\dfrac{\eta_2 - 1}{\eta_2}\right) \end{displaymath}

(5)   \begin{equation*} \lambda\eta_1\eta_2 m_\text{prop} = m_\text{initial} \left(\lambda\eta_1\eta_2 - \lambda\right) + m_\text{payload}\lambda\eta_1(\eta_2 - 1) \end{equation*}

then substitute the RHS for equation(5) in equation(4) and collect terms

    \begin{displaymath} m_\text{initial}(1 - \phi\eta_1\eta_2 - \lambda\eta_1\eta_2 + \lambda) = m_\text{payload}(\eta_1(\eta_2 - 1) + \epsilon\eta_1\eta_2 + \lambda\eta_1(\eta_2 - 1)) \end{displaymath}

further simplifying

    \begin{displaymath} m_\text{initial}(1 - (\phi + \lambda)\eta_1\eta_2 + \lambda) = m_\text{payload}(\eta_1(\eta_2 - 1)(1 + \lambda) + \epsilon\eta_1\eta_2) \end{displaymath}

With all terms relating only to initial mass and payload mass, a general expression for payload fraction can at last be defined:

    \begin{displaymath} \dfrac{m_\text{payload}}{m_\text{initial}} = \dfrac{1 - (\phi + \lambda)\eta_1\eta_2 + \lambda}{\eta_1(\epsilon\eta_2 + (\eta_2 - 1)(1 + \lambda))} \end{displaymath}

Posted in Rocket Design Theory | 2 Comments