Developing Orbit Part 3

guest blogger john hare

Recent events make it easier to describe one scenario for getting orbital costs down. This specific example almost certainly won’t happen and just stands in for the dozens of possible ways that orbit could become affordable.

Masten and XCOR have a joint venture for developing a methane lander. Consider the possibility of future cooperation. If in five years Masten and XCOR are operating profitable sub orbitals out of Mojave, it will be past time to start on orbital hardware. If both companies have 300+ kg payload capacities to above 100 km, then the Lynx  will have the capacity to lift one of the earlier Masten vehicles (Xombe 0.5?)to above 100 km. A fairly rapid partially retrograde development could have the small Masten vehicle capable of very long suborbital flights if properly assisted.  Say, Mojave to Kodiak, Oklahoma, or New Mexico. The Lynx lofts the Masten vehicle to mach 3-4 at high altitude and stages. The Masten test article finishes the boost for one of those spaceports while the Lynx returns to it’s runway.

The experience gained in test flights with almost existing equipment will go a long way toward convincing investors that orbit is well withing the reach of a conservative  three stage to orbit transport. (Pete suggested this as two stage over on Transterrestrial) The reentry problems can be explored for mach 10 to 15 including turnaround time and environmental impact. Reentry noise can be one of the largest hurdles for inland recovery of second stages. If, and only if, inland recovery can be done in an acceptable manner will this concept work. A tiny smallsat stage might be launched from the Xombe 0.5 if required to make the point.

For a second set of tests, Masten launches a three barrel vehicle with their full size suborbital craft. The two outers deliver the same assistance to the center vehicle as Lynx does to the smaller craft. The full size suborbital vehicle lands downrange at one of the available spaceports. Once this is explored, a 300 kg upper stage is used to place a useful smallsat in orbit. Some testing is done on recovering the upper stage.

If the above tests confirm that the technical concept is sound and acceptable to the uninvolved public at the recovery spaceport, then funding can be sought for a full orbital system. A much larger XCOR vehicle (Panther  for this post) is developed to launch the full size three barrel Masten (Xorbit for this post) assembly. This time the two outer vehicles cross feed propellant to the center vehicle and land at one of the downrange spaceports. The center vehicle goes on to deliver 300 kg of payload to orbit. This allows XCOR to focus on the much larger suborbital Panther while Masten focuses on improving existing vehicle performance and reentry. The Panther could also be the rent-a-booster as Clark suggests over on Space Transport News. The technical risk to orbit from that point would be comparable to funding XCOR and Masten to a full suborbital operation now.

A launch organization has four main costs. Development costs. New vehicle costs. Fixed operating costs. And marginal costs. Only marginal costs are mostly unaffected by flight rate. On a gas-n-go vehicle propellant is a major marginal cost.

On this three stage to orbit with each stage having a mass ratio of three, (total MR=27) there would be about 36,000 kg of propellant or about $18,000.00 per launch. Other marginal consumables would double this to $36,000.00 per launch or about $120.00 per kg in marginal costs.

Starting from a base of operating vehicles and recent development experience with an intact team, I speculate that the development cost would be about $250M. Expected interest on the money for this high risk field could easily be in the 25% range for a development cost of $65M per year to service the debt. If there is one flight every two weeks, $2.5M per flight to service the debt. At two flights per week, this drops to $625K. With multiple airframes flying twenty payloads per week, this drops to $63K per flight to service the debt.

New vehicle costs for the four airframes (Panther and 3 Xorbits) could be in the $20M range after the bugs are out. At a flight every two weeks, it would take $770K to pay off a set in a year. At two flights per week per airframe, it drops to $193K to pay off in a year. If some financing can be arranged, then costs can drop considerably more. (It could be much less in the reality as the Panther could lift 20+ times per week while the Xorbits could possibly do two each.)

 My first guess on fixed operating costs is $30M per year keeping two or more spaceports fully operating with all support staff, including transporting the Xorbit stages back to Mojave. These numbers are based on this guess. At a flight every two weeks, $1.154M per flight. At two per week, $289K per flight. At twenty flights per week, $58K per flight figuring that staff doubles at that flight rate.

flight frequency   debt    vehicles      fixed              marginal           total      

two weeks           $2.5M    $770K       $1.154M        $36K            $4.46M     

two per week    $625K      $193K         $289K          $36K            $1.143M    

twenty per week$63K      $193K         $58K              $36K              $350K     

If it worked out like this, then a flight rate of once every two weeks would break even at a little under $7K per pound, while twenty flights per week could get it down to around $530 per pound. A lower flight rate under every two weeks would make the plan unworkable while a higher flight rate than twenty per week would make it somewhat cheaper than $500.00 per pound. These numbers assume that interest only is paid on the development and that the vehicles must pay for themselves in one year.

This is very much a first generation space transport. Obvious places to get the cost down further are to reduce development costs while still getting an acceptable vehicle group. Getting a better interest rate. Improving propulsion and dry mass fraction to get the mass ratio down from 27 to 16 or so with less dry mass to lift in the first place. Getting it down to two stages to orbit to eliminate supporting downrange spaceports and the extra stages. Flying each airframe more than the twice a week I have here. Reducing the cost of new airframes and spreading the payback time over more than one year. Reducing the number of support personnel required per flight. And so on to get costs insanely low compared to the present day.

 Developing orbit into an economic driver is going to take a lot of work and intelligent handling. Orbital development to date with existing transportation is a mere shadow of the possibilities given transportation that is economical, reliable, and convenient. Everything changes when a decision can be made and executed reliably and affordable in a matter of weeks or possibly days. When was the last time you scheduled anything years in advance? How many times in your whole life do you schedule something years in advance and know that it might cato before it gets started? That is what the whole launch industry is forced to do at this time.

Basing on this cooperative venture creating a somewhat convenient transportation method to orbit, speculation on markets becomes possible. The first markets are somewhat limited in orbital inclination by the restrictions of the downrange spaceport requirement. The choices are a somewhat retrograde high inclination orbit, or a normal orbit with an inclination at  Mojave’s latitude. Anything to ISS is out due to inclination restrictions. Tourism is out due to the small vehicle size. What can you do?

While proving the vehicles, expensive payloads are out, so the focus must be on things that have low intrinsic value with only being in orbit making them valuable. Propellant is the first obvious choice. The first launch to each available inclination carries the largest propellant tank that it can as payload with subsequent launches filling it up. Or possibly one of the ELV companies leaves an upper stage in the right orbit that can serve as a depot. This is only good if there is a market for propellant from that particular orbit though. I dislike the concept of buying propellant, or water, or sand in orbit for the purpose of artificially creating a market.

The first stopgap depot would need to be in a normal orbit at Mojave’s latitude. When it has sufficient propellant delivered to make it worthwhile, one of the majors launches an un fueled GEO bird from the Cape to rendezvous with it. A 25 ton GEO bird unfueled could take on 50 tons of propellant before boosting to it’s final orbit. This would make for a very massive satellite compared with the current operational GEO sats. This would take about 170 launches of Panther/Xorbit. A contract for a million a launch would be profitable to both sides. The GEO provider would have the capability of a 75 ton launch vehicle for an extra $170M per GEO sat while only risking a 25 ton launcher. The Panther/Xorbit companies would need to get the launch rate above 3 per week to make a profit and pay off their debt. The 3 launches per week to reach profitability would take over a year to complete for each GEO bird that takes advantage of the capability. Assuming there are at least two of these monster satellites per year to refuel,  the launch rate is at nearly 7 per week. If deliveries could reach that flight rate, about 40% of each launch would be for profit or paying off the debt. Debt payoff would be under 3 years and would further reduce the launch costs by a factor of almost two, which would make the launches even more profitable at that price point.

Propellant is not the only possible cheap on the ground expensive in orbit payload though. Military surveillance would benefit from the ability to place fleets of cheap observation satellites in orbit. The gaps in coverage of the existing sats are most likely known to the people with something to hide. They probably time most of their sensitive moves to take place in these gaps. The military could easily spec an LEO surveillance sat that could be mass produced for under half a million. Though these birds would be far less capable than the high end machines they use now, a thousand of them would provide wall to wall coverage. At a million a launch and half that for the actual satellite, they would be looking at $1.5B for the whole program not including collecting and integrating the data. Allowing three years for placing the constellation, flight rates and costs match that of the propellant market only. Under three years to payout and higher profit thereafter.

Communications is frequently brought up as the target for early market. With cheap reliable launch as suggested here, less capable LEO satellites could be used in constellations with spares on orbit and on the ground ready to be launched in days if required. It is quite conceivable that these 300 kg comsats could be mass produced for a half million just as the surveillance satellites are. A billion and a half for a robust LEO comsat constellation of a thousand birds should be a business plan that would close. A three year launch schedule has the same numbers as the propellant or surveillance constellations. Under three year payoff and 40% profit after at a million a launch.

Astronomy and microgravity science and Earth observation satellites should be a market to match the last two.  Though the individual capabilities are less with the smaller satellites and cheaper construction, a cast of thousands could make up the difference. The ability to launch follow up or replacement experiments in a week or less would make commercial use of LEO much more user friendly. Over a hundred countries with thousands of universities and tens of thousands of private companies would be the market this time. With similar launch rates to the other users, the money works out the same.

If all four of these markets were to emerge though, a flight rate of 30 or so per week would drop costs still further. Costs of under $350K per flight would allow a charge of a half million per flight to make over 30% profit available for the debt payoff or investor ROI. It would also create even more incentive for the users of the service to make less expensive satellites for an even lower total cost. $4.5M per week profit would pay off the development debt in just over a year.

The assumption that these satellites would be inferior to the $Dirksen Galactica models launched by the current providers might not be true. The current birds are just too expensive and hard to replace to fail so every possible effort is spent making them reliable. This means literally gold plating some components and using only hardware that has been “space rated”. The time required to space rate components added to the long lead time on the current launch capability means that by the time any component reaches orbit, it is incredibly expensive, and it is obsolete by standards on the ground. The point has been made many times by many people that many of the current satellites, especially comsats,  are so expensive that the cost of launch is not all that important. With replacement launch on demand, satellite reliability becomes somewhat less important. It becomes possible to launch the latest electronic devices without “space rating” them at all. If they survive, they are space rated, if not, they are not. Henry Spenser has noted that the commercial stuff launched in the Canadian satellite that he worked with has lasted for years. How much capability is in $10K worth of May 2010 commercial electronic components components compared to $100M of “space rated” 1995 electronic components?

When failure is not an option, success can get expensive. We have all read that. How many have also noted that when failure is not an option, success also gets much  less capable?

With launches available on demand for half a million, many high risk or complicated capabilities can be tested and possibly implemented. A 300 kg tug could do considerable work in LEO. It is past time to deploy tethers for extensive testing. Debris cleanup vehicles can be tested and then employed. Fairly small service vehicles could do good work in GEO. National Geographic could afford a 300 kg Lunar, Mars, or NEO probe that was launched on one flight and fueled up by others.

It just gets better and the sky is not the limit.

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I do construction for a living and aerospace as an occasional hobby. I am an inventor and a bit of an entrepreneur. I've been self employed since the 1980s and working in concrete since the 1970s. When I grow up, I want to work with rockets and spacecraft. I did a stupid rocket trick a few decades back and decided not to try another hot fire without adult supervision. Haven't located much of that as we are all big kids when working with our passions.

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28 Responses to Developing Orbit Part 3

  1. Pete says:

    This all looks very plausible to me – some thoughts:

    A “panther” class XCOR vehicle is presumably still at least ~5 years away. It would be nice to start exploring something close to orbit before then.

    I am concerned about how long it would take to do dinosaur-esk depots. The dinosaur side of that equation may take many years and would be very slow to grow in any case – very long lead times. Propellant depots for small fast response satellites may actually be a more near term prospect. A travelling party of mammals and dinosaurs can only move as fast as the slowest member.

    The thought of launching a Masten rocket vehicle from a Lynx and starting to explore the reentry regime near term sounds very appealing. Reentry is still perhaps the 500lb gorilla in the room for new space, demonstrate something in that area and the investment environment may suddenly change very quickly for the better.

    Developing small modular depots, tugs, satellites, etc., that are assembled in orbit at the almost “university” level sounds very interesting, specifically, it may even just happen naturally from such a small low cost high frequency launch capability. It is an easy and natural way to grow satellite and space mission capability without having to go dinosaur on it.

    With the three big markets you talk of (sub orbital, propellant and small satellites), build it, and the market may actually just come.

    A fourth possible market might be for the construction of a space station (a natural growth of a depot). From what I could tell there was no part of a space station that could not be launched from small payload modules, indeed it might even be easier because it could be far more responsive and up to date in the design and construction. I suspect one might almost be able to find an angel investor willing to back such a program (perhaps cheaper and more capable than what Bigelow is funding) – with modules being redundantly designed and contracted for from most anywhere. It sounds look it could be a robust, highly redundant and highly flexible business model.

  2. Surprised you didn’t mention cargo resupply of Bigelow stations. I expect that having a steady flow of consumables as well as emergency items, even if delivered in small packages, would be quite welcomed.

  3. Pete says:

    A dinosaur tug might be useful – something that could travel/interface between a new space depot and an old space whatever, allaying safety concerns without slowing/restricting development.

  4. john hare says:

    Surprised you didn’t mention cargo resupply of Bigelow stations. I expect that having a steady flow of consumables as well as emergency items, even if delivered in small packages, would be quite welcomed.

    You are right as usual. These posts are kind of feeling my way and I am not 100% sure that there would be reliable human spaceflight as this gets started in this particular scenerio. Each time I start one of the posts in this series I think it is the last and by the time I finish I realize that I’m going to need to figure out another one. The next step is affordable human spaceflight as I am trying to think it out. What I am looking for is scenerios that can be used to persuade some of the people in ‘our’ camp that don’t see how it might be possible to get there from here.

    Thank you for commenting here. Sometimes I wonder how many hour a day you devote to keeping us all informed/motivated/entertained on your blogs.

  5. john hare says:

    A “panther” class XCOR vehicle is presumably still at least ~5 years away. It would be nice to start exploring something close to orbit before then.

    I am a bit concerned about too early, too fast, jumping into something beyond current capabilities. If a pile of cash fell out of the sky today, I would not fund an orbital capability. I would try to support a diverse group of companies that are going for suborbital transports. The ones that survived and succeded would be the ones I would fund for orbital ops. Six years ago, Scaled Composites seemed to have it sewed up. Today, they don’t. If they had been handed a massive pile of cash then, they apparently still wouldn’t be in orbit today. If, and only if, they prove by doing with SS2 would I jump in with them for orbital development.*

    I think it is time to put Darwin on our space development team. I don’t think it is an insult to XCOR, Masten, Armadillo and company to say that the winners move on to the next round.

    *Hypothetically of course. I’m broke.

  6. A_M_Swallow says:

    Reprice the launches every 3 or 6 months. This allows the price to reflect the actual demand. As demand increases your price drops. Always aim to make a profit.

    Assume that there is a two or three year delay between a new service becoming available and customers appearing. The delay is due to the customers having to raise both the money and build the payloads.

  7. john hare says:

    I would assume that the launches get repriced almost every time the sales staff negotiates a new customer contract. If there is enough demand and no credible competition, you might just leave the prices a little higher for a while to get the debt paid down. Not quite whatever the market will bear, but with a strong view to maximising profits in both the short and long term. The long term can also suffer if you carry too much debt for too long. On the gripping hand, you have to entice enough customers to keep the wheels in the wells, which suggests keeping price as low as you can while still generating sufficient ROI. The short version is that constant attention must be paid to pricing and the ripple effects it has.

    It will take time to get the new customers on board. I think we might all be surprised at how many old customers have unlaunched payloads sitting around. Propellant though is a known requirement and can be delivered even in advance of customer availability. When there is a stockpile in a known orbit, people that can use it will start think of ways to save themselves money. Airframes on the ground are a dead loss in three out of four categories, the most expensive ones.

  8. gravityloss says:

    Can you fly 1000 km horizontally from 100 km altitude with some Xombie-like vehicle? I really doubt it..
    And you start your upper stage burn anyway at 20 or 30 km altitude, you don’t coast to 100 km because that will just add gravity losses.

    Cross country flight is really really hard with rocket vehicles.

  9. gravityloss says:

    Let’s assume a 2000 km horizontal distance. Let’s use a parabolic arc with a flat Earth assumption.
    If the horizontal speed is 1 km/s then the time it takes is 2000s, half parabola in 1000 s (50 km height), so you need, at 1 gee the vertical deceleration, an initial speed of 10 km/s. Ouch! Horizontal flight is really demanding because of the huge gravity losses.

    4 km/s horizontal speed, gravity is still three quarters so the time is about 250 s from start to apogee and needs an initial vertical speed of 2.5 km/s.

    It becomes quickly apparent that you need really high delta vees for stuff like this.

    It’s of course seen at first glance already. A craft that can travel 100 km vertically is a different thing than one that can travel *ten times* that horizontally.

    But the point to point myth will be happily continuing without any attachment to reality.

  10. gravityloss says:

    Gah, I made a mistake there.
    a 1000 s dive is 0.5*10 m/s^2 * (1000s)^2 = 5 million meters or 5000 km. That’s the apogee height. Here of course a constant gravity and flat earth assumptions stop working.
    It’s still a hard problem.

  11. The maximum range of a ballistic trajectory is:


    So to go 2000 km you need about 3200m/s delta-v. Your maximum altitude is given by:


    Which works out to 500 km. Not an unreasonable altitude for a rocket vehicle. Interestingly enough, 3km/s is about where many of the suborbital vehicles max out. Aerodynamic losses will decrease range significantly, but the way to make rockets work well is to treat them like rockets. Don’t try to calculate a “horizontal flight” component.

    The trip takes 7 minutes, by the way.

  12. Oops – I just saw that I put in an extra 2. The maximum range is given by:


    So you need 4.5 km/s delta-v, the maximum altitude is 1000 km, and the total trip time is 10.5 minutes.

  13. john hare says:

    You do not launch the upper stage from a static vehicle at 100 km. You launch it from 30-40 km when the Panther is climbing at~500m/s with perhaps 1,000m/s horizontal velocity. The second stage has 3,000m/s in it’s tanks to extend that.

    Point to point is dumb. These stages land empty downrange after boosting the upper stage.

  14. Tom DeGisi says:

    What about barnstorming rockets at county fairs which take you up a kilometer for $100? Those will get you some low operating costs and frequent flights.


  15. Dick Eagleson says:

    Point to point is dumb. These stages land empty downrange after boosting the upper stage.

    Perhaps I’m missing something that ought to be obvious, here, but isn’t landing downrange pretty much the definition of point-to-point ballistic flight?

    And, for that matter, so would be gassing these 2nd-stage Xorbit puppies up again and launching them back to their spaceport of origin which I assumed – perhaps incorrectly – was the implied way of “transporting” them back to where they came from.

    I smell a Homer Simpson moment here somewhere. Is it mine?

  16. Pete says:

    Six years ago, Scaled Composites seemed to have it sewed up. Today, they don’t. If they had been handed a massive pile of cash then, they apparently still wouldn’t be in orbit today.

    I share your disappointment.

    Along the evolutionary analogy, evolution happens fastest at small scale – bacteria evolve much quicker than whales and growing physically large tends to be a one way evolutionary trip to stagnation and extinction. The evolutionary argument invariably pushes me back to the how do we reduce the entry barriers and scale requirement of space so that more smaller companies can give it a go and evolution can better do its thing.

    The space Lego approach definitely has a lot of merit – with bricks sized so that more people can play.

    An interesting question is would it be cheaper to demonstrate low cost access to space at very small sizes – below those needed to carry people, or should one assume a size sufficient to carry people? Presumably if one can demonstrate low cost access to space then it does not much matter whether one does it carrying people or not, either way, we would have low cost access to space, and the world would have changed (scaling up would presumably quickly follow).

    If low cost access to space can be physically demonstrated at smaller scale (and I suspect it can) then it might be possible to do so using the Lynx and a Masten derivative vehicle (or two). I do not think there is any shortage of potential market at these smaller scales if one actually has dramatically lower cost access to space – so the business model would seem to be there. Hence I suspect that low cost access to space can be demonstrated within five years and ~$10m per program, (preferably by more than just XCOR and Masten) and that this would be the shortest path to large scale low cost access to space.

    If this scenario is possible, then I think five years is a reasonable time frame and obviously having to spend ~$250m can be avoided. Up front investment is greatly reduced and debts can be paid back much faster. Money need not be wasted on large long term projects with little hope of success. If this much smaller scale approach is possible then a space Lego brick infrastructure in the say 30-50lb may need to be contemplated (micro satellites, tugs, propellant depots, etc.), this is quite different from a human sized space Lego brick infrastructure.

  17. gravityloss says:

    By “horizontal flight” I just meant not landing where you launch. If you check out, they are parabolas.
    The radiation environment gets interesting at 1000 km height.

  18. gravityloss says:

    And in a parabola, the horizontal velocity is constant.

  19. john hare says:

    These are stages landing downrange without payload. The point to point that we are calling dumb is the launching of cargo on ballistic trajectories. That is as hard or harder as orbital flight and is quite unlikely to pay as well. Note Gravitylosses comment on the radiation environment on the ballistic trajectories.

    I had assumed airfreight or trucking the second stages back to Mojave for the next flight. That was a main driver for the two per week flight rate of the upper stages.

    I think the Doh! moment is mine. This post was too long and wasn’t clear on the assumptions, which led to several misunderstandings on the intent.

    The smaller sizes would be demonstrated with the Lynx/Xombe 0.5 in the post. If there is enough profit at those sizes, it could reduce the financing required for the next round. The intent of the post was that IF XCOR and Masten were operating successful suborbital vehicles in a few years, then $250M would be the probable size of the financing round to move them forward.

    Giving them $250M in five years was not the intent, and giving them $250M today was the reverse of the meaning here. The suborbital companies are on the East bank of the river building canoes. The ones that get there canoes built and paddled across to the West shore can negotiate loans from the bankers to build keelboats. The ones that successfully get keelboats built and get a cargo downriver to the brokers can negotiate steamboat production contracts.

    These are not ballistic missle trajectories. These are second stage recovery options. The flight paths are very different. The second stage may peak at 100 km altitude and 4,000 m/s before starting down. I really think we agree on the technical points, but are talking past each other on intent.

    The reason for a second stage recovery option is to hold down the mass ratio requirements per stage to allow margin for RLV operations. The first stage is a RTLS stage, which is Panther. If this were a two stage to orbit, the upper stage would need a mass ratio (dense fuels) of perhaps ten depending on achieved Isp. With reentry, cross range, and landing requirements, that very likely would have no or even negative payload margin on the first try. The recovery of a second stage might permit RLV operations at a much lower investment and technology level. If you have a better way to get that performance while holding the investment and required technology level down, we would be interested.

  20. Pete says:

    The smaller sizes would be demonstrated with the Lynx/Xombe 0.5 in the post. If there is enough profit at those sizes, it could reduce the financing required for the next round. The intent of the post was that IF XCOR and Masten were operating successful suborbital vehicles in a few years, then $250M would be the probable size of the financing round to move them forward.

    But maybe, just maybe, they could demonstrate cheap access to orbit at much smaller scale within say three years for an added say $25m. Further, they could launch thousands of small payloads in so doing. This would be cheap access to space, no need to move forward from here – mission accomplished. Are you placing the bar unnecessarily high?

    We have a tendency to assume that cheap access to space means carrying people. Is this really the only possible solution? What about simplifying things as much as possible and concentrating just on achieving cheap access to LEO. Worry about launching people later (or leave it to the ELVs), after all, people make up an almost negligible mass proportion of global annual payload to orbit. If cheap access to space really is the tipping point, then should we not just be focusing on that?

  21. john hare says:

    If they can demonstrate low cost space access at that scale and make a profit doing it, more power to them. At some point there will be a scale up. Why not by the ones that have proved they can do it at smale scale?

  22. Pete says:

    If they can demonstrate low cost space access at that scale and make a profit doing it, more power to them. At some point there will be a scale up. Why not by the ones that have proved they can do it at smale scale?

    Sure, but by that point, it does not really matter. Cheap access to space would already be demonstrated and these companies would already be flying cheaply at very high flight rates and making their millions. If one can demonstrate cheap access to space in such a manner, then one kind of already has cheap access to space. Scaling up becomes a second order consideration based on economic optimization and expanding into other market niches.

  23. john hare says:

    Fair enough.

  24. TimC says:

    ‘getting the mass ratio down’. The Atlas I believe originally used a body which was unable to support itself when empty. The SRB uses a thin shell to contain the solid propellant. A perfect mass ratio for a booster would presumably make the booster ‘shell’ out of propellant itself. A solid fuel popsicle?

  25. Dick Eagleson says:

    A solid fuel popsicle?

    Sounds more like an ATK skinless frank. I go through a bunch of miniature such items every July 4th. They’re called sparklers. Lots of sparks. Not much thrust.

  26. Pete says:

    A perfect mass ratio for a booster would presumably make the booster ’shell’ out of propellant itself. A solid fuel popsicle?

    For example, a self supporting “cheese” of solid rocket fuel chord that gets feed into a rocket engine at its base. Unfortunately the ISP would likely not be great.

    I kind of like the idea of inflatable external tanks, they can be structurally self supporting, vehicle independent, and can weigh less than 1% of GLOW. Structurally integral balloon tanks as per the Atlas I tend to be overly fragile and inconvenient.

  27. Actually, Pete, that is exactly what my company (Universal Transport Systems) is doing. We are seeing good results – Isp isn’t everything. We can get incredible mass ratios! Hopefully we will have some fun information/videos to show soon.

  28. I would love to hear more about progress on the Skylon project (successor to Hotol). Given the advantages of it’s unique engine it would be a shme if the technology ended up being mothballed again.

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