Mass Value

I was on the space show recently to discuss my research on mass value, and its implication for space policy. I will also be presenting this at the upcoming Mars Society virtual conference, which is shaping up to be a big event. I thought I would add this blog to the discussion.

My background is in software engineering and astrophysics. I did not consider the large scale expansion of humanity into space a realistic possibility for my adult life – I had read breathlessly enthusiastic books about our future in space as a child, but viewed this mostly as a fantasy when I became an adult.

However, in December 2015, the first landing of a Falcon 9 booster changed my mind. Many projects had promised to revolutionise space in the past – the Shuttle, DC-X, VentureStar – but all had turned out to be vapourware. Now there was a company who were able to actually realise these dreams. SpaceX changed the landscape of launch. It made the settlement of the solar system a real possibility in the near future – and I wanted to investigate this idea further.

My first angle of investigation was to study mass to low Earth orbit per year, as a means to check on progress in space. Using sources such as NASA, Gunter’s Space Page and Encyclopedia Astronautica – combined with some educated guesswork, estimating payload mass from rocket size – I managed to pin a mass to LEO on every single launch in the first 60 years of spaceflight

The solid line is the US, the dashed line USSR/Russia, the dash-dot line ESA and the dotted line China.

This is useful; major events of the space age so far can be quantified. The lead in mass that allowed the US to win the Moon race, the loss of capability after the Challenger disaster and similar after the dissolution of the USSR. A spike at the end (these data finish in 2017) also shows the rise of SpaceX.

But this isn’t telling the whole story. I want to determine how close we are to settling the solar system, not how close we are to settling LEO. To send something to Mars, the Moon or elsewhere, you need to send some mass into LEO even if momentarily. But the amount of mass that you need to put in LEO to get a kg to a destination depends very much on how this is executed. Using higher performance propulsion systems means less mass of propellant is needed. Techniques such as gravity assist and aerocapture can further reduce the amount needed.

More importantly, as we move out into the solar system, In-Situ Resource Utilisation (ISRU). This is a dry NASA acronym for a process that will be a key driver of human progress in the coming centuries. It simply refers to using matter from the rest of the solar system for a mission instead of taking all the mass with you from Earth. This can include things such as manufacturing propellant for the return journey using materials at the destination or burying surface habitats under rocks for radiation shielding.

As an increasing proportion of the material goods we use in space comes from materials in space, we will be able to expand our presence there without having to launch more and more mass from Earth (which is expensive). None of this mass gathered from other celestial bodies will show up in a study of LEO mass at all.

To deal with these issues, I developed a concept I term mass value. It is detailed in this paper that I published earlier this year in New Space. The mass value of a mission is the amount that would have to be placed into LEO to transport the mission payload to its final destination, using a standard basic method. That method assumes a specific impulse of 300 seconds, using Hohmann transfer orbits only.

The basic method is hard to define, you do not want real missions to underperform it unless they are truly terrible. The physical assumption is that storable, hypergolic propellants will be used in a drop tank; no additional engines are assumed and thrust is not considered at this point. The drop tank approximation supposes a spacecraft similar to that presented by Werner von Braun in his 1955 film Man and the Moon

Werner von Braun presenting a proposed circumlunar spacecraft. The cylinders surrounding the rear end are drop tanks, containing nitric acid and hydrazine. Credit: Disney

The objective is not to compare a mission to a hypothetical one done using the basic method; it is to compare missions to each other, using the basic method simply to set the scale.

In the paper, I discuss the Mars Direct mission proposed in 1990 by Robert Zubrin. Each human Mars landing would involve 2 heavy lift launches, equivalent to 240 tonnes to LEO, and yield over 5000 tonnes of mass value, by virtue of the fact that the Earth Return Vehicle generates around 100 tonnes of propellant on the surface, mainly from local resources. The 5000 tonne value is a realistic estimate of what it would take to directly place the fully fuelled ERV on the surface using the basic method. For reference, a typical Apollo Moon landing mission yielded a mass value of just under 200 tonnes, for 140 tonnes delivered to LEO.

Mar Direct was conceived as a response to the flawed ’90 day study’ Mars mission plan that was produced by NASA in 1989 for the Space Exploration Initiative (SEI). In that plan (in addition to many non-Mars related tasks being frontloaded to the point a landing would have taken 30 years to accomplish) there was between 550 and 850 tonnes placed in LEO per Mars expedition. The landed payload was only 25 tonnes, giving a mass value of less than 1000 tonnes. Comparing these two missions on LEO payload, the SEI plan appears to be superior. In terms of mass value though, not only does Mars Direct deliver 5 times more, it also does it with a fraction of the launch resources.

Of course, this point could also be made just be directly comparing the landed payload of each mission; but the advantage of mass value is that it is entirely agnostic with regards to destination. Missions to the Moon or to asteroids can be compared to Mars missions on a single scale.

This is a good way of comparing missions, but I have in mind also its use as a policy tool. A government fund that provided a per kg award for mass value would, I believe, stimulate cost effective space development and mission design. There are two approaches; either a flat cost per kg mass value is offered, or a fund of a fixed sized that is divided up between participants in proportion to the fraction of total mass value they produced. This latter method would provide more uncertain payments but would incentivise participants to check either others work (in order to boost their own fraction of the total mass value) and so reduce the problem of fraud.

If, as I had done with LEO mass, the total mass value a nation produced over a year wear added together, an aggregate performance figure could be reached, that would meaningfully measure how much that nation was contributing towards the creation of an interplanetary society; something akin to a GDP or GNP figure. I intend to start compiling and publishing such figures myself.

Calculations of mass value for increasingly complex space settlements would become difficult rapidly. A fuel production plant is simple enough, but if many independent enterprises spring up and start trading with each other in local currency, how can we calculate the mass that would have to be put into LEO to achieve the same output?

In the novel Artemis by Andy Weir, a Moon base is depicted where the local currency units are called Slugs or ‘soft landed grams’. These are exchangeable tokens that entitle the bearer to have 1 gram of cargo delivered to the base. If such a currency is implemented in an off-world settlement, it potentially provides an easy way of working out mass value.

If someone buys an item using this currency, for instance paying 10g, that means that they value this item equally to 10g of negligible cost cargo from Earth. Whoever provides this item will have some inputs they either pay for or import; so the correct figure for the equivalent ‘payload’ is the value added of the transaction – the price minus the cost of the inputs. This equivalent payload mass combined with the location of the settlement can be used to calculate a mass value for that transaction. Aggregating these values – this would require the additional information about the payload per cargo delivery, which should be available – would allow a mass value for the entire settlement to be calculated.

We should expect mass value to grow exponentially. Things that can generate mass value off world (such as fuel plants, bioregenerative life support systems, and entire settlements) have a mass cost themselves, and their mass value would be proportional to how many of them there are. This would make the rate of mass value production proportional to the mass value generated so far – which is the definition of exponential growth. The book I am working on will go into more details on the implications of this, and I may post some excerpts on this blog later.


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