Recently this website, Build the Enterprise, hit the news because of the author’s rather quixotic call to build a real interplanetary version of that most famous fictional starship lineage. Unfortunately the site’s Forum-ware is very cantankerous, so I posting my discussion of necessary redesigning of the concept (slightly reworded for clarity)…
Running the numbers, the figures are wrong, wrong, wrong.
Here’s a preliminary list.
(1) Wet mass is quoted as 84,822 tons. Propellant load is 12,474 tons. Yet elsewhere, in pounds, it’s 187 million/55 million. Inexplicably the propellant mass has been halved. To get to Mars in 90 days with the quoted mass-ratio, (187/(187-55))= 1.42, means a very high exhaust velocity is required. Exhaust velocity and jet-power are inextricably related by:
P = 1/2.T.v
where P is the jet-power, T the thrust and v the exhaust velocity. To get to Mars in 90 days requires a high delta-vee (dv) – enough to travel to Mars on a short trajectory, against the Sun’s gravity, then matching to Mars’ orbital velocity. With a VASIMR that low mass-ratio might get it to Mars in 90 days – with a dry tank. The 0.002 gee acceleration quoted however is IMPOSSIBLE. Thrust, T = M.a i.e. mass (84,822,000 kg) times 0.0196 m/s^2 = 1,662,511 newtons thrust. With a bit of algebra we find that with a 1.5 GW jet-power the exhaust velocity is an impossibly low 1,262 m/s. A reasonable exhaust velocity (high-thrust VASIMR mode) is 15,000 m/s – meaning a maximum acceleration of ~0.00024 gee or a jet-power of nearly 25 gigawatts.
However a lot more propellant will be needed if the vehicle thrusts all the way at that exhaust velocity, so on a typical trip to Mars a VASIMR steadily builds up the exhaust velocity to a maximum 300 km/s at the half-way point, then a steady decline as the vehicle slows down for Mars arrival.
Often people will say VASIMR can get to Mars in 39 days. They don’t often say what power and fuel that requires. To reach Mars in 39 days also required that particular VASIMR option to aerobrake into orbit around Mars – something not recommended for a large vehicle like “Enterprise”. The required propellant mass would be 230,000 tons, and the power source would mass 48,285 tons, while delivering 96.6 GW of electrical power to the engines. A 90 day mission is far less challenging in technological terms.
[Additional note: time under power over the same distance is related to the power by the 1nverse cube – thus taking 90 days means a power-supply that’s 8% the size of the 39 day trip.]
(2) In many ways the shape of the Enterprise is quite good. The frontal area is low, thus presenting a smaller target for potential meteoric impactors. Handy when going at high speed through our rather junky solar system. The original 1960s design also placed the antimatter reactors on booms as far away from the habitat as possible. The movies, and all later Trek, rather idiotically had the antimatter warp-core in the middle of the secondary hull – not a healthy idea at all. And plasma conduits all over the place… asking for trouble.
There is a major issue not addressed by the TV spaceship creators. Waste heat. Specifically for the Gen-1 “Enterprise” the VASIMR is essentially an externally powered fusion rocket – hydrogen plasma is heated and directed just like in an operating magnetic-mirror fusion reactor. The difference is that there’s no attempt at energising it all the way to fusion conditions. In theory, a VASIMR could be up-graded to be an actual fusion rocket. But without actually making its own fusion power, the VASIMR needs to get power from fission reactors, and they all put out excess heat. There’s only one way to get rid of excess heat in space when it’s not being thrown over-board in the rocket exhaust gases and that’s via radiators.
And the “Enterprise” – Gen1 or the fictional versions – don’t have them. A real “Enterprise” will need a set of “wings” – big radiators – to handle the heat or else the whole lot will cook.
(3) The back-up fuell-cells are a good idea, but for use in space they need an additional supply of oxygen of their own. A MW bank of fuel cells will use a lot of oxygen in a hurry, so you need to have a bank of liquid Oxygen (LOX) tanks to supply it.
(4) Why is the “Enterprise GEN-1” 3 times bigger than the fictional version? The fictional upgraded “USS Enterprise” was just over 300 metres long, yet its proposed namesake is ~950 metres long. I suspect an imperial-to-metric conversion error.
My preliminary, and hopefully friendly, critique. I look forward to dialogue with the concept creator.
As usual, everybody forgets about the heat radiators.
And while it is clever to use the saucer section to house a large artificial gravity centrifuge, would it not make more sense to mount it with the spin axis coincident with the spacecraft’s thrust axis?
Hi Adam,
I’ve been working for a while on the GEN1 Enterprise concept and most of the issues you raised have been or are being corrected by the author.
1) The acceleration was way off. there may have been confusion between 0,002g and 0,002m/s, obvioulsy quite different beasts! The numbers at this point are a more reasonable 0,001 m/s at 5000 ISP, giving 2,5 GW electrical power requirements.
The mass is 85 000 kg with 25 000 kg fuel. The return trip requires a fuel depot in Mars orbit, or local extraction of Argon from Mars. The orbital calculation need more work.
2) The waste heat issue has been addressed with two solutions: Liquid droplet radiators between the two nacelles, or large conventional radiators on the nacelles. The end result still looks like the Enterprise (IMHO).
3) With the level of power required, there will be plenty of backup from the nuclear reactors themselves! The ship is also so big that it can support about 100 MW of solar cells without much visual impact. The secondary system requirements are tiny compared to the engine requirements.
4) The Enterprise is big because the author feels the centrifuge should rotate at 2 rpm. And because if you’re going to build big, why not build really big?
As M. Nyrath mentions, the sideways placement of the main hull is not perfectly logical. It looks much nicer though, and the drawback, a small regular variation in gravity, is certainly no worse that what people on sea going ships had to put up with!
40 years of functional design since the Ernst Stuhlinger proposals of the early sixties seem to have lead to a dead end: a Spaceship to Mars will never be cheap enough, or safe enough for the critics. So the BTE idea takes a different route: build the excitement into the design. Make the trip as grand as it can be. Make fun one of the functional goals.
Worth another look, perhaps?
Regards,
Michel Lamontagne
Hi Michel
Happy to give it another look. I think my biggest issue is the mass, which seems excessive, and the fact that the design is constrained to *look* like the “Enterprise” but is x3 bigger. That seems kind of arbitrary.
I would like to see some more figures and a bit more analysis of the probable mass, since a simple scaling from a totally different Mars lander design concept is very likely incorrect. As for the orbital calculations, the orbital mechanics of such low-thrust trajectories as a vehicle fights against the gravity of Earth, the Sun and Mars is a non-trivial calculation. There is a paper or two on good approximations, plus lots of professional software for direct computation. From memory, I suspect the delta-vee is significantly more than the current mass-ratio will allow.
Hi Adam,
Yes, the ship is massive. In a way, it’s the first 100 missions to Mars all rolled into one. The main constraint leading to this huge size is the desire to have a 1g centrifuge at low RPM.
The web site has added a forum and a Wiki. At the current rate, I expect most of the answers to your questions will be in there in the next six months. Depends on the enthusiasm (and expertise) of the volunteers!
Regards,
Michel Lamontagne
Hi Michel
Refreshing my acquaintance with the literature and I find that the delta-vee to reach Mars in 90 days is quite a lot – about 28 km/s. And that’s at fairly high thrust. Tends to be higher at very low thrust. The “Enterprise” needs a beefier propellant tank or a bigger power-source.
Hi Adam,
Thanks for the information. Yes, the deltaV at this point is lower than that. More fuel, most likely, or a lighter ship. The power source is already as big as we want it to be! The orbital calculations are obviously the next step towards a realistic design. The first calculations done where too optimistic.
Fortunately, liquid argon, the planned fuel, is pretty dense, so there is plenty of space for it on the ship if more is required.
Regards,
Michel Lamontagne
Hello Adam,
Are you sure about that number? Earth to orbit is only about 10 km/s after all. A quick Wiki search at http://en.wikipedia.org/wiki/Delta-v_budget brings up 4 km per second but probably with a quite different orbit and with chemical thrust. The 28 km/s includes a breaking allowance for the high arrival velocity of a 90 day transfer? Maybe already for electrical thrusters?
Anyway, detailed orbital calculations are obviously a must.
Regards,
Michel Lamontagne
Hi Michel
Here’s a paper that’s handy for some quick delta-vee numbers:
Rapid Interplanetary Round-Trips at Moderate Energy
…a scan of the mission tables will show you that delta-vees of 20-30 km/s are needed for relatively quick transfers to Mars. Low-thrust engines result in higher delta-vees due to much higher gravity-losses when climbing out of gravity-wells.
I’m working on an Excel spread-sheet model of the orbits in that Wertz paper, which might prove useful. There’s lots of good soft-ware out there too.
Hi Adam,
Thanks for the table, very interesting. So the 90 day travel time seems a bit short at this point, unless we decide to go for more exotic nuclear reactors and really high levels of power. I expect there must be a series of intermediate solutions between the Hohmann transfer and the 90 days ones in the table that give less demanding requirements, and will look for some software. Aerobraking would give low deltaV solutions, but I wouldn’t try it with our design! Although we could try aerobraking the saucer only…. 😉
Regards,
Michel Lamontagne
Hi Adam,
I’ve downloaded the Trajectory planner, a stand alone free program that is an add-in to the Orbiter Space Flight Simulator. It gives solutions for Earth-Mars orbits, transit times and deltaV information. It agrees with the data in the table you referred to me, with all the intermediate solutions as well.
So we should be OK on that point at least, and can go ahead designing a range of missions for our GEN1 Enterprise, from the original 90 days to less demanding 120 and 160 days transit times.
Thank you for your help, regards,
Michel Lamontagne
A whole year has gone by!
Well, I’ve learned a bit of 3D modeling and here is the result of the work done on the ‘interplanetary’ Enterprise. It’s still gigantic, and i’ll be working on a smaller version now, but I wanted to complete the original vision of BTE-DAN, the originator of the idea.
Any work spots on the Icarus project for amateurs?
http://btewiki.org/index.php?title=GEN1b
Regards,
Michel Lamontagne
Hi Michel
Truly amazing! You’ve blended BTE Dan’s aesthetic with practical astronautical engineering.
regards
Adam Crowl