Jan 29th, 2013 by Jonathan Goff
In many discussions of rocket technology, a skeptic will often make some comment about how things would be so much better if we had Warp Drive. But the reality is that we don’t really need Warp Drive for things to be interesting. We just need Sufficiently Advanced Propulsion Technology™ (name derived from Clarke’s Third Law).
While all sorts of arbitrary definitions for Sufficiently Advanced Propulsion Technology™ can likely be suggested, I would suggest the following high-level requirements as a minimum for a candidate technology:
- Can operate safely in a biosphere.
- Has an Isp high enough to enable an SSTO vehicle with a Mass Ratio comparable to existing jumbo jets
- Has an engine thrust/weight sufficient to enable a high enough vehicle thrust/weight for minimized gravity losses while still keeping total engine mass comparable to the engine mass fractions of existing jumbo-jets
My logic for this is simply that if you had a propulsion system like this, the airframe and systems requirements for the vehicle would end up being not that much higher than that of an airliner, and most likely the service life of the system would be measured in the low tens of thousands of flights. While I think that it’s possible to design a fully-reusable TSTO and maybe even SSTO rockets using existing rocket propulsion technology, in order to hit the mass fractions necessary, you either get really miniscule payload fractions, or you end up getting very very challenging mass ratios, or most likely both. But with SAPT™ you get very high payload fractions, with very unchallenging mass ratios. Now realistically, you would want to spread the pain a little bit more between the propulsion and the vehicle structures/systems, but I think this is a good first-pass that’s easy to analyze–i.e. this is what a propulsion system would need to do in order to enable easy highly-reusable SSTO vehicles without having to push hard on any of the other technical areas.
So what would this mean as far as performance specifications? Let’s start with stats from a representative jumbo-jet. I’ll use the 777 Freighter as an example of an existing, long-range jumbo jet.
Payload Fraction: .29
Propellant Mass Fraction: .43
Engine Mass Fraction: .05
Let’s start first with a “dense” propellant SAPT™ system (say something like subcooled ammonia or water), since for that you could assume that the propellant storage and handling dry mass is similar to the 777′s propellant handling and storage dry mass. If you assume a required 9500m/s of delta-V to orbit (a reasonable SWAG including gravity losses and drag losses with a high-Isp propulsion system), and a .43 pmf (which works out to a MR=1.75), the required Isp is approximately 1750s. If you assume a vehicle launch T/W of 1.4 (to keep gravity losses modest in spite of the very slow change in vehicle mass with respect to time), that comes out to an engine T/W of about 29. By comparison, most solid-core NTRs have Isps with subcooled ammonia of only about 450-500s, and T/W ratios in the 10-30 range (with the better numbers being for more theoretical designs and the worse numbers for ones that actually were closer to operations back when the US did NTRs).
For a LH2 propellant SAPT™ system, you’d need to do something to factor in the much lower density of LH2 compared to Jet A, liquid ammonia, or water. If you want to keep the payload fraction the same, and assume that the propellant-density-scaling parts of mass ratio (tanks and tank volume-driven structures) make up 10% of the non-engine dry mass ratio, that gives you ~2.3%. With LH2 being ~10x less dense than Jet A, and assuming you keep the payload fraction, engine fraction, and the non-propellant-density-scaling dry fraction constant (ie you take all the extra tank mass out of the propellant fraction), that drops the propellant fraction to about 22%. Using a slightly higher required delta-V of 10km/s (to factor in higher drag losses due to lower system density, and higher gravity losses due to the higher Isp engines not accelerating the vehicle as fast due to slower mass change), you get a required engine Isp of about 4100m/s.
Are there any even semi-sane propulsion technologies that come anywhere close to these numbers? The only ones I can think of that meet all three criteria might be Bussard’s QED ARC engines or variants that run off of something like Winterberg’s micro-chemical fusion bomblets…both of which involve not-exactly-proven-out fusion technology. Pretty much you’re stuck assuming some sort of advanced fusion or anti-matter system. Solid-core nuclear thermal or laser or microwave thermal are all too low of Isp. Gas Core Nuclear thermal could be high enough Isp, maybe, for an open cycle design. But finding a way to do that without flagrantly violating requirement #1 would be tough. Some of the pulsed nuclear propulsion ideas get up into the right Isp and T/W ratio range, but all involve EMP levels that would fry anything in LEO. There may be something else that none of us are thinking of, but nothing that looks even remotely near-term.
So in conclusion, this was a fun and somewhat silly exercise, but it does show you that in order for a propulsion system to be advanced enough to make the rest of designing a high-flight-rate SSTO-class vehicle “easy”. We aren’t talking about warp drive per se, but we are talking about technologies that are sufficiently advanced compared to the state of the art to still seem somewhat magic.
Until we have something like SAPT™, it looks like we’re probably ought to have focus on technologies that allow us better T/W ratios for our engines, much better mass ratios for our vehicles (due to better materials), keep living with much crappier payload mass fractions, and live with more Rube Goldberg launch methods (like TSTOs, boostback, airlaunch, tether assisted launch, etc). It isn’t the end of the world, and I think there’s a ton of room left to be squeezed out of existing, boring, chemical propulsion. But yeah, the inner sci-fi nerd in me hopes that some genius wunderkinden out there are working on propulsion technologies that are indistinguishable from magic.