What is the best way to access Low Earth Orbit (LEO)?
I have been reading various technical papers again this week and some are supporting different space transportation systems, so I thought I would cover the basic options for accessing LEO and what I think of each. I’m only going to cover fully reusable systems for Earth-to-Orbit (ETO) transportation systems because only fully reusable systems will have the cost/pound to orbit low enough that you or I could afford a trip to space. The candidate systems I will discuss includeSingle-Stage-to-Orbit (SSTOs), Two-Stage-to-Orbit (TSTOs) both ground-launched and air-launched, Space-Tether enhanced systems both in-orbit and ground-based, and finally air-breathing launch systems both subsonic and hypersonic.
ETO transportation is difficult because, unless you have access to a space tether, all transportation systems have to beat the rocket equation. The simplest form of the rocket equation is DV = Isp*g0*ln(m0/mf) where DV is the maximum change in velocity, Isp is the vacuum specific impulse of the rocket motors, g0 is the Earth’s gravitational constant, m0 is the vehicle liftoff mass, and mf is the vehicle burnout mass. The total DV to get from the ground to LEO is roughly 9.7 km/sec for an SSTO. This includes gravity losses; air drag losses and steering losses. Figure 1 below shows the effect of Ispand m0/mf for various SSTO type vehicles.
Figure 1 – Effect of Propellant Fraction and Isp on SSTO Ignition Mass
What Figure 1 showed was how critical propellant mass fraction and Isp were to SSTOs, both vertical takeoff and landing (VTVL) and horizontal takeoff and landing (HTHL). Propellant mass fraction is the fraction of the total mass that is propellant and is equal to 1-mf/m0. Obviously, for achievable propellant mass fractions near 0.90 one needs specific impulses above 430 seconds to build workable SSTO’s.
For many years cost-estimating tools have said that SSTOs are the lowest cost solutions for Earth-to-Orbit (ETO) transportation systems. This was because you only had to build one type of vehicle and there was no reassembly required before launch. We never actually built and flew an SSTO because the cost estimation tools we had back then were still predicting that SSTOs had DDT&E (Design, Development, Test & Evaluation) costs too high to make a reasonable operational profits as commercially developed ETO system.
Recently, technology improvements in automated design and build, plus materials advances, have reduced the DDT&E costs of rocket engines and airframes to 20% to 30% of previous estimates using the old cost estimation tools. In other words, we could now build an SSTO and expect to make money. So why isn’t it happening? First some background. Without cost to orbit less than $500/pound you can’t support commercial human endeavors. This was proven in the Commercial Space Transportation Study back in 1993 done by the New Business Organizations of all six major Aerospace Companies. This is best explained by the curves in figure 2 below.
Figure 2- ETO Yearly Launch Market is Elastic wrt Launch Cost/Pound
As you can see in figure 2 the yearly flight rate to ETO is essentially flat above launch costs of $750/pound. At $500/pound and below the ETO market can support 100 flights/year. The problem is the total yearly launch revenue (red line) is very low for the level of investments required, so unless the launch providers can get some of the total launch plus on-orbit revenues (yellow line), developing a new reusable ETO transportation launch system has a marginal payback. Hence, despite forty years of trying we still don’t have a Fully Feusable Launch System (FRLS). That appears to be changing in that Space-X is working towards a 2021 launch of their Starship FRLS (BFR), and Blue Origin’s New Glenn may not be too far behind. Elon Musk and Jeff Bezos are funding their FRLS programs despite the marginal economics because 1) they are billionaires and can afford the risk, and 2) they believe that it is humankind’s destiny to move out into the solar system and beyond. We should all salute them.
Musk and Bezos are building VTVL TSTOs fueled by methane. How does that matchup with the conclusions above? Both Musk and Bezos are seeking cost-driven solutions and at this point in history it is much cheaper to develop and operate a liquid methane-fueled reusable rocket than a liquid hydrogen-fueled reusable rocket. They are willing to pay the penalties for a two-stage system to enable a good payload capability with the lower performing LOX-Methane rocket motor. In the future it may become practical, and even cost-effective, to build lightweight reusable liquid-hydrogen tanks, but that is not the case now.
There are two basic future mission for ETO vehicles. One is tourism and the other is freight. The different requirements will drive different solutions. Tourists want to go to a nearby airport, board the vehicle, and arrive at their final destination in no more than a couple of hours (after all zero-gravity causes most of them motion sickness and they are in cramped quarters). Freight doesn’t care how long the wait is as long as the ride is cheap.
For tourists we want the first stage to be an airplane capable of operating out of standard commercial airports. The airplane takes off and flies to the ground track of the intended tourist station in LEO and launches such that upper stage can rendezvous with the station in a matter of minutes. Obviously, the airport takeoff was timed so that the station would be in position for the quick rendezvous. The return from space is a bit less constrained since the upper stage is winged and has significant cross-range capability. My proposed TSTO tourism launcher uses an Air Collection and Enrichment System (ACES) and is shown in figure 3 below.
Figure 3 - Cutaway of Tourist ETO using ACES Technologies.
ACES collects air as the airplane flies to the ignition point on the ground track separates out the oxygen, liquifies it, and stores it in the upper stage and airplane propellant tanks. To accomplish this, it uses the enthalpy in the liquid hydrogen stored on board as fuel for the jet engines to burn during the transit from the airport to the space station ground track. This process was demonstrated during the 1960s in a USAF black program and again in 2003 by Andrews Space under a DARPA contract.
The primary driver for freight delivery to LEO is cost, not time, so fast rendezvous is not a requirement and VTVL works just fine. The near-term solution is LOX-Methane VTVLs and we have two examples under development right now. Further in the future we should have a satisfactory liquid hydrogen tank solution and I can see a tri-propellant SSTO as the lowest cost competitor. An example tri-propellant SSTO is shown below in figure 4. This was proposed in my U of W class back in 2012.
Figure 4 – Example VTVL SSTO Architecture using deployable Heat Shield
The SSTO proposed has numerous features to improve its economics. First, it uses existing SSME and AJ-26 rocket engines to minimize DDT&E costs. (AJ-26s are Russian engines bought by Aerojet for the Kistler K-1 Launch Vehicle described in Chapter 6). Since, the SSME uses LOX/Hydrogen propellant and the AJ-26 LOX/RP-1 propellant it is obvious that our SSTO is a tri-propellant design, as shown in figure 4 above. A tri-propellant rocket that burns the low performance kerosene (RP-1) fuel first enhances DV performance, and the VTVL configuration reduces the SSTO empty weight, and development and unit costs. The five SSMEs and the twelve AJ-26s burn from liftoff to about Mach 4.5 where the AJ-26s shut down and the SSMEs extend their nozzles. Two of the SSMEs shut down about 415 seconds into the 492 second ascent burn to keep acceleration under 4 gravities. This SSTO has a Gross Liftoff Weight (GLOW) of 3,931,000 lbm (1783 mT), a payload of 72,750 lbm (33 mT), and an empty weight of 288,000 lbm (130.6 mT). It is a vertical take-off, vertical landing (VTVL) configuration built almost entirely of advanced composites with a deployable, inflated base re-entry shield made from flexible TPS materials similar to those used on the proposed IRVE shown at the bottom. This approach allows for both low cost and the extremely low dry weight.
The only way to beat the rocket equation is to reduce the total DV to get to orbit, and there are ways to do that. One way is to increase the speed at launch. High speed sleds and catapults have been proposed to do that. Another way is to reduce the speed needed to achieve orbit and that is the next topic I want to cover.
Space tethers can be used to provide a rendezvous target in LEO that is moving slower than an object in orbit at that altitude. In fact, if the tether is long enough (> GEO altitude) the bottom could be stationary on the Earth’s surface and we have a beanstalk on which a cable-climbing car could carry a payload from the surface of the Earth all the way to geosynchronous orbit. Unfortunately, there are numerous fatal flaws. There are currently no materials strong enough to build a beanstalk, everything in orbit will eventually impact the beanstalk, and at a crawler speed of 60 meters/second (i.e. a fast elevator) it would take seven days per trip and the number of crawlers operating at once is limited by weight considerations. In addition, the first day in a crawler is spent in the heart of the Earth’s Van Allen belt where an unshielded person would achieve a fatal dose in hours.
I propose a far simpler solution, namely the Skyhook Tether shown in Figure 5 below.
Figure 5 – Proposed Skyhook Configuration
The total tether length is 2100 km and it is stored and deployable from a LEO station at 1300 km altitude. The Concept of Operations (CONOPS) for the Skyhook system is to launch and assemble an initial 200 metric tons to LEO and this initial station self deploys using electrodynamic forces on deployed tether lengths to get itself to operational altitudes. After that it can bootstrap itself up to full operational conditions. For the configuration shown the ETO launcher total DV for ETO has been reduced from 9.7 to 7.1 km/sec significantly reducing the mass fractions required. I visualize a 450,000-pound SSTO carried by an airplane to the point where it can rendezvous with the skyhook catcher and transfer a 10-metric ton tourist capsule. Once the transfer is complete, the capsule experiences approximately one third G and is pulled up to the intermediate winching station which has observation windows and refreshments. Three hours later they are at the space station. More details are available in Chapter seven of the book.
The last topic I wanted to cover is air breathing ETO systems. There has been some excellent work done in this field over the years, but it requires really high-temperature, light-weight materials and those haven’t come along yet. The original NASP project was way over-sold for the solutions they had in hand. Recent work is more realistic and the SABRE (Synergic Air Breathing Rocket Engine) work in England is of special interest because their numbers check out and their HOTOL concept might conceivably work. Unfortunately, they are still laboratory testing so we’re just going to have to wait and see.
I tried to cover an enormous amount of material in six pages, but most of it is in the book, so you know where to go with questions. The bottom line is that there are numerous techniques for achieving ETO. Musk and Bezos are pursuing an optimal path with today’s technologies, but tomorrows technologies could lead to even cheaper paths to achieving ETO. I’m too old to go to space but many of you are going to have that opportunity, and I wish you well.
Thanks for your attention.