The projected NASA budget for the X-33 program as currently envisioned is as follows (dollars given in millions; years given are fiscal years; includes monies in NASA supporting technology work) --
96 97 98 99 Total NASA $ 49 $152 $325 $345 $871 USAF $ 0 $ 0 $ 0 $ 0 $ 0Two types of SSTO are being investigated by the Phase A contractors: Vertical Take-off, Vertical Landing ("VTVL") & Vertical Take-off, Horizontal Landing ("VTHL"). To illustrate, Shuttle is VTHL -- it takes off vertically using rockets & glides to a landing using wings. VTVL would take off using rockets & would land on its tail on a pillar of fire using rockets. The wildly successful DC-X unmanned test vehicle demonstrated VTVL multiple times, flying out of White Sands, New Mexico. On the Moon, humans have performed vertical rocket powered landings 6 times (albeit with no atmosphere to contend with, & in 1/6th Earth's gravity). Robot spacecraft have also successfully landed on the Moon this way, as well as on Mars (Mars has a thin atmosphere & 1/3rd Earth's gravity).
A VTVL SSTO would convey many operational advantages. Consider, for example, the many times the Shuttle has had to stay longer in orbit due to bad weather at one or both of its landing sites. Similar weather concerns would not affect a VTVL vehicle. On the other hand, however, the only vehicles we have experience with are VTHL spacecraft; i.e., the Shuttle & some test lifting bodies (ASSET, PRIME, etc.).
The underlying rationale for reusable SSTOs is as a means to be able to get to Earth orbit cheaply & frequently. It is assumed that any such operational SSTOs would be commercially built & operated. It is precisely this vision of cheap, frequent, commercial access to space that is motivating SSTO proponents to be SSTO proponents. Any operational commercial SSTO, then, must be sensitive to commercial concerns. One such concern is "time is money." VTVL SSTOs would be much more sensitive to this commercial aspect than would VTHL SSTOs.
Thus, a dilemma. A VTHL SSTO would probably simplify the goal of demonstration of reusable single stage flight to orbit & return, but would also tend to lead to commercially subobtimal operational SSTOs. A VTVL SSTO X-vehicle might be a little bit technically more risky than a VTHL SSTO.
Another aspect to the dilemma -- two of the three Phase A contractors are looking only at VTHL vehicles. The 3rd team, McDonnell Douglas/Boeing, was originally looking only at VTVL, but is now looking at VTHL, & will probably bid VTHL, dropping VTVL -- despite their team having the only experience with VTVL via their DC-X "X"-vehicle. All work on VTVL would cease.
Luckily, there is an answer -- one time-honored in the aerospace industry. It is to build two different types of vehicles, & then have a competitive "fly-off."
This answer comes with a price. A competitive fly-off between a VTVL & a VTHL SSTO X-vehicle would cost $1.85 billion, versus the current program of about $1 billion (the cost, roughly, of two to three Shuttle flights). The cost breakdown per year would be --
96 97 98 99 Total NASA $ 90 $300 $600 $600 $1590 USAF $ 50 $ 70 $ 70 $ 70 $ 260v. 1 Wednesday 22 March 1995