The explosion must be powerful enough to lift, but controlled enough to be useful.
The tanks must be pressurized to hold as much propellant as possible, but not so much that they leak. The metal must be thin to reduce weight but not so thin that it bursts under pressure.
The propellants must be energetic, sometimes poisonous, but not so caustic as to make them impossible to handle.
The most powerful and useful propellant and oxidizer are elements necessary for life: hydrogen and oxygen. But at the temperatures and pressures at which they are kept for rocket travel, they are below freezing and deadly. Their cold is nearly the cold of space of space itself. Liquid hydrogen, allowed to burn, creates a fuel-air bomb mixture; liquid oxygen is aggressive in its corrosiveness. Ignite them and they produce a fire to rival the surface of the sun.
The shape of the rocket is determined by the nature of its flight. It must present as little surface area as possible moving forward to reduce wind resistance as it plows through the atmosphere at unbelievable speed to break free of Earth's gravity. To steer the contraption, directional controls are located in the rear, an arrangement that produces clumsiness on land. The controls must react quickly, faster than a human being, really--so that the travelers are at the mercy of their hopefully well crafted machines.
Most of the mass must be dedicated to propulsion--getting the beast off the ground, along with its fuel and control systems. And somewhere along the line, the rocket must carry a useful payload. Without it, the rocket is merely an experiment in ballistics and Newton's Third Law.
The best rockets built have a mass fraction of 0.90. That is, 90 percent of the rocket is dedicated to non-propulsion hardware. And most times we cannot even manage that. We must sacrifice scientific payloads for life support systems or stronger structures.
Then there is space itself. Materials that behave one way in the relatively benign and predictable atmosphere of Earth outgas or turn brittle in hard vacuum. Thermal management is a pain. On the sunward side of a spacecraft, the temperature is 250 degrees Fahrenheit. On the shadowed side, the temperature is -250 degrees, so spacecraft must have sophisticated radiators to release heat into the bitter cold, or they must rotate like a chicken on a spit, and then the structure must cope with repeated expansion and contraction through differential heating.
Did I mention acceleration? Human beings can withstand, at best, four or five times the force of Earth's gravity before their performance is impaired. We black out at 7 g. Mere machines can be built to handle many times that value, but every weld must be perfect, every angle accounted for, every scrap of junk and dust removed to ensure that the circuitry and wires not merely survive, but do what they're supposed to do.
Then there's radiation. The sun is a big nuclear reactor. Protected by the Earth's atmosphere, magnetic field, and SPF 15 sunblock, sunbathers can still get cancer. In space, beyond the Van Allen Belts, little protection exists, and shielding adds weight. Radiation mutates biological tissue. It kills. It disrupts electronic computers and communications. It weakens metals, making them brittle. Radiation is not kind to space travelers.
The space environment itself has deleterious effects on human beings without spacesuits. Exposed to vacuum, human lungs explode, blood boils and freezes. The longest an unshielded body can survive in space is 30 seconds.
The distances and speeds involved in space exploration are outside of most people's experience. You can talk about being 250 miles above the Earth, but to circle the entire planet--over 24,000 miles--in an hour and a half doesn't really compute. The distance to the Moon is about 240,000 miles--ten times around the Earth. You can almost wrap your head around that number. The distance to Mars, though, can be as far as 250 million miles away. It's so far that a radio signal traveling at the speed of light would take 22 minutes to reach Earth one way. And some people dream about going to the far planets or the nearest stars, where light-speed distances are measured in years. Crews must work out most of their problems on their own or on a substantial time delay.
All of the worlds in our solar system have atmospheres unsuitable to human life. Like the empty space between them, they will kill. If the death isn't by vacuum, it might be by pressures greater than the deep ocean, temperatures hot enough to melt lead, or poisons that would scald the lungs.
So this is this environment space advocates seek to enter--this is the universe we intend to explore. It is so far lifeless, barren, hostile, and unwelcoming. It requires supreme acts of technological virtuosity to reach, let alone survive in space. There is little immediate promise of welcome or gain.
And yet we dare to go. Despite the danger. Despite the loneliness and isolation from others. Despite the cramped quarters, limited life-support ssytems, and potentials for crew conflicts.
We go to learn and to challenge ourselves. At some deeper level, we even go because of the danger, to prove to ourselves or others that we are not afraid, that we are willing to challenge fate. That we truly have the minds, the moxie to go where no one has gone before...and return to tell the tale.