Question

Describe some optimal control problems in your major field or in another area of interest to you. How difficult would it be to find the optimal controls?

Solution

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A moon lander\textbf{A moon lander} is an optimal control problem which asks us how to bring a spacecraft to the moon's surface, using as little fuel as possible?

We are going to use a following notation:

h(t)height at time tv(t)=h(t)velocity m(t)mass of spacecraft(changing as fuel is burned)α(t) thrust at time t\begin{align*}&h(t) - \color{#4257b2}\text{height at time t}\\& v(t) = h'(t) -\color{#4257b2}\text{velocity }\\& m(t) - \color{#4257b2}\text{mass of spacecraft(changing as fuel is burned)}\\& \alpha(t) -\color{#4257b2}\text{ thrust at time t}\end{align*}

where α[0,1]\alpha\in[0,1].

From Newton's law we get

mh¨=gm+α\begin{align*}m \ddot{h}=-g m+\alpha\end{align*}

where the right hand side being the difference of the gravitational force and the thrust of the rocket.

This equation can be modeled as a system:

{v˙(t)=g+α(t)m(t)h˙(t)=v(t)m˙(t)=kα(t)\begin{align*}\left\{\begin{aligned} \dot{v}(t) &=-g+\frac{\alpha(t)}{m(t)} \\ \dot{h}(t) &=v(t) \\ \dot{m}(t) &=-k \alpha(t) \end{aligned}\right.\end{align*}

These equations can be represented as

x˙(t)=f(x(t),α(t))\begin{align*}\dot{\mathbf{x}}(t)=\mathbf{f}(\mathbf{x}(t), \alpha(t))\end{align*}

where x(t)=(v(t),h(t),m(t))\mathbf{x}(t)=(v(t), h(t), m(t)).

Since we want to minimize the amount of fuel used up we have to maximize the amount remaining once we have landed, so we get the function

P[α()]=m(y)\begin{align*}P[\alpha(\cdot)]=m(y)\end{align*}

where yy is the value which represents the first time that h(y)=v(y)=0h(y)=v(y)=0.

We have a problem here because the deadline is not predetermined, so we have the following conditions: h(t)0,m(t)0h(t) \geq 0, \quad m(t) \geq 0.

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